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• • • •

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41.

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ELEMENTIRT INTRODUCTION

NAUTICAL ALMANAC,

ASTRONOMICAL EPHEMEBIS;

EPITOME OF ASTRONOMY,

WITH SIXTY ENQRATED DIAGRAMS.

BY G.P.PAYNE,

OPTICIAN, LIVKBPOOL.

LONDON:

Cbarlbb Wilson, Lbadknhall St.
LIVERPOOL:
Melling & Payne, South Castle St.

it S3.

PEEFACE.

The scheme of the following pages is to afford instruction without the aid of
any other work than the ^'Nautical Almanac, and Astronomical Ephemeria/' to
which it is professed they are an Introduction.

Writers on Elementary Astronomy generally either reserve the information
most eagerly sought hy the curious, or detail it on the supposition that the rea-
der is a mathematician. Undouhtedly no great progress can he made in Astrono-
my without the aid of the mathematics, hut there may he a medium between a
perfect acquaintance with these abstruse studies and a sufficient knowledge to make
astronomical data easy of comprehension. The Author, therefore, in pursuance of
his plan has, he beUeyes, familiarised and introduced as many scientific explana-
tions as may enable any student to comprehend the facts and complicated theories
of the Celestial Phenomena. It will be immediately evident to the scientific rea-
der that any attempt at such explanations beyond an outline would not be
consistent with the scope of so small a work as the present.

The alphabetical arrangement has been adopted for the convenience of reference*
and care has been taken to give in every article the words to which the student
must refer to complete his information on any one subject.

EPITOME OF ASTRONOMY,

Astronomy may be defined as the science which teaches the order, motions, dis*
tances, magnitade,eolipses and appearances of the heavenly bodies, and their relation
to the Earth. The study of this science consists of systematic observations of the sun
moon, planets and fixed stars; and to assist these observations there are books and
tables of general calculations, maps which delineate the apparent places of the starry
host; and the celestial globe, on which we see the stars, not as they appear in the
heavens, but reversed for the purpose of working problems. The stars on the celestial
globe would present a picture of nature if the globe were hollow, the stftrs pierced
through the shell, and we were to view these apertures from the centre instead of the
circumference. The symbols and abbreviations used in Astronomy are the foUowingir

The Run.

Conjunction. Signs of the Zodiac.

The Moon,

Quadril. Hours of

Celestial

New Moon.

Opposition. Riffkt

Longi-

First Quarter.

Trine. Ascension,

tude,

•

Full Moon.

•X-

Sextil. r

Aries. - - ^ 0°

Last Quarter,

Ascending Node. I

Taurus. -. - 30

Mercury,

Descendii^ Node. II. n ,

Gemini. - 60

Venus.

North. III. ffi

Cancer, r - 90

S,©ore

The Fiarth,

South. IV. St

Leo. . - - 120

Mars.

Ea-st. V. 1TJJ

Virgo. - - 150

Vesta,

West. VI. ^

Libra. - - 18U

Juno.

Degrees. VII. tn.

Scorpio. - 210

Pallas,

Minutes of degree. VIII. f

Sagittarius. 240

Ceres.

Seconds of degree. IX. Yf

Capricomus. 270

Jupiter.

Hours. X. ns

Aquarius. - 300

Saturn.

Minutes of Time. XI. K

Pisces. . -330

The Georgian.

Seconds of Time.

B.A. Bright Ascension.

Dec. Declination. Long. Longitude

. Lat. Latitude.

Greek characters to denote the magnitude of fixed stars in any particular constella-
tion: a Alpha, First magnitude; /3 Beta, Second; y Gamma, Third; 8 Delta Fourth; e
Epsilon, Fifth; f Zeta, Sixth; i; Eta, Seventh; S Theta, Eighth; i Iota, Ninth; k Kappa
Tenth. These characters are sufficient to distinguish the principal fixed stars, but
the whole of the Greek alphabet is used ,and, when exhausted> the Italian and Boman
characters, and the common numerals are employed. When a Greek character and a
numeral are both used they denote tht^t there is more than one star of the same mag-
nitude in the constellation; as a* Geminorum, (Castor J the star Pollux being a^
Greminorum. a. Alpha, also signifies Ascension, and ^, Delta, Declination.

€ AB. AC. AG. AL.

ABERRATiON>(^a deviation.) Light occupies 8" 13" (8 minutes 13 seconds of time)
in passing from the Sun to the Earthy and during this time the Earth has moved
20'', (20 seconds of a degree, the entire orhit of the Earth round the Sun heing
reckoned as a circle of 360 degrees) . At any instant, therefore, the rays of light
which reach the Earth are those which left the Sun 8" 13* previous, when the Earth
was in a different position with the Sun to that which it occupies at the time
of observation. This phenomenon is called aberration, and the luminous circle which
mi^ be seen around the real disc (face) of the Sun (caused by the aberration of
the solar rays) is called the crown of aberration. And as the aberration of the
Sun is owing to the motion of the Earth in its orbit, so the aberration of a Pla-
net is due to the ownbiii^ moUofas of that Planet ahd the Earth in their respec-
tive orbits. It is therefore said, that the. aberration of a Planet is equal to the
geocentric ( See geocentric, ) motion of the Planet, or the space it appears to
move ovef as seto from the Earth, during thi timb (^' light ocec^wes in piss-
hxg {\rom the Planet lx> the Earth. ¥^ tiie method «f estinMiJig Ae amomil; cf
abemAion see mens^ratiok.

Acceleration. Either a real or ap^a^t iiitii^ase of Uhc veloctty dt cdlMltiifl
bodies. It is idito applied to th^ difibrence belfweito a sblkr and sidereal di^, {ISeis
TIME ) or the time by which th^ ttxed Hti*s art seen froih the Earth, oh €Btt9t
arrival at some pbint, such as the mendMi point {iSf€^ MERii^rAiv) to anticipate !3ie
anival of the Sun at the same point. A Star which i^ses with the Sun one day wW
rise 3°* S6* Sooner than the Sun the sncceefing diay. This nebehraihn is caused: by
the Eftrth revolving in its orbit round the Sun, so that we View the'^n StSy in it new
point wit& relation to the Fixed Stars, which are at such an inxKmott'^ble distance
that the entire circle of the Earth's path round the Sun ( the diaaiieter of the circle
being 190 millions of miles, ) is but a speck when compared wiih idie imidensity of
the Starry sphere. The acceleration of a Planet is a realintoveaiiib ofTielooity in one
part of its elliptical orbit over its velocity in ichother part. The ddoelerMon of the
Moon is a very slowly and gradually increasing motion of tke'MolDh Youkid the £arth>
the cause of which is only attempted to be explained by mechanieal laws of great com.
^lexity. Betardatixm is the term contrary to acceleration, and signxdes eiflier a real
or apparent slowness of velocity at one time compared with anothsr.

AcRONicAL, or AcrOnychal. a term given to the rising of a^itair above the ho.
rizon as the sim sets, or to its setting below the horizon as the sun rises.

Age of the Moon. See Moon.

Altitude, is measured on the arc (of 90 degrees,) between the obser^r's zenith
and horizon, {See angle, zenith and circles). Thus the Star in* the fimt diagram
is at an altitude of 45^. ^

This apparent altitude differs firom the true td- S

titude by the dip, parallax and refi'actum,(wkidh fil'

loords see), and the allowance made for these differ-
ences is called the correction of the altitude.

When the apparent alti-
tude is corrected the tme

altitude is an angle between OhseriHr I J Sortie n

two lines drawn from the centre of the Earth, one of them being
the rational horizon. {See circles.) Cirhles t)f dtitude iwe at
right angles with the horizon, and intersect each other at the zenith. IPorcffi^fo-of U-
titnde are parallel to tlic horizon.

AM.

AN.

South/

AitfftiTUDB. W^ p^t of the <compm «t.iiir]Uch a jbefn^y b^dy rinses or ;^ts, or
ibe Ai0t9»oe of tl^ jpvwiptl^m -tlie ^t'or W^t^^^t^nrar^ t^^ NqrOi^ Soutb.

l>h»% m-.i^ 4iliginKm» o be^ng 4]ie position of the

observer^ the am|Attt4eoftheStar.risipg|>r^isel7

hdf way between the £iist and Nov|hjH>iQ^^will be

45^N. (45,<il^a^ sN^orth) and thje amplitude of the

West 4 A \ Easl other Star settiiig one quarter of the distance from

the West to the South will be 22"" 30' S. (22 degrees
30 minutes, or 22^ degrees South.) Amplitude may
be called the horij^ontal azimuth. (See azimuth) .
Circles of amplitude are at right angles with the
horizon and ix^m&ct each other at the zenith.

ANGiiSj (in Greometry)j is the incHnation or opening between two, straight lines that
meot at one extremity. The point jwhere the lines meet is called the vertex or angu-
lar p^t, and is taken for the oentre of a circle^ on the circumference of which the an-
glejusoneasured m degrees and parts of a degree^ the whole circle b^ing divided into
360?. The vBiie of ^-circle by whjich the angles are
meas^ff^ is immaterial; the aras, or parts .of the cir-
comference, of the largest and smallest circle described
from ^ angular point, (that is drawn by one leg of a
pair of compassesiwhile the other le^g is on the ai^2;ular
pointy) and included between the lines forming the an- I
g\e, being proportionate to each other. Thus one'
fourth and one eighth of each circle in the figure are lii-
•videdby the Imfis^ forming the angles 45° and 90° .
iXhefigQrc^JieiNrQsento aiigles of 22^°, 45°, 90°aQdl35°,
the ^e or radius forming in eadi caae one J^ of the
.«n^^ jghile. tjbe measurement by degrees. is comn^on :to both drdes. It will be seen
«itttajjghM|ce that an angle.cannot amQuntto.as.mi^y as 180°> the two.radji or lines
^ from 0° and 180°.uotfoi]a9i^^rf^9]^}e« but becoming. one continued
striught line. .Astranovucal.moQsurements.are performed by observing
the angle included between any two points, as a Stiur and the nearest point
of the horizon, a Star and the nearest point of the Moon's disc^; the eye
of the observer being the angular point. In tig. 2. e. Leaad e. b, are the
legs of the angle, e,a. the altitude, I, a. b. the base and e* the angular
jpoint; the measurement of the angle taking place at the base or arc L a. b.
Anomalistic Tear. See time.

' An&s. When the Planet- Saturn is in sodi n position that we see the thin edge
only of the Ring whidi suiTOunda him the term ofiMB is given to the Rii^ which then
appears only as two horns or handles projedang fix>m each side of the IHanet.

^ , , -fir Antbcedkmce. Although the FLgiets rise daily in

the East r and. s^t in the West, an appearance owing to
the rotation of the Earth on its axis, th^" are in reality
pursuing the opposite course from West toE^t. There
is an exceptipn to this rule however occasionally to
be observed; the Fhmets appduring either to be sta-
tionary among the Fixed Stars for a short period, or
to retrograde towards the West. This is owing to the
different positions of the Earth and Flanets with respect

IR0

Xasl

West to each other in different parts of their orbits. Thus,

8 AP« AQ. ABu AS.

With reference to the diagram, if the Earth proceed in its orbit, -from West iP
Ea^ty whilst Jupiter is passing from W. to E. Jupiter will appear to he travelling
westward, fot when the Earth is at West Jupiter is seen at W. towards the East, and
when thg Earth is at East Jupiter is seen at E« towards the West. This apparent
retrograde or hackward motion is called the antecedence, or motion in anteeedentia,w}ale
the true motion is called the motion in eonsequentia or direct motion.

Aphelion and Apogee. See Apsidcs.

ApsitkES. As the orbits of Planets are elliptical, and the Sun not precisely in the
centre, part of each orbit Is farther from the Sun than the opposite part, and the
extreme points of the farthest and nearest parts are called the apsides. The most re-
mote part is called the aphelion or higher apsis, oitBXa?
and that part the most adjacent is the perihelion ,..-..•.—... — ^^^^
or lower apsis. The line of the apsides is drawn ..•'* \ .
through the centre of the sun to the aphelion and ^'/\J» ^\ -%
perihelion, as in the diagram. Instead of aphelion i!i / ^ CjWa '^\ ^T

and perihelion the words apogee and perigee axe ^i\i; -.- ^J^aO' — v""'^!®*-

used to designate the farthest and nearest points ^\^ **^if yuia^.j^. ^

of the orbit of the Moon round the earth, apogee '--^ ^/ ^

being the farthest and perigee the nearest point. '••.. y

The Sun is also said to be in apogee or perigee q^' '^"5^

when the Earth is at the farthest or nearest point A P I«-A- ^

of its orbit round that luminary.

Apparition. See Heliacal.

Aippt^LSE. When two heavenly bodies can he seen at one view through the telescope.

Aquarius, (the Waterman) The sign of the Zodiac which the Sun enters about the
19th of January and leaves about the 16th of February. . .

ARp. Part of the circumference of a circle. If at noon precisely an observer look
towards the Sun and imagine a line in the heavens passing perpendicularly from the
centre of the Sun to the horizon he will imagine an arc of his meridian.

Aries, (the Ram.) A sign of the Zodiac which the Sun enters about the 21st of
March andi leaves about the 19th of April.

Asc&NDi^G. lUsing from the horizon. When a Planet's latitude {See latitude,) is
increasing towards the North i^.is said to have an ascending latitude, hecause in the
Northern parts of the World the Planet will appear to be proceeding farther from the
Sorizon, and when a Planet's latitude is increasing towards the South it is said to
have a descending latitude. The reverse of these terms would be appropriate in the
houthem Hemisphere. The ascending node {See node) is that point of a Planet's
orbit passing the Ecliptic towards the North.

. Ascension. See Right Ascension.

AscicNSioNAL Difference. See Right Ascension.

Aspect, (ff Stars and Planets, is their situation with respect to the Sun or to each
other. There are five aspects: Conjunction, when they are in the same degree ;
Sextile, when they are 60 degrees distant from each other; Quartile, when they are
90°, or one quarter of a great circle apart; Trine, when they are 120°, or the third
part of a great circle asunder, and Opposition, when they are opposite to, or a semi-
circle distant from each other.

AS.

ATI.

AX.

AZ.

Astronomical Day. See Time.

Astronomical Horizon. The Rational Horizon. See circles.

Asteroids. Four Planets lately discovered; Juno, Ceres, Vesta and Pallas* See

BOLAR SYSTEM.

Austral. Soutliem.

Augmentation. There is an apparent and cominonly observed increase of mag-
nitude of the Sun and Moon when near the horizon, the cause of which is in dispute,
but may be owing to the vast quantity of atmosphere through which we see those
bodies when we look towards the horizon at them. A similar apparent increase of
size takes place if ashilUng be put m a saucer with a small quantity of water, when
the shilling will appear of its real size, but if the saucer be filled with water the shill-
ing will appear much enlarged. We may consider the atmosphere as a large magni-
fying power (or convex lens, as the optical term is,) weakest in the zenith, (See
ZENITH,) towards which we look through the least quantity of atmosphere, and oPthe
greatest power at the horizon, towards which we look through the
greatest quantity. Thus to a spectator at a, of the Moon in the zenith M

there is a much smaller quantity of atmosphere between the Moon and * ;' / \
his situation on the Globe than there is to a spectator at b. The ? A •

figure will show how small the surface of the atmosphere, covered by Ij \ \

the reflection over a, is to that angular reflection caused by the rays
proceeding to b. It vnUl be seen therefore that this apparent increase ^^0Si^?T?
<of magnitude is only an optical illusion. The Moon, indeed, is decreetsed
instead of increased in apparent diameter as she approaches the horizon
from her greatest altitude. This is caused by the Moon being so much ff
farther from the observer at b than from the observer at a. For the Moon being at a dis-
tance of only 240, 000 miles from the Earth, and the distance from a toe, or one half
the diameter of the Earth, being near 4000 miles, the observer at a is one sixtieth part
of the entire distance nearer the Moon than the observer at b. The Augmentation of
the Moon is 1 second of a degree when the Moon has an altitude of 5 degrees from
the horizon, 10 seconds when at an altitude of 40°, 15" when at an altitude of 80°, and
1 6" when at the zenith.

Axis, of the Earth, An imaginary line conceived to pass through the centre of
the Earth from North to South, and around which it revolves.

Azimuth. The distance of a celestial body from the North and from the East or
'West points of the Horizon of any place. When the body is so far from the North

« 1

^v.

as to be near the South the Azimuth is sometimes
reckoned from the South point; that is, the sup-
plement {See SUPPLEMENT.) of the Azimuth is
taken instead of the Azimuth itself; but this is im
material, for the number of degrees between the
North and South being 180 it is of the same im-
port whether we say a celestial body is 1 70° from
the North or 10° from the South. The Azimuth
"VVest of a Star is technically described as an angle at
the zenith (See ZENiTH,)of a spectator contained between his meridian (See merid-
ian.) and the circle of altitude (See altitude.) pasang through the star. If we
suppose the star.* on the circle of altitude in the diagram to be precisely half the

£a3^

'Sotjuth,

10 BE. BO. tSA. CE.

distance between the West and South its Azunuih inXL be N. 135° W, {Northf 135
degrees West.) or S. 45° W. (Sautk, 45 degrees West.) When the circle of altitude
or Azimuth circle cuts the Horizon at the East and West points it is called the
Prime Vertical. The star at the Eai^ in the diagram is on the Prime Vertical. The
Magnetic Azimuth differs* from the Tnce Azimuth according to the variation of the
compass.

Beard of a comet. The rays of light around the nucleus or central body, distin-
guished from the tall, or himinou^ appearance emanating from that part of the
Comet opposite the Sun.

Bissextile, or Leap Tear. "Rx^ty fourth year, when a day is added to the calen-

dar> the twenty mnth of February, for the cerrectioil of time. But as the excess of a
Solar year (See time,) is not quite the fourth part of a day above the 365 days of
a common year, and as the adding a day to the calendar every fourth year would ex-
eeed the amount of correction required by 44 minutes in 4 years, ( or 1 1 minutes
annually,) it is arranged that eeerg hundredth year shall not be a bissextile year but
only three of every four centesimal or hundredth years. The leap years of the pre-
sent century ace easily found by dividing the number of years since 1800 by 4; if
remain it is leap year; if 1,2 or 3 remain it is 1,2 or 3 years after bissextile.- Exam-
ple: Is 1849 a leap year? Answer: 4 times 12 are 48 and 1 remains; consequently
the year 1849 is 1 year after bissextile.

BoREAf.. Nofthem.

Boreal Sn&NB. Theses of the Zodiab North df the Equator, Aries, Taurus
Gemini, Cancer, Leo and VirgOi m which the Sun is ntuated from the 2l8t. of March
to the 21 St. of August.

Calendar. An Almskiac. Fbr' an exphflHtioii of the **R^eipal Articles of the
Calendar^', in Ate Naoiical Almduae, see T»f«.

Cahcfr, the Crab-. The sign of the Zodiac which the Sun enterr about the 2rst.
of June, and leaves^ about the 22nd. of July.

Capricorntjs, the Goat. The sign of the Zodiac whioli the Sun enters about tlie
22iid. orDecember andieaves abdut the 19th. of January.

Cardinal Signs. The Signs of the Zodiac in which the Sun appears at the
commencement of Spring, Summer, Autumn and Winter; Aries, Libra, Cancer and
Capricorn.

Catoptrical. Bielating*to catoptrics; the science of^reflected vision. See reflbc-

tion.

Central Forces. The centripetal andcentriftigal forces, which are antagonis-
tic to each other, and' by means of which the planetary motions are conducted. The
centripetal force, if acting by itself^ would gravitate the MoOn and the other satell-
ites to their primary Planets, and the primary Planets to the central Sun; but the
centrifugal force, of equal power with the centripetal, and exerted in a conttary di-
rection^ (that of repelling or scattering from the centre,) brings about an equihbriUm
of motion. < The scattering discharge of mud from a coach wheel in motion is the
effect of a centrifugal force, and the &11 of a heavy substiance to the ground^ ir cau-
sed by the centripetal or gravitating force.

Celestial EauATORi The Equinoctiid cirde in the heavens. (See circles.)

Celestial Meridians. Great Circles in the heavens due North and South,

and intersecting each other at those points of the heavens immediately opposite the

CER. CIR. 1 1

'■■■■■ - , .

North, and South pales of the Ei^nth, wd termed the celestiAl poles. If -the reader
will pla€e himself ^th his hack to thp Nprtb he wijl h»ve the West at his. right
hand, the East at his left hai^d and he will look towardsithe. South* A line imogioed
to he drawn under his feet towards the North and Squjt^ ^11 be his Meridia)i ob
the Globe, and an arc in the heavens opposite this lii^e, fronji the zenith (the poinJi
overhead, ) to the North or South will be an arc of his Celestial Meridiaft. The^uper-
ior meridian of an observer is that ar<^ of his Meridi^ Circle which is above tho-
elevated Pol^ (See poles,) the arc belqw the Pole beir^.his.ti^erMii' meridiaa* This
distinction is nepessary when the transit of the Meridian, hj the celestial bodie3 is
mentioned, as thpy all mal^e two trans^s of every Meridian Girdle da^y; one transit
taking place at the superior or upper, arc of each Meridian and the other at the in-
ferior or lower arc.

Ceres. One of the four Spheroides. iS'e^ solar sx^TPMf

Circles^ A^t^onome^s ims^^e ma^y Great Circles iu the heavens^ but three priu^
cipal ones, from whic^^ t^ey.fprm the bases of all observations ,of the heavenly, bodies.
Every circle i? called Qreat the plane of which* passes thcough the -centre ot the.
Earth. The j^^an^ of ^ circle is an .ii^ag^nary number of ra^u • HUingrUp. the circle:
tjms the sppkes.of a.wh^^lare the, radii of the circle of the. wheel, and thexadii with
the axle compose the j^/awe: the parchment of a dr^m is,th^ jo/a/ie of the circlex)n which .
the parchment is stretjch^d. The, first of the three Great Circles is the Rational Hori-
zon. That circle made round our field of view at. sea by the watpr apparently touching,
thei heavens is called the apparent or visible^ horj^QJV a circle .drawn horizontally^, or per-
fectly level, from th0 S-ElSr.Shl'B^lS;

eye of an observer is "^ ^^i'

the «e?m5/&,horizpn» i'^sSS^ —
and a cirjcle whose '^
plane pas^e^ through

the centra of the earth ' ^ . ^ .

is the ra^ionc? horic B.A.TIOKAX, '^ T-IORi 7>OK^^

zon {See tfie figure, anc( the word de^resion) The sejcond Great Circle is the
Equinoctial, or Celestial Equatpr, a circle of th^ heavjens, the plane of which 4s in
continuation withthe plaae of the Equator of the Earth. The Sun is jon the Equi-
noctial about the 2 1st. of March and '2 1st. September, when equal days imd nights
are over the Earth. {See seasons.) The Equinoctial is traced by observing certain
remarkable .Stars, to which the Moon in her course seems ta appxpach, and also. by
observing the position of the Planets. The third Great Circle is the Ecliptic, or ap-
parent annual path of the Sun, which also is traced by observing icertain .aremarkaUe
Stars, the distance of which Jfrom the E^pty;. Jtia]^ i>^^ i^QHe«il%- fiae^iitaised^ Se^-r
SEASONS and earth.

Circles of the Sphiire. Th)BL appoB^j^^^e ofih^^^e^s^em^Bom. the Earth risgl^ -
bular or spherical,, and any. circle in^agii^fsd in the heavens is a Circle of tiie Sphere*

Circles of Altitude,.^, Am plitupe, Aziuvjn, DEeLiNATjoN, Lativude,
Longitude. See, Altitupe ^c.

Circles, Diurnal. Circle^apparently deserihed by the jb^eas^enly bodies in thei^
daily passage over the Earth frojji East .to West,

Circles of Excursion. Circles par^-Uel to, simI at^uqh a distance (about 10 de-
grees) from the Ecliptic as to.be the boundmri^s ^of the excursions of the Plimets.

Circles of Perpetual Apparition and. Perpetual Occultation. The in-
habitants of the equatorial parts of the Earth have the whole of the Celestial Sphem •

CIV.

COL.

Celestial
South Pole.

Celestial
North Pole.

turned towards them every day, but those living North or South of the Equator see only
a part of the sphere. This will be understood by imagining the globe in the first dia-
gram to be revolving from W to E, when it will be evident that a person situated oa
the Equator will have every part A

of the Sphere presented to his
view in 24 hours, but a person
situated at the North or South
Pole of the Earth would always
have a Celestial Pole overhead,
and all the Stars of his hemi-
sphere (half a Sphere) perpet-
ually describing circles around
parallel to his horizon. The cir-
cles D G would be diurnal cir-
cles (See CIRCLES diurnal) of Stars seen at N; and A O would be the circle of
perpetual apparition and perpetual occultation to N and S; the hemisphere from A
O to the Celestial North Pole being in perpetual apparition to N. and in perpetual
occultation to S and the hemisphere from A O to the Celestial South Pole being
in perpetual apparition to S and in perpetual occultation to N. In all places distant
from the Equator only one Celestial Pole is visible, or in perpetual apparition, the
other being invisible, or in perpetual occultation. The second diagram represents
the circles of perpetual apparition and occul-
tation of an observer in latitude 45° North.
The Globe must be imagined to be revolving
from W to E., as in the first diagram, but,
the observer being now at I instead of at the
Equator the circle A B will be in perpetual
apparition to him, and the circle C D in per-
petual occultation; the extreme North point of
his horizon {b of the dotted line) being always
directly under some part of the circle A B, and
the extreme South point {c of the dotted line)
being always directly over some part of the
circle C D.

Civil Day. The day commencing and ending at Midnight. See time.

CoLt7RE8. Two Great Circles of the hea-
vens supposed to intersect each other at right
angles at the Celestial Poles, and to pass
through the Solstitial and Equinoctial points
of the Echptic. The Solstitial points of the
Ecliptic, are those points which are farthest
from the Equinoctial, or Celestial Equator,
and the Equiuioctial points are those points of
the Ecliptic which intersect the Equinoctial.
In the diagram, A A is the Equinoctial, B B
the Ecliptic, P S P S the Solstitial Colure, ^
P N P N the Equinoctial Colure, P P the Ce-
lestial Poles, S S the Solstitial points and N N
the Equinoctial points.

■~l

COL. COM. CUL. J3

__ )

Comets. Wandering bodies, the nature and utility of which in the general scheme
of the Universe are entirely unknown. In default of a better surmise however, we
may consider them to be electrical bodies binding our Sun or Solar System to the
other nearest Suns (or Stars ) by which means our System may not, even in an in-
finite series of ages, float unchecked over the wide expanse of creation, and approach
some other system so as to endanger the stabiHty and equiUbrium of our centripetal
and centrifugal forces. (See central forces and solar system) t

Complement. In Geometry, such an addition to an arc as will suffice to com-
plete 90 degrees, or one quarter of a circle. In Astronomy it is the number of de-
grees required to complete the 90° of an arc between the horizon and zenith, or be-
tween a Celestial Pole and the Equator, &c. Thus, if a Star have an altitude of
40° the complement will be 50°.

Configurations. The relative positions of heavenly bodies with each other as
seen from the Earth; principally appHed to the configurations of the satellites of Ju-
fiter

Conjunction. The meeting of two heavenly bodies, which need not f^pear abso-
lutely to touch each other for this term to be used: it is sufficient if they are in the
same degree of the 360 of a great circle of the heavens. When the Sun is between

^J2? lEarlA, 9,.-

O • Mereunf

a Planet and the Earth, or a Planet is between the Sun and the ^arth, then that
Planet is said to be in conjunction with the Sun. When the Earth is between a Pla
net and the Sun, the Planet is said to be in opposition with the Sun. In the diagram
both Venus and Mercury are in conjunction, whilst Mars is in opposition. Conjunct-
ion and opposition are sometimes termed the syzygy of a Planet. The apparent con-
junction of two Planets is called a Grand Conjunction.

' Consequentia. See Antecedence.

Constellation. A series of stars near each other and supposed to form parts of
some figure as a bear, a lion ^ The number of constellations is 91. See fixed

STARS.

CojJSTANT of Aberration of the Sun. The amount of aberration independent of
the very small change produced by the variation in the dbtance of the Earth from
the Sun. See Aberration.

Copernican or Pythagorean system of astronomy. The system first propoun-
ded by Pythagoras, and afterwards fUlly established by Copernicus, explanatory of
the motions of the Earth and other Planets moving round the central Sun.

Culmination. Stars appear, like the Sun, to describe portions of circles in the
heavens; that is, they appear to rise daily in the East, and pass gradually towards
the West after having risen above the horizon a certain distance according to their
position in the Sphere. The greatest altitude a Star reaches is at the celestial meri-
dian of the observer, and its transit of that meridian is called its culmination. But
Stars within the circle of perpetual apparition of any place (See circle of perpet-
ual apparition, ) do not set below the horizon, but appear to describe entire

cus.

DAY.

DEG.

circles around the elev^ed pole; ( the celestifd pple
mtihin tjus drclf; and consequently pexpetuaUj ^*®"
Tated ahoye the horizon). Ii;i tl^e case of s\^ch Staxs
when their transit of th,e meridian takes ph^ie above
the pojje it is Cftljed tlj^e ugper culpoinatip^, aiid th^
passage or. transit heipw tl^ pole is caft^d the lower
cijibuinatiop. If ij N, in the ac^D^p^nyuig e^^a^,
be the northern Horizon of a place in north latitude,
^ will be tfc celestid north I^ole, C t^A thp orcle
of peipe^^f app^ Bf C i) D an arc of the^nj^e^
ric^ and' 1!^. P w^^ the arrows a cirde of th^* cefcs-
tial sphere parallel to the Equator and circle of perpetual appaiition. Now the appa-
rent d^ily motion of the sphere being in the direction pointed by the arrows the
Mo6ti 'aud Stftf at 1\J; and'C are miking their upper culmination, and the Stars be-
n&lh' the T61e E ai'e afeout to make their iower culmination; in the latter case one^
of the Stars being above and the other fteZou; the horizon. The -^Q Rf 2 q 7^
term^ u^pct aM tew^t teulniinAtion are flsb siised to Henote the
irrivjfl of a Star at Ae MeiieEkii bimeatk the horizon of any
place. Let the second diagram r^resent Ihe horizon of a place
with its circle of perpetual occultation beneath it. The Star
just below the horizo^ will then be making its upper culminfition^ and at the most
depressed part of the circle another Star is making its lower culmination. The
terms Culminating, Transit of the Meridian and Meridional Passage are synony-
mous; and the word Southing is generally used in north latitudes for the culmination
of the Moon. Moon culminating Stars are those Stars which on the day of obser-
vation cilhmnate about the same time as the Moon; as the Star at C in the diagram.
Cusps. The points of the illuminated horns of the Moon or Yenus.
Cycles. See time.
Day. See time.

Declination. The distance of a heavenly body North or South from the Equi-
noctial Circle. Degrees of declination are reckoned on th$ Celestial Mmdian* Cir-
cles or parallels of declination are parallel to the Equinoctial.

' Degree. The 360th. part of ^ cirde of any dimension. A degree is subdmded
into 60 minutes/ and each minute into 60 seconds. A second is sometimes diyided
into tepths, hundredths and thousandths of a second: 4° 16' 5 9"* 9 are read ^ 4 de-
crees 16 minutes 59 seconds 9 tenths; -69 as 9 hundredths of a second, 'SOjS.j^ls 806.
thousandths of a second. A degree of longitude varies in dimension at eveiy distance
from the Equator, where it is 69-5- miles, to the Poles, where all the circles of longi-
tude meet, and wherie, consequently, there is no longitude. A degree of latitude is a
trifl^iess than a degree of lon^tude on the Equator; the diameter of the Earth at the
Equator being gfeater than a diameter measured between the Poles. This fact is as-
certained by the foUOi^in^ xo^thod of measuring the arc of a meridian. A Star is
observed at one extremi^ of a kmg plaiie to have a certain altitude when on the
meiidian of that place. The observer then travels Noith or South, so as to continue
under the same itieridii^ until the altitude lof the Star becomes one degree more or
less than at first. The exact distance the observer has travdled is a degree of terrestrial
latitude, but it is evident that the measurement of some known part of a degree would
answer the purpose equally as well as the measurement of an entire degree. Now,
this actual measurement of degrees and parts of degrees has been accomplished in va-

DEP. DlS. DIA. 15

— — .— ----. .•■■ ■--

nous ktitades, and the rescdt is that tUe lengthLcf a d^ee^ oik the Eail^ Ikicreases
"with the latitude^ being the greatest near the Poles and the le$^t. at jtbe Equator;
from which fact it follows that the Earth's curvature is greater at the Equator than
at the Poles, oV, in other words, the Earth is
flatter at the Poles than at the Equator*
Let F A represent a flattened arc of meridi- .
an oh tlie GTobe, as at the Poles, (but pur- •^
{k>sely exaggerated for this explanatidn,} and \
P c s A an ate of meridian of greater curve,
as at the Equator. The zenith of a place
being precisely in a direction upwards in
continuation of a plumb line hanging to-
wards the centre of gfavi^, and the centre
of gravity being varibd in t^e instances FA
and c s according (but not corresponding) to the centres of the two curvatinre^ Z
will then; represent thezenith of F on the flattened and of c oi| file curved . suiface,
and Xwill represent the zenith of s on tibie curved^and of Aon tji^ flatt^^aed surfae^.
The altitude of the Star * is the complement of its zenith distance, {See cojn ple-
MENT aind ZENITH,] and, consequently, the Star wpuld have the sfucne altitude. at, F
as at c, and itt s as at A, and the measurement of the curved and flattened, surfaces
on the Globe coincident to the arc Z "^ X of the Celestial Sphere will be. dissimili^r,
being the difference' between !]^ A and c s> the great cury^ure c s being of less extent
in length than the smaller curvature or flattened surface FA..

Depression, or dip of thf horizon. An angle psntalned l^etween the ajyjtf^i^t
and sensible horizons, aiid the eye of an observer. (jS^ee circles). In the diagram
the line and riig\uhd,^r
c represents a phmulaet
showing^ the tf 1X6 per-
pendicular; CA ik aline
drawn horizontpUy to it
^m the eyeofan obser ^
ver at c, and is the sensible horizon, whilstc B,a lin^ ^eddng appoint wluere the .sky
and water appear to meet, is the visible hori^n^ ^d'A c b id the an^e of d^i£99tQ9i
or dip. Again: the ship being the place of observation, and d the sensible horizon
or a level line from the eye of an observer on, deck, whilst b is a line to the visible
horizon, the angle between the observer's 0ye.and d » is. the depression. It- will be
seen that the greater t^e 4titude of the place of observation the ferther the visible
horizon is removed from the observer.

Depressed Pole. The opposite Celestial Pole to that which is elevated. (See
Culmination.

Descension. The apparent passage of a cdlestial body from its greatest altitude
towards the western hbriion either in a Right or Oblique Sphere. (See Right As-
cension.^

Diameter. The appd.fent diaiiieter of a celestial' body is the an£ile xa^det which
it appears when viewed from the Earth. (5^^^ angle.) A' great circle of the heavens
(See circles) being divided into 360 degrees, aud each dejgfee into 60 minutes, fis
many minutes or parts of a minute of this circle as are covered by the celestial body
will be the apparent diameter of the disc.

16 PIS,

DiOPTKiC, Beating to tl

Dip. See otnanov.

Disc. He Tuible part o

X of redacted raioB. See BcnucnoK.

T &m of the Son, the Mood or a Planet, as seen &om
the&rtb.

Distance. J9e« mehsubatiom.

DiUBNAL. B«Utiiig to a day. The dtnraal motion of a Planet is its real daily mo-
tion from Wetl to Eatt in its orbit roond the Sun. llu dinmal tranat oi a nanet
ia ttt qiparent daily paasage from E<ut to Wett, caused by the rotalioa of the Earth,
during which the Flanrt deseribes ac arc erf' the sphere wbicb i* called the Planet's
diniDolarc.

Dominical letter. See time.

Dkagon'b bead and db.agon'b tail. Tiro Astronomical characters; Q Dragon's
Head, signifj'ing the ascending node, and fs Dragon's Tail, the deaeending node.
See MODE.

Dynamic^ Part of the aaemx of Mechanics; naed in Astrooomy >s it conUins
the theory and mechanical laws of bodies in motion.

Eakth. {See also bolab btsteh). The Earth is divided by ge<^Taphers into
lire zonei; one torrid, tvo frigid and two temperate.
The torrid it that division to part of whicli the Sim ia
perpendicular at all times. It extends 23° 28' (nearly)
on eadi side of the Equator. The northern limit of this
tone is called the Tropic of Cancer becanse the Sun,
when perpendicular to it, is in the sign Cancer, and the
southern limit is cslled the Tropic of Capricorn for a ri-
milAr reason. The frigid zones, or Arctic and Antarctic
circles, are 23° 28' from each Pole, the Arctic being at
the North and the Antarctic at the South. The tempe-
rate zones he between the frig^ and the torrid xones.
The imagioaiy divisions of the Earth which have a n
tronomy are i ~

-^ofO^

^'^ttEIvJ.

e particniar relation to As-
, the Equator, a great circle equally distant from the Poles, and di-
riding the Earth into the northern and aonthem hemispheres. A celestial continu-
ation of the plane of the Earth's Equator is the Celestial Equator, or Equinoctial,
which divides the heavens into the northern and southern
hemispheres, ii. Meridian tines, or hour circles, passing
through the Equator, from Pole to Pole. (See Fig.2) A
Meridian on the Earth is a semi-circle, alt the ' places on
which have the same longitude, which is reckoned on the
Equator from some particular meridian, as Greenwich or
I Paris. In this diagram the meridian lines or semi-circles
I are drawn through every 15° of longitude, which number
8 equal to one hour of Time, or one twenty fourth part
of the circumference of the Earth. Reckoning from the
East, every place on either of these semi-circlea will have
noon one hour earlier than the places on the next semi-cir-
cle to the 'WeBt. The perspective of the meridian hnes m this diagram would be in-
correct if the Earth were to he conceived at rest; but the student must imagine the
Earth revolring, a-.d each meridian semi-circle to be passing separately under his
view. Tlie word meridian has been derived from mid-da'j. The Sun is SMd to be in

""-Hi ■^.o'

EA.

EC.

the meridian when he touches the highest point of the arc he appears to describe
daily in the heavens, that arc being pecuhar, or existing only in respect, to the
place where it is observed, iii. The Ecliptic, a more imaginary circle over the
Earth than the Equator, or the meridians, and, therefore, more difficult to be con-
ceived. Let the flame of ' the candle A represent the Sun, and let B C be a string

strained oh supports, behind which string place a ball or globe, inclined as in the dL
agram, and let the upper part of the ball represent the North. N will then represent
the North Pole of the Earth and S the South, and the wires at N S the axis of the
Earth. Then turn the ball or globe on its axis and you will perceive that the string
will every instant divide the globe in a different manner to the previous instant, but
will always maintain the same relative position to the whole globe. Such is the
Ecliptic when imagined as the hue of the Sun*s annual path over the whole Earthy
projected, or depicted on the globe. As the globe revolves the circle E F will be
turned to the flame of the candle, and the Northern Hemisphere will have more of
the hght and heat than the opposite; but remove the candle to B and the reverse will
be the case, and if the candle is placed at H it will be parallel with the Equator O Q
in every part of a revolution of the globe. This explanation will assist in familiar-
ising the phenomenon of the Seasons. (See Seasons). The line from F to A will
. represent the radius vector of the Ea^h when the Sun is at C, and if the Moon he
imagined at B, the line from B to I will represent the radius vector of the Moon.

See RADIUS VECTOR.

East. The exact point where the Sun rises at the times of the Equinoxes; that
is, at the times when the Sun, in his apparent diurnal course, passes over the Equa-
tor of the Earth, the Equator being at an equal distance from the North and South
points.

Eccentric. Out of the centre. An eccentric circle is a circle the centre of which
is not concentric with, or on the centre of, another circle. The eccentricity of a Pla-
net's orbit arises from the orbit being elliptical and the Sun not being in the centre.

Eclipse. The passage of the Moon over the Sun or any of the Planets; of the
Planets over the satellites; and of the shadow of the Earth over the Moon. Figure 1

rip-1

J^arth

represents an eclipse of the Sun; the Moon being observed from A, on the Earth, to

18 EL^ EM^ ER

pass between the Earth and the Sun. Figure 2 represents an eclipse of the Mooui

Ficr 2

V^cLvO.xi^ ) o Moon

the Earth passing between the Sun and Moon, and throwing its shadow on the
Moon. An a«;i?//flrr eclipse of the Sun is when the centre of the Moon appears on
the centre of the Sun, and a ring of the Sun's disc appears entirely round the
Moon's disc. An annular eclipse occurs when the Moon is at her greatest distance from
the Earth, her apparent diameter then being not so great as when nearer. A central
eclipse occurs to an observer when the centres of the eclipsing and eclipsed bodies aj>-
•j • 3 pear exactly to coincide. Tlie echpse in Fig. 1. is annular, and Fig. 3.
^ ^ represents an eclipse of the Sun both annular and central. Calculations

of the ecHpses appear in the Nautical Almanac, and the echpses of Ju-
piter's Satellites by Jupiter are of much service in determining the
longitude of places on the Earth.

Ecliptic. (See circl-es, zodiac and earth). The Echptic has its name
from the circumstance that all the eclipses of the Sun and Moon are performed either
actually in, or very near that circle.

Elevation of the Pole. The distance of a Celestial Pole from the zenith of the
place of observation, by which the exact distance of the place from the Pole of the
Earth and from the Equator can be ascertained. For, if the North Pole of the hea-
vens is obsened to be 40 degrees from the zenith of any place that place will be in
SO*" of North latitude; 50° added to 40° completing the 90° from the Pole to the Equa-
tor. When the North Pole is below the horizon of any place that place is in South
latitude. The elevation of the Pole is said to be the complement of the elevation of
the Equator. Thus if the Pole be ascertained- to be 35° from the zenith, the Equator
will be 55° from it, 55° being the complement of 35°; that is, the two sums added
complete the 90° from the Pole to the Equator.

Elliptical. Relating to an ellipsis or oval. See kepler's latvs.

Elongation. The difference in motion between ^^-^ J^lamet

the swiftest and slowest of two Planets, as seeif from .
the Earth, or the space one has passed over be-
yond the other. The word elongation is also used to
denote the apparent distance of a Planet from the
Sun. The angle of elongation is contained between
lines drawn from the centres of the Sun and Planet to
the centre of the Earth, as in the diagram.

Emersion. When a Planet or a satellite has been "Xi&rlTfu

eclipsed by a heavenly body, and appears to^ come

out from behind that body it is said to emerge, and the fact itself is an
emersion. Immersion is the commencement of an Eclij)se, when the body is immers-
ed, or sunk within the influence of the eclipsing body.

Epact. See time.

EO EV^ FA^ FL 19

Ephemeris, Astronomical. A diurnal account of the situations ^c. of the heav-
enly bodies.

Equation of Time. See time.

Equation of the Equinoxes. As the equinoctial point (the first point of Aries,)
is continually shifting its place on the EcHptic (See precession,) the places of the
heavenly bodies referred to the Equinox, that is, their Longitude, or their Rigkt AsC'
ension {See these terms,) is also continually shifting. The diiference between the true
and mean place of the Equinox is a quantity termed the Equation of the Equinoxes.
Equation of the Centre. An allowance for the difference between the true
place of a Planet and its mean (average) place in its elhptical orbit.
Equator. See earth.

Equinoxes. The times when there are equal days and nights over the whole
Earth. The Equinoxes are two in number, the Vernal and the Autumnal; named so
to correspond with the Spring and Autumn Seasons of the Northern Hemisphere,
But the Vernal (Spring) Equinox to us in the North is the Autumnal Equinox to the

South. In the North, the Sun appears daily
to describe in the heavens an arc, such as 1,
in June, a smaller arc, 2, (the Equinoctial}
during the Vernal and Aatumnal Equinoxes,
and the smallest arc, 3, in December. While
these appearances are presented to the North
the contrary are presented to the South, and

the phenomena at the Equator are very dissimilar For, when the Sun describes
to the North the arc of the Equinoctial, as depicted in the figure, the Sun at
the Equator is vertical, passing directly overhead; when the arc 1 is described
to the North the ascension as seen at the Equator is only 23 j degrees North from
the Equinoctial, and when the arc 3 is described to the North the ascension at the
Equator is only 23^ degrees South from the equinoctial.

Equinoctial. See circles and earth.

Equinoctial Colure. See colure.

Equinoctial Points. The first points of Aries and Libra. See Colures.

Equinoctial Time. See time.

EvECTioN. When the Sun is in Perigee, (nearest the Earth) in January, the Sun
has a greater, and the Earth a less, attractive power over the Moon than in July, when
the Sun is in apogee (farthest from the Earth). As the Earth's attractive power dimi-
nishes, and the Sun's power increases, the Moon recedes from the Earth and her orbit
is enlarged, the effect of which is seen in her periodic time of performing a lunation,
which in January is 35 minutes longer than in July. The greater the dilitation
(enlargement) of the orbit the longer is the period in which a revolution is perform-
ed in it.

Facul^e. Spots on the Sun, which are brighter than other parts of the disc.
MaculcB are spots less bright, or even dark.

Fixed Stars, (the Suns of other Systems) are at such an enormous distance from
the Solar System that scarcely any perceptible change is made in their relative posi
tions as seen from the Earth. There is no doubt however that they move, but as our
Sun,, and all the Planets, occupy but an inconsiderable speck in creation, auJ the near-

20 FIXED STARS.

est of what we call the Fixed Stars is at least 200,000 times farther from us than the
Sun is, the change of position of a few only of the Stars is perceptihle even by the
most careful and accurate observations. Although their number, when view-
ed through powerful telescopes, appears infinite, yet the naked eye can ne-
ver see more than 500 at one time. The Stars of the first magnitude are
only 20 in number, those of the second, 7^, and those to the sixth magni-
tude inclusive, only 3128. For the purpose of arrangement, most of the Stars
are supposed to be situated within some figure, called a constellation, the remainder,
not included within any figure, being termed informed Stars, Twelve of the constell-
ations are in the Zodiac, 34 at the North, and 45 at the South of the Zodiac, The
Stars, called fixed, have first, an apparent motion over the Earth in 24 hours, an ap-
pearance caused by the rotation of the Earth on its axis;, secondly, a very small ap-
parent motion in Right Aseenaion and Declination, caused by the Precession of the
Equinoxes, Aberration and Nutation, (See the explanations to these terms,) and,
thirdly, some of the Stars are seen to have a real, or proper, although very slow mo-
tion, by which they not only change their relative positions sUghtly, but become of
greater or less magnitude at one period than at another. We infer indeed that every
Star (including the Sun) has some progressive motion, but the inconceivable distances
at which they are placed preclude our appreciating the motion except in a few in-
sauces, and, as we accompany the Sun, his very gradual change of place, distributed
as it is over thousands of years, becomes perceptible only in respect to a period like
that occupied by the precession of the Equinoxes. If the small circles jS^ S, in the fi-

gure be taken to represent the circuits performed by any two Stars in an indefinite
number of thousands of years, and the circle E a circuit performed by tlie Sun and
Solar System in 25,000 years, it can be readily imagined that the angles under
which we shall view the Stars in different parts of their circuits, *S S, will be percep-
tible only of a very minute change in any short period.

sii:i:

I*'

FIXED STABS. 2I

To find the constellations Ursa Majoris, (the Great Bear J and Ursa Minoris,
(the Little Bear,) is the first lesson in the study of the Fixed Stars to those living
in the Northern hemisphere. The following diagram contains the principal Stars

^ *-

"^ ^ $

or- ^ *• ^ ^^

JPi;U Star . -^^

in these constellations, the most important of whieh is a Polaris, or Alruccabah,
the North Pole Star or Cynosure, in the constellation of the Little Bear. The
student who for the first time essays to find these stars must bear in mind the
daily revolution of the Earth, and must not, therefore, expect to find them always
in the situation they appear to the reader of this page; for although their relative
distances from each other are always the same, yet each hour produces an evident
change in the positions of the constellations when compared with the horizon or
zenith of an observer. The Great ^ear, for instance, which at one time may be
seen emer the Pole Star, as in the above delineation, will, six hours after, be to the
left or west of it, six hours more will be under it, and in another six hours will be
at the right or due east of the Pole. These constellations, and others which surround
the North Pole of the heavens, never set below the horizon to observers in the nor-
thern p^ts of Europe, America and Asia, and never rise to the inhabitants of many
of the southern parts of the world. The constellations surrounding the South Pole
never set to the inhabitants of South America, Africa and Australia, and are never
seen from Europe, Asia and North America. (See circles of perpetual appari-
tion.) The Great Bear is sometimes vulgarly called the Waggon and Horses; the
tail of the Bear becoming the horses, and the Star Duhhe the hind wheel of the wag-
gon. Bearing this in mind it is easy to remember that Dubhe and the other Star
forming the back of the waggon become the Pointers to the Pole Star. The follow-
ing map may be taken for lesson the second. It contains several principal Stars, an
acquaintance with which will form a good foundation for pursuing the subject with
the aid of regular celestial maps.

FEffiD STARS SURROUNDING THE NORTH POLAR POINT.

* »(.^^ ,y * *

* * ^ J

C^-^^r.

JPqCa^

*Alcle-rci//nv^C^ ■*

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The Pleiades, or Seven Stars, are in the constellation Taurus; Caput Medusae,
Algol and Algenib in Perseus; Capella in Auriga; Castor and Pollux in Gemini; Al-
maach in Andromeda; Schedir in Cassiopeia; the Pole Star in the Little Bear;
Dubhe, A'lioth, Mizar and Benctnasch in the Great Bear; Seginus in Bootes; a Draco
and Rastiban in Draco; Deneb or Arided in Cygnus; and Alderamin in Cepheus.

GA.

GR,

HE.

The Nautical Almanac contains the Apparent Place8(the Right Ascension aad De-
clination,) of 100 Fixed Stars, an Ephemeris of the Moon Culminating Stars, ( See
EPHEMERis and CULMINATION,) and a Tahle of the Phenomena of the year, parti-
cularly the conjunctions of the Planets and Fixed Stars. By a frequent reference
to these parts of the Almanac, and hy comparing the Phenomena therein desclibed
to their actual occurrence in the heavens a gradual progress in the study of the Fix-
ed Stars may be made.

FoRMULii.. A form, rule, or method, of working a problem or ealcnktiim.

Galaxy or Milky Way. There is a white (milky) broad belt df Mght over the
heavens which is ascertained to emanate from an innum^raMe host of Fixed Stars,
only visible through the most powerful telescopes. A very slight observation of the
heavens on a clear night will serve to recognise this belt, in a very small part of
which 258,000 Stars w^ere once observed. Its breadth varies from 4° to 20°.

Gemini, (the Twins.) The sign of the Zodiac which the Sun enters about the
1st. of May and leaves about the 20th. of June.

Georgium Sidus, Urantjs, Herschell. Three names of the Hanet discov-
ered by Dr. Herschell. See solar system.

Geocentric Place. The position of a Planet as viewed frem the Earth. Hclio-
xi-rsi poUt^ centric longitude and latitude of a Planet is its

position supposing it to be viewed from the Sun.
In the diagram the geoeentric piece of the Planet
(omitting its decliBation,) is, R^ht Ascension
14** 30', and its heUocentric lon^tade 165^. (See
declination, right ascension and longi-
'^ >fj TUDE CELjnsTiAL). For, as the circle with the

/*^»*i^

First Point of Aries represents the circle of
Right Ascension, divided into 24 hours, or 360
degrees, the geocentric place of the Planet, or
that point of the circle in which the Planet app-
cars when viewed from the Earth is 14^*30°* (14
hours, 30 minutes of Right Ascension,) from
the First Point, and, at the same time, that point of the circle in which the Planet
would appear if viewed from the Sun is 165° (degrees) distant from the ^irst Point.
The Roman numerals, VI. XII. XVIII. denote three of the hours of Right Ascen-
sion, and the figures 90. 180. 270. three points of the celestial longitude.'

Gibbous. A term appUed to one of the phases of the Moon. See moon.

Gravitation, See central forces.

Heliacal. Stars, when they approach too near the place of the Sun, are lost
to sight by the superior effulgence proceeding from the solar luminary, and are said
to rise and set heliacally. When a Star emei^es from the influence of the Sun's rays
it makes an heliacal apparition or appearance; and when immersed in the solar rays
it makes an heliacal occultation or obscure position. The Moon rises and sets helia-
cally when 1 7° distant from the Sun.

Heliocentric. See geocentric.

Hemisphere. The half of a sphere, orb or globe. The great circuit of the hea-
vens visible at any time is the Celestial Hemisphere. The visible discs of the Sun

24 HO. IM. - JU, KE.

and Moon are the Solar and Lunar Hemispheres turned towards us. Any half of
the World is an hemisphere: thus we say the northern and southern^ or the eastern
and western hemispheres.

Hour Angles^ and Horary Circles. See time.

Horizon. See circles.

Horizontal Parallax. See parallax.

Immersion. See emersion.

Inclination^ Angle of. See noi>e8 and obliquity.

Inclination of Moon's Orhit. See Moon;

Inequality. Unequal motion: a deviation in the motion of a Planet or Satellite
from its mean motion.

Informed or Unformed Stars. See fixed stars.

Ingress. The entrance of the Sun into any of the signs of the Zodiac.

Intercalary Day. See bissextile.

Juno. One of the Asteroids. See solar system.

Jupiter. See solar system.

Kepler^s Laws. Three laws respecting the motions and positions of the celes-
tial hodies of the Solar System^ first announced by Kepler and since demonstrated
by the calculations of Newton: they are considered the foundation of Astronomy.
To explain the first law recourse must be had to that part of Geometry called Conic
Sections* a knowledge of which is essential in many parts of Astronomy.

Conic Sections are curved lines formed by the intersection of a plane and a cone.

A plane, or plane surfiice is one perfectly fiat and even.

A cone is a solid body of the shape of Fig. 1, circular at the base A and angular

or pointed at the^ vertex B. A straight or right line drawn from the vertex to
the centre of the base, as D C, Fig. 2, is the axis of the cone All right lines
as D E, D F, Kg. 2, drawn from the vertex to the circumference of the base, are
sides of the cone. If a cone be cut by a plane parallel to its base, as P P, Fig 3,
the section will be a circle, because the base is a circle. If it be cut so that the
section be parallel to a side, as 1 1, Fig. 4, is to L L, the section will be a parabo-
la. If it be cut perpendicularly to the base, and so that the plane cutting it, as H H,
Fig. 4 if extended upwards would meet an extension of a side, as L N, the secti-
on is an hyperbola. If it be cut from side to side obliquely (not parallel,) to the
base, as I O, Fig. 4, the section is kd. ellipsis. Figure 5 is a parabola^ Fig. 6 an
hyperbola and Fig, 7 an ellipsis.

KEPLER'S LAWlg.' 25

- —

The orbits of all the Planets^ and some of the Comets^ are elliptie, whfle other
Comets are supp osed to describe parabolic and hyperbolic eurves round the Sun
nerer to return.

A curvilinear figure is composed of curved lines, as the ellipsis^ The Periphery
is the circumference of an ellipsis, or any other regular curvilinear figure. The
axis of an elHpsis is a right line dividing it into equal parts, as A B, the longest

axis, and D E, the shortest axis of the ellipsis. Fig. 8.
The longest is called the transverse or major axis ; and
the point C, where the longest and shortest axes inter-
sect each other, is the centre of the ellipsis. The foci
of an elUpsisare two points in the longest axis. To
find the foci of an ellipsis draw the longest and short-
est axes, take half the longest in your compasses and
and set one foot in the shortest axis, at an end joining the periphery, as e. Fig. 8:
the other leg will then intersect the longest axis in either focus, 6 or h. This
latter explanation will serve to elucidate Kepler's first Law : -

THE PLANETS ALL MOVE IN ELLIPSES OF WHICH THE SUN OCCUPIES

ONE OF THE FOCi'.

The Second Law is as follows : -

THE MOTION IS THE MORE RAPID THE NEARER THE PLANET IS TO THE SUN,

SO THAT THE RADIUS VECTOR ALWAYS DESCRIBES
EQUAL SURFACES IN A GIVEN TIME.

In explanation, first see solar system and radius vector. Then, if A B C D

Fig. 9, be the orbit of a Planet, and S the Sun in one- of
the foci, the motion of the Planet, in its orbit at A, near
the Sun, is greater than at D, remote from the Sun, and
the Radius Vector will sweep over the surface, or curve,
from D to B, or from C to D, in the same time as it
will describe the surface, or curve, C A, or A B.

The Third Law requires an explanation of the terms
^ squares and cubes, besides that of Major Axis of an

Ellipsis. (The latter has been already given in the explanation to Fig, S.)

Squares, or square numbers (in Arithmetic,) are the product of numbers mul-
tiplied by themselves : the square of 2 is 4 and of 6- 36.

Cubes, or cube numbers, (in Arithmetic), are formed by multiplying any num-
bers twice by themselves : the cube of 6 is 216 ; thus algebrically produced : 6x6
= 36. 36x6 = 216. (6 multiplied by 6 are equal to 36, and 36 multiplied by 6
are equal to 216.) The Third Law, then, is thus announced:

THE squares of THE TIMES OF THE REVOLUTION OF ANY TWO PLANETS

ARE TO EACH OTHER AS THE CUBES OF THE MAJOR AXES OF THE ORBITS.

- - • » ...

The consequences deduced from these three laws are demonstrated to be, that
the force {See central forces) acting on the Planets is directed towards the
centre of the Sun, and regularly decreases in amount as the distance from the Sun
is increased, (or, more properly expressed, is in the inverse ratio of the square of
the distance of the centres of the Planets from that of the Sun ;) and, lastly, that
the force is proportionate to the mass. From all these consequences it results
that the Sun is the centre of an attractive power, that all the Planets revolving
round the Sun are, like him, endowed with the power of attraction ; and that in

LE.

LI.

LO.

Moon
o

the SoUr Sjrsttm all thd particles ^i matter mutually attract each other with a
forde proportkmate to their masses.

LAflTyoi!, CetiitiaL The distance of a celestial body fh)m the Ecliptic

North or South. The Sun's latitude is a very
slight variation of his apparent place, as seen
from the Earth, caused by the Earth being at-
tracted towards the Moon. If the Earth, at-
tracted by the Moon, be at A, on one day of
the month,the Sun will be seen in the heavens
at D, and, on another day of the month, the
Earth being attracted by the Moon to B, the
Sun will be seen at C. This di£Perence, how-
ever, is not so great as the diagram makes it
appear to be, and but rarely amounts to l'^,
which is less than the millionth part of a great
circle. The Sun's latitude is technically eeilled
the angular distance of the Sun's centre from the plane of the Ecliptic, North or
South. The latitude und dscUnatum of a celestial body are likely to be confounded
by the student unless it be impressed on his mind that Latitude is the distance from
the EoUpttei and Deolination the distance from the Celestial Equator or Equinoctial.
Circles or paraUels of latitude are circles parallel to the Ecliptic.

LfiAP YfiAR. See BtSSEItTtLE.

Leo (the lion). The Sign of the Zodiac which the' Sun enters about the 23rd. of
July and leaves about the 22nd. of August.

LtSAA, (the Balance or Scales). The sign of the Zodiac which the Sun enters
about the 23rd. of September and leaves about the 22nd. of October,

LtBUATtoN. See NtJTAfioN and moon.

LtMii. A word applied to one part of a heavenly body when it is necessary to
distinguish it from another part. Thus, we say the upper or lower Limb ; the east-
em or western Limb of the Sun or Moon, in allusion to the upper or lower, the
eastern or western part of the Sun or Moon as seen from the Earth.

LiMtTiND Parallels. A term used to denote those parallels of celestial latit-
ude beyond which an occultation of certain Stars by the Moon cannot possibly
occur. See occultation.

LoOAtiiTBM of the Radius Vector of the Earth. An artificial number
representing the length of the Radius Vector, or the distance of the Earth from the
Bun. Logarithms are artificial numbers used for £EMnlity of calculation where tl^e nie-
thod with the common arithmetical characters would be tedious and full of com*
plexity.

LoKGiTUii«> Ceteetiat. The distance of a celestial body from the Equinox or
first point of the sign Aries^ measured on the Ecliptic. The celestial Longitude
and the Right Ascension are likely to be confounded by the ttudent, who must im-
press ou his mind that the longitude is measured on the EeUptk in degrees, minutes
and seconds of space ; while the Right Ascension is measured on the Equator in
hours> minutes and seconds of time, and in degrees, minutes and seconds of
space. Circles of Longitude are synonimous with meridian circles.

Longitude Stars. Those Fixed Stars which are generally selected for ascertain-
ing the longitude of places on the Earth by comparing the observed distance of the

LU.

MX.

ME.

Moon from one of them with the Greenwich time at which a Lunar Distance ia pre*
dieted in the Nautical Almanac to occur*

Lunation and Lunar Distance. See. ^ooif .

Mars. See soi.ar system.

• Mean Time, Mean Solar Day, Mean Noon. See timk.

Mean places of Fixed Stars. When the apparent Biffht Ancmmon and JD#-
clination of a Fixed Star are corrected for Precession^ Aherratimi and Nuiationt(S€€
the erptanations of these terms,) the product is the mean ^i/ac(?.

Mensuration. The act or art of measuring lines, Buperftci©8, (surfao^if) and
solids. Under the word angle will be found the astronomical method of mensara-
ting by the arc of a circle, and under the word pegrxs the method of measuring
the arc of a meridian. In this place will be explidned the iystlpms adopted for esti-
mating the progressive motion of light, and the distances of the Sun imd Planets
from the Earth. The student is first requested tp read the articles ASBRRATioir,

ANGLE, ARC, CONJUNCTION, DEGREE, piAMK'rf:|l, fttUC, Slft£RSIOIf| MERIDIAN,
PRBIT, PARALLAX, SATELLITES and SpJLAR SYeTKMt

The method of aseertafadng the distaooe and magaitude of

the Sun or a planetary body is by obserrations of the paralU
ax, and may be thus Atmiliarly explained. Let A B repres^t
an arc of any meridian on the Earth, 6 » an arc of the celestial
sphere and P a Planet. To an observer at A the Planet ap-
pears at a, and to an observer at B the Planet ai^pemrs at b i
half the arc a & is thereft^re the parallax rf the Hwiet. Now,
the angle A P B is equi-angular, or similari to the angle a V b,
and as the base A B of the angle A P B can be measured so
the altitude, P C , of the angle can be ascertunedi and this al-
titude is the distance of the Planet from the centre of the
Earth. The distance beii^ thus found, the magnitude of the
celestial body is calculated by observing ttie angle subtended,
or drawn under the disc of the Planet to the eye, M d C d, C
in this instance representing a place on the circumference of
the Earth, P the Planet §een at the horigon of that place, and
d d the base of the Angle subtended. In the case of the 8urii
for example, the distance being 95 millions of miles, and the
angle subtended from his disc being, on an average of the entire
year, 23' 3" (a little more than half a degree, ) the base of the
angle is the diameter of the Sun, 882,000 miles. An aa|le to
represent this might be drawn in a field 800 feet long, in which,
every inch being reckoned as 10,000 miles, the dtitttde of the
angle would be 791 ft. 8 in., and the base of the angle^ repres^ting
the diameter of the Sun, 7ft. 4*2 in. It will be immediately evi-
dent that tmce the distance of the Earth from the Sun is the di-
ameter of the Earth's Orbit, 190 millions of miles. Aaother me-
thod of measuring the parallax of the Sun, and consequently his
distance and diameter, is by observing the transit of the Planet
Venus, or Mars, across the Sun's disc. This problem is rendered
complicate by the motion of the Earth and Planet during the pro-
gress of the observations; but the principle may be thus e^plaincdi
Let S (l^g,2.) be the Sun, Y Vpnus and A B an arc of the

ME.

MI,

MO.

ctrcqmfeience of the Earth. An observer at A ^11 see Venus projected on the
disc of the Sun at a, the same instant that an observer at B will see the Planet pro-
jected at h, And iht angle a Y 6 is equal to the angle A Y B.

Again : let a b, Fig. 3, be an arc of the celestial
sphere, S the Sun, Y N Yenus in two points of her
orbit, and E R the situation of two observers on the
Earth. WTien the observer at E sees the Planet pro-
jected on the centre of the Sun, Yenns will be at V,
and when the observer at R witnesses the ' same phe-
nomenon Yenus will be at N. The sidereal time occu-
pied by the passage of the Planet from Y to N will be
equivalent to the celestial arc a I, as the great circle in
which the Sun appears is divided into 24 hours. If
then the obser^^ers duly note the separate instants when
the Planet appears to them to occupy the Sun's centre,
the interval between the two observations can be con-
verted into space, and will become the measurement
of the angle V S N, which is equiangular with a • b the angle measuring the Son's
parallax.

In explanation of the method of ascertaining the progressive motion of light, let
S, Fig. 4, be the Sun, J, Jupiter in his orbit, and E e two
positions of the Earth in its orbit. When the Earth is at
E, Jupiter is seen in conjunction with the Sun, and when
at e we see Jupiter in opposition. The diameter of the
Earth's orbit E ff is upwards of 190 millions of miles. If ]
then we note the time of an immersion of one of Jupiter's i
Satellites when at E, and another when at e, and calculate
the time elapsed between the two, and theji again note the
time of an immersion when at e and one when again at E,
and calculate the time elapsed between the two last, we
sjiall find that the two last were performed in a greater pe- -^ ^S ^

riod than the two first, because between the two last the Earth receded from Jupiter
190 millions of miles, and between the two first immersions the Earth advanced the
same space nearer Jupiter. The exact difference would be IC minutes 26; thus
showing that light occupies that time in traversing the diameter of the Earth's orbit,
and half that time is the amount of the aberration of the Sun. But a plainer expla-
nation may be afforded if we conceive an imaginary case. Suppose the Sun and Ju-
piter to be stationary, the Earth to perform a circuit from E to e. Fig. 4, in six months^
and an eclipse of a Satellite of Jupiter to take place at noon daily. Then, if froni
the Earth at e, we observed the immersion at the instant of noon, we should not
witness it at E, 190 millions of miles farther from Jupiter^ until 1G"» 26' past noon.

Meecxjry. See solar system.

Meridian. See circles and earth.

Minor Planets, or Asteroides. Ceres, Juno, Pallas and Vesta. See solar

SYSTEM^

Month. Sec time.

vc-

•E

'•■..••«•

THE MOON, ^gi

Moon. Next to the Sun the Moon is, to us, the most remarkable of the heavenly
bodies. She is an attendant Planet, or Satellite of the Earthy to which she affords
light by reflecting the light of the Sun; and she revolves from West to East, and to
the same point of the heavens from which she monthly sets out, in ap average period
of 27 days 7 hours 43 minutes and 1 1 seconds of mean solar time ; {See time,)
which revolution is called fiL periodical month; but from New Moon to New Mpon
again, the month consists of 29 days 1 2^ 44" 3", called the Synodical month or
lunation. To perceive the cause of this variation it must be considered that while
the Moon has been traversing her oihit round the Earth, the latter has been ad-
vancing near a twelfth part of its orbit round the .Sun. The Moon, therefore, to be
in the same position after a revolution, not .only has to pass once round the Earth,
but has to accompany the Earth in the twelfth part of its orbit. The .diameter of
the Moon is 2,200 miles; her mean distance from the Earth only 240,000 miles;
A distance not much more than thirty times the diameter of .the Earth. Her orbit
is elliptical (oval,) and the inclination of the plane of the orbit ^Kt an oblique angle
with the plane of the Equator, and nearly coinciding with the EcHptic. The word
lunar signifies pertaining to the Moon. A lunation is a lunar or synodical mbntii,or
one complete revolution of the Moon round the Earth. A lunar year is twelve
synodical revolutions of the Moon. A lunar day is the time occupied between the
Moon's leaving the meridian of any place and arriving at the same point again the
succeeding day ; which apparent daily passage is performed in 24^ 48°^ 46' of
mean solar time, so that a lunar day is 48*" 46' longer than a solar day. The ap-^
parent daily passage of the Moon over the Earth (from East tp West, and contrary
to her true motion,) is caused by the daily revolution of die Earth. Lunar distance
is the distance of the Moon from the Sun or any of the Planets or Stars, as seen
from the Earth; the measurement of which by instruments, with the aid of ca}cu«
lations to be found in the Njiutical Almanac, will determine the longitude at sea
within about 15 miles. The calculations in the Nautical Almanac relating to the
Moon are of the following phenomena^ i. The sefni-diameter, or measurement of
half the diameter of the Moon, in minutes, seconds and tenth parts of a second of
a degree for noon and midnight of every day in the year. The apparent diameter
of the Moon is daily undergoing a change in size, on account of her distance from
the Earth being variable, ii. The Horizontal Parallax, (See parallax), hi. The
Longitude, {See longitude celestial), iy. The Latitude, (See latitude
celestial), y. The age of the Moon in days and tenth parts of a day, at meaa
noon of Greenwich, reckoned from the time of the New Moon, that is, from the
time when the Sun and Moon are in conjunction, or have the same longitude, yi.'
The time of the Moon's meridian passage at Greenwich. (See guliwinatign). yii.
The Bight Ascension of the Moon's bright limb, (See right ascension), yiii.
The Declination of the Moon's centre in the heavais. North or South ^om the
Equinoctial or Celestial Equator, from which she is never distant more than 26
degrees, ix. The sidereal time qfthe semidiameter passing the meridian, (See
TIME, SEMIDIAM1STER luid MERIDIAN). X. The mean longitude of the MotnCs as-
cending node, (See nodes), xi. The Moon's daily motion, the velocity of which
is continually undergoing a change, xii. The occultation of the Planets and Fixed
Stars by the Moon, (See occultation). xiii. Moon Culminating Stars, (See
culmination). XIV. The mean time of the greatest libration of the MoovCs appaj

90 THE MOON

rent di$e. The ]ibrati(m of the Moon is a motion she has [See nutation] by which
ike disc generallj presented towards the Earth is partly remoyed from sights an-
other part occupying its place. The Moon always presents the same hemisphere
towards the Earth, and the libration is the only change of the Moon's disc. "We
have now only to explain the pheues of the Moon^ and other phenomena therewith
connected ; for which purpose we use the diagram, page 31, requesting the student
in the first place to bear in mind that the Moon is at an insignificant distance from
the Earth, compared with the Moon's or the Earth's distance from the Sun,
This diagram represents the positions and {)hases of the Moon for every
day, from the third day of the lunation in November to the New Moon of Decem-
ber 1844, The positions on the first two days are omitted to prevent confusion.
This lunation has been chosen because there is a partial Eclipse of the Sun within a
few hours of its conclusion.

The dngiam also represents ; i. The circle of Right Ascension (See right Ae->
CBiraiON). II. The first Point of Aries, from which the Bight Ascension is reck^
oned in hours nmmtes and seconds, iii. The Rational Horizon of Greenwich at
tke ietting of the Sun on the Idth. of November, and 9th. of December 1844 ; the
Horizon at the Smi's setting on the intermediate days lying towards some points
betwcCT the two lines, iv. The Bight Ascension of the Sun on the Idth. of Nov-
ember, and 9ih. of December 1844; the apparent change of the Sun's position being
caused by the actual variation in the position of the Earth in its Orbit, v. The
Bight Ascension of die Mooa each day from the 13th. of November to the
^, of December 1844. On the 13th. of November, at the setting of the Sun at
Greenwich, an observer will have the Sun in R, A. (Ritfht jheensionj 15^ IG'^yand
the Moon, the age of which will then be 3 days, in R. A. 18^ 31°*. The difference
between the R. A. of the two bodies will be 3^ 15"*, and the Moon will set about 3^
15°* after the Sun. The limb of the Moon nearest the Sun will, at this time, be
slightly illuminated, and this illumination, in the form of horns, will increase until,
on the 17th. of November, the Moop's age being then 7 days, half the disc will be
iiluminated ; this is the termination of tjie first quarter. Aflber this the dark part of
the Moon will appear homed, the illuminf^tion being then called gibbous, until ou
the termination of the second quarter, on the 24th. of November, the Moon's age
being 14 days, the Moon will be full or completely illuminated. The Moon vrill
then pass through the third and last quarter, and pesent the phases as in the dia-
gram, until the 9th. of December, when the Sun's R. A. being 17^ 7^ 30", and the
R, A. of the Moon being 17^ 10°* 10", the Moon will have completed a lunation, the
whole of her disc will be dark, or only reflecting the feeble light afforded her by the
Earth and Stars, and, between 18°* past 6, and 44°* past 9, of Greenwich Mean
Time, the inhabitants of the greatest part pf North America, and of the islands in
the North Pacific Ocean, will witness a partial Eclipse of the Sun. If the path of
the Moon were precisely in the Ecliptic [the apparent path of the Sun,] there
would be an Eclipse every month, on the occasion of new moon, but as this is not
the case the Moon, when in conjunction with the Sun, is generally a little to tlio
North or South of the great luminary and an £clip6e only occasionally takes place.

THE MOON.

jl«f ^

*^^ 0^^*

^5.^

^'^^^

NA.

NO.

NU.

^ADIR. See ZENITH^

Nodes. Those points of the orbit of a Planet which cross the Ecliptic. That
point where the Planet passes from South to North is called the ascending node, and
the other is the desceTiding node. The angle which the plane of the Ecliptic makes
with the plane of the Planet's orbit is the angle of inclination, as A B, Fig. 1. A
line from one node to the other is the line ofnodea, as in Fig. 2. The nodes of
a Satellite are those points of a Satellite's orbit which cross the orbit of its primarv^
as the orbit of the Mooa crossing the orbit of the £arth. In the case of the Mood^
the nodes are continually changing their positions on the EcUptic, and there
are calculations in the Nautical Almanac of the longitude of the MooifCs ascending
node, that is, the distance at any time of the ascending node from the Equinox

A. JB^

jr^zi

Jig 3

or first point of Aries, the line of the Moon's tioctes making an etitire reTolution
round the Ecliptic in 18^ years. If E C, Fig. 3, be an arc of the Ecliptic, E the
place of the Equinox, or the place of the Sun on the 21 st. of March, and the Moon
be observed in one lunation to cross the Ecliptic at a, towiards the North, a will be
the place of the Moon's ascending node, and the longitude of the node will be
E a. But the next lunation, the Moon, instead of crossing the Ecliptic at a, will
cross at b, and the node is therefore said to shift from a to 5, from which, during
the following lunations it will shift to c &c. until the whole circuit of the Echptic
will in turn have received the nodes. The Equinox also (See precession,) is
gradually shifting its place, and the correct longitude of the node will be the
mean longitude of the Moon's ascending node from the mean Equinox. The longi-
tude of the (L's ascending node for 1 Jan. 1843 is 281° 34' -1 ; for 1 Jan. 1844,
262° 14' -4 ; for 1 Jan. 1845, 242° 51' -5.

Nutation. A motion the Earth has in its annual revolution, by which the axis
is twice slightly inclined towards the Ecliptic and as often returns to a positioii in
which it is not so inclined. The Moon has also a similar motion called its libration.
These phenomena appear to be connected with the precession of the Equinoxes.
(See OBLiauiTY and precession.) A ''Term of Nutation involving 2 }>" is the
amount of Nutation occurring in two months.

OBL.

OPP. .

PAR.

Nebulous, cloudy. The nebulous Stars are of the sixth magnitude, which show,
a dim hazj light, and are scarcely visible to the naiked eye; but all Stars of this mag-
nitude are not nebulous. See Nebula.

Nebuljs. Misty congregations of matter observable throx^h the telescope aronnd
some Stars, and also in many parts of the heavens apparently distant from any Star.
In the constellation Andromeda a nebula can be seen with the naked eye, the app *
earance of which has been described as that of a candle shining through horn, the
condensation being greatest in the centre where the greatest brilHance is observed.

Nucleus. A body around which other matter is gathered, as the .central body,
of a Comet.

Obliquity of the Ecliptic. . The angle which the EcHptic makes with the
Equinoctial, which varies slightly in different years and different times of the year.
Thus the obliquity on the 1st. of January 1843 will be 23° 27' 36" -58 ; on the 1st.,
of January 1844, 23° 27' 33" -02, and on the 1st. of January 1845, 23° 27' 34'''24.
This change of obliquity is called the obliquity of the obliquity of the EelipHe. '

Occultation. [See heliacal.] The word occultation is principally applied
to the obscuration of. Jupiter's SateUites within the illumination around that
Planet.

Opposition. See conjunction.

Orbit. The path in the heavens of the Earth, or any of the Planets or Comets
round the Sun, or of a Satellite of a Planet round that Planet, as the path of the
Moon round the Earth.

Pallas. One of the four Spheroides.' ' Siee solar system.

Parallels of Altitude . See ai^titude .

Parallels of Declination. See declination.

• *

Parallels of Latitude. jS^ee latitude.

Parallel Latitude. A similar latitude.

Parallel Sphere. See sphere. «

Parallax. The apparent difference of posi-
tion of a heavenly body by its being seen from
different parts of the Earth, or at different heights
above the horizon. Thus the Moon M, if seen from
A will appear among the Stars at E. The Moon
cannot be seen in her true place unless she is in
the Zenith, or as seen from S, when an imaginary
line drawn from the centre of the Earth wiU show
to the spectator at S the Moon among the Stars at
C ; S bemg then the nearest point of the Earth to
the Moon. The angle, or difference, between C.
and E is the Moon's Parallax to the spectator at
A, and the angle, or difference, between D and C
is the Moon's Parallax to the spectator at B.
When we have to look horizontally at the Moon
she has an Horizontal, or the greatest Parallax,,
and it is evident by the diagram that when in the
Zenith she has no Parallax. It will be seen that centre.

34 PAB. PER, PHR

Fualla^ makes a odestial body appear too low ; the calmlation for it is therefore
added to an tqipareiU ahitode, Tlie Rxed Stars hare no Parallax ; even the Sun
and Planets (excepting Mercory and Yenns) hare Terj little ; it is considerable only
with the Moon, owing to that body being so near the Earth, and, conseqaentlyy at
an enormoos distance from the story sphere in whidi she appears to shine. The
Parallax of the Bfoon at the horiaon, or the Moon*s horiiontal Parallax, is upwards
of a d^Sree ; the Son has an hoiiaoulal PtoaDax of onhf 8^ *5.

PAKALUkx, Ankval* The ang^ whidi the Earth's Orbit makes with distant
parts of the heavens* "When we conader that the orbit of the Earth is 190 millions
of miles in diameter .we mig^t caqiect that, from different parts of the orbit, points of
the heavens woold be seen under different angles. Let the circle A B represent

the oibit <rfthe Earth, Now, if the Pked Stars were proportionate^ as near to
onr orbit as the * * in the diagram are to the drd^ they would subtend the ang^e
20^ at B, and the angle subtended by the same Stars at the opposite part of die
orbit. A, would be 8^, {See angle). If a variation of the angle, or annual parallax
of the fixed Stars amounted to only one second, or the 216,000th. part of a degree
the distance would be fiurieem millume of naUume of miles; but it is doubtful if
even thu small variation has been estabhshed by observation. The distance is so
great that the entire orbit of the Earth is but a point in comparison.

Parhslion. a mock Sun, occasionally seen and apparently owing to the reflec-
tion of the Sun's l^t on thick and frozen clouds.

PSNUMBRA, A partial shade between the perfect shadow and the full light of a
heavenly body under an eclipse ; the time occupied by the rays of light in reaching
us preventing our seeing the eclipse at any one instant precisely as we should do if
there were no aberration. {8ee abbrration.) It is certain that the Sun is eclipsed
to the full extent of the penumbra before that partial shadow is visible i^t the Earth.

Perigxe. See apsides.

Perihelion. See apsides.

Pertxjrbations. Certain irregularities in the motions of the Earth andPlanetSi
such as the Nutation of the Earth and the Libration of the Moon {See nutation),
the *' obliquity of the obliquity '' of the Ecliptic {See OBLiauiTr), the Precession (tf
the Equinoxes (iSfee precession), the Acceleration of the Moon (fi^etf accelbra<«
tion), ^.

Phases. iS^eeMOON.

Phenomenon. An extraordinary appearcqice [phenomena, appearance8|] ii}
Nature. Every Astronomical occurrence is a pb^pQipenon.

PLA* POL. PEE. »5

Pisces (the fishes). The sign of the Zodiac which the Sun enters aboat the
19th. of February and leaves about the 20th. of Mardi*

Planets. See solar system^

Plane. See circles.

Poles. There is a Fixed Star in the North which appean always to bear the
samey or telry nearly the same relative position to the Earth ; the Pole Star^ which
shiiies with a steady, but dead hind of light, a little less than 2 degrees from the
true North Polar point. Whilst all the other Fixed Stars appear, like the Suti, to
describe diurnal circles in the heavens, (an appearance owing to the daily revolution
of the Earth,) the Pole Star but slightly changes its position, and indeed appears to
remain stationary. Instrumental observations, however, show that the Pole Stiur
describes daily a small circle of 3^ diameter, and that therefore it is 1^^ degreeist
from the polar point, which is the centre of the circle. At the North pole of the
Earth the North pole of the heavens is immediately in the zenith. At the Equator
the North and South poles of the heavens both appear at the horizon, and to be
parallel to the centre of the Earth. Half way between the Equator and the North
pole of the Earth, (or in 45°North Latitude,) the North pole of the heavens is el^
vated, or at an altitude of, 45°, and appears half way between the zenith and the
horizon. The poles of the Ecliptic are points in the heavens 90° from every part
of the Ecliptic drcle, and about 23^° from the poles of the Equator, or North and
South poles.

Precession of the EaviNOXES. To enable the student to have any cenoqptioB <f[
this interestmg subject he must first have a clear impression of the motion of the
Nodes of a Planet, or Satellite, as attempted to be explained in the instance of the
Moon, under the word nodes. Now, the Precession of the Equinoxes, (or the
Becemon of the Equinoctial Points, as the phenomenon is called by some Astrono*
mers,) is analagous to the motion of the Nodes of the Moon , but with this differ*
ence to commence with, that whereas the Nodes of the Moon complete a revolution
about the EcHptic in 18|^ years, the revolution of the Equinoctial Points about the
Ecliptic occupies no less than 258 centuries ! If, when the Sun arrives, about the
21s»t. of March, on the Equinoctial, his distance from some Star, measured on the
Ecliptic, be accurately ascertained, and compared with an observation taken on the
same day of the previous year, it is found there is a differencCi or increase of the
Star's Longitude, amounting to 50" '1. This difference, so small for a single year,
becomes great when accumulated, so that in a century it is 1° 23' 30" , and in about
25,800 years the difference will be 360^, or the entire circle of the Ecliptic. Jjet
A B C D , Fig. 1 . represent different positions of the Sun with respect to the Fixed
Star S, at the Sun's rising above the horizon of an observer at the Equator ; the ho-
rizon, in each case being depicted by horizontal waves. Let A be the position, at
sun rise of the Vernal Equinox, the diurnal direction being A E. BCD will then
represent the Sun rising on other days of the spring months, as the Sun, in his an-
nual course, is seen to advance in a northerly direction, A being due East, and *the
diurnal direction of the Sun bemg B G, C H, DI. The line P A B C D wiU be an
arc of the Ecliptic ; the Sun, A, will be in the First Point of Aries, on the Ecliptic,
and the Star S, being referred to the Ecliptic, (by a line drawn perpendicularly from
the Ecliptic to the Star,) will have no Longitude that year ; the Longitude being
reckoned from A in the direction BCD. But on the next occurrence of the vem*

PRECESSION OF TWIg v qnii^ry^c^
1 H G iE

"■«»— •■^•B

d eqnmox the Srni will have receded, or not have advanced as ftr as A. by 50' -1

Tl' -La^^^^ *^* P'*^** «f *•>« Equinox, or the First Point of Aries .nTrtl
l^onptude ofthe Fixed Star S. wiD be P A. or 1°23' 30*. ft Sh tii^Sf t!

fteJS^ td SIT' fn^^ °*« P»»t» of theEdiptic^ be likewis/kwS
tneu- piaoes, and the constellations of the Zodiac, after which the Sinis of the Edin

^et: ^e "Zfmr. f .g-eoincide with those Signs. Snchirf^tsIS
Ari^dl^J^ilZ i ^'^' 2«00 y^ ago. ^ in the stellar constelh»tion

kSn o?S,V 7 J^ ^ ^ ®r """ ^ *" """^ "l"^"" t-* »»««« in that const^U

lation of the Zodiac ) bnt from the effect of Precession it is now in the co^.

lataon Rsces, and. 2000 years hence, will be in the constellation A^^Z

S^E^'.^fT'^ the original Signs although their positions no lom^i^^^'w^

the same cause, is a slow movement of the celestial
poles, or such a motion of the poles of the Earth
as to make them point successively, in about 25,800

r^' V r^ P"* °^ » '^«''«' ^»^g the. pole of
• ^S'S, " * *^*'*'' ""* *h« <*^eter of which .
" J .V ^ °'' *^** *h® ""g^® between the Ecliptic
and the Equator. See Fig. 2, in which • within the
«rele IS the pole of the Ecliptic, P the position of
the present Pole Star and North Polar Pomt, and L

attraction by the Planets of the protuberance o7^ir^' "^ ^^ ** ''""^
Equator. («e* degree.) P^nioerance, or redundancy of matter, at the

T,y2

PRI.

RAD.

BEF.

P. L. of difF. Proportional logaritliin of difference. (See hOGAKtruu,)

Primb Vertical. {See azimuth.)

Radius Vector. A radius of the elliptical orbit of a Planet or SateUite, as in

O^^BIT

the figure^ in whicli all the lilies diveiging from
the body near the centre of the orbit are the radii*
The radius vector (or traveUin^^ radius^ ) of the
Earth measures its distance from the Sun, which
varies so that at one peri9d of the year, ( the
winter of the Northern, and the summer of the
Southern hemisphere,) the Earth is 3 milUons pf miles nearer the Sun than when at
the most remote point. Calculations of the length of the radius vector of each
Planet, for every day in the year, are in the Nautical Almanac. : . ,

Reduction of Stars. The correction of the Right Ascension and Deelination
of the Fixed Stars for Precession, Aberration and Nutatidn« ^See these terau.) .

Reflection, of the raye of Light. The rebounding of -the' rays, or the Angle

they make with a reflecting' surCice^ such as wa«
ter, 6r a polished surface of metal or glass. It is
said that the angle of inddenoe is eqtial to the
angle of refleotiotf . Thus, in the figure, a rAy of
light from Af after striking a reflecting surfabe at
8y will rebound, or reflect to JB ; ^ iS^ 22 is, then,
the angle of incidence and B 8 H the angle <^
' reflection ; these angles being of similar extent;
In like manner, a ray from C will be reflected to DiiC 8E being the an^e of inci-
dence mdDSHihe angle of reflection. A ray falling perpendiculaiiy on ft reflect-
ing surface/ as from' E to iSf, will be reflected back in precisely the same, direction *;
the incidetice making no angle neither will the/ reflection^

RIbfraction; of the rays of Light. An oj^tical illusion, which makes a heavenly
body appear at a greater altitude thiiui it wot^ld be seen at^ thc^e- were no aitoos^
phere over the Earth. Light deviates frohi a^stniight line according to the densifyi

« « M t

I '

::».•-*.•_•:

•I / 1 1
A.

« ji

t!..^**:- .•

i<-

5 A. R- T jj

or quantity, of atmosphere throng wjfiich it passes. When the Sun is seen to rise
or set the quantity of atmosphere, through which his rays proceed to an observer, is
much greater than when the Sun has ascended fi considerable (dtitude. This wUl be
seen by the diagram. The Sun, when at 8^ sends Ins rays several thousand miles
throng the atmosphere frpm ^ to J{ to ^ei^^J^^an qbjservei; at B ; but if the Sun be
in or near the zenith , as z , the, jcays bftve imly tp ^penetrate the atmosphere a few
miles, or. the altitude of the atmosphofe above the jB^rth > aj^ frqo^ D fo E9. ^ I'cach
an observer at E% When we look at the Sun rising or setting we look through such

KIGHT ASCENSION.

an enonnoot quantity of atmosphere that we are enabled to gaze at him with eur
naked eye ; but if the Sim be at a eonaiderable altitude the quantity of atmosphere
we look through is so mueh less that we ate then only enabled to bear the fieiee
glances of the solar rays thn^h a darkened or colored medimn. These remarks
are necessary previooa to giting an explanation of the term refraction, which is the
new direction the rays of light move id after passing, in an oblique direction , from
one medium or element into another. In passing from a rare into a dense medium
rays of light are refracted towards the perpendicular. J B , Fig. 2. , represents
a ray of light passing through a rare mediunf ; B G the
same ray continued through adense medium, and D £ J>

a perpendicular line ; the ray at B G being refraeteJ ^^-^^^^^^^ I

toward$ the perpetuUeular. On the eotitrary, a ray of ~

light passing from a dense into a rare medium is refrac-
tedyWisi the perpendiculaf. If a ray proceed from C,
Fig. 2. and emerge at B, from a dense into a rarer me-
dium it wiUbe refraefced towards A, from the perpendi-
cular. Beverting to Fig.I, a ray from the Sun 8, after passing through tlie rate s^micc
between the Sun and £arth» will be bent on arriving at the dense atmosphere, owing
to which an observer at B will see the Sun, by refraction, before that kumnary has
actually risen above the horizon^ Befractioa is (noncy) at the zenith, aiid 34' (34
minutes of a degree,) at the hcmzen. When the Sun just appears to rise or set his
centre is 34' below the rational horizon. {See circles.) Since refraction, therefore,
causes a celestial body to«ppear too high the amount of refraction has to be snb-
traeted from the apparent altitude to correct it for the true altitude*

Bstardahon. iSee AccsLBRATioif •

Rbtolvtion. The movement of a Planet, Comet or Satellite from one ftitA
until it returns to the same again, as the annual revolution of the Earth round the
Sun : or it is the movement of a body on its axis, as the dhimal revolution of the
Earth. The revolution of the sphere is the apparent daily passage of all the celes-
tial bodies over the Earth, from East to West, an appearance caused by the Earth's
diurnal revolution from West to East.

Right Ascension. An observer situated at the £quator sees all the Fixed
Stars rise and set perpendicularly to his horizon ; those rising due East passing over
his zenith and setting due West, and those rising 45^ from the East setting 45^ from
the West, after reaching an altitude of 45^ in the heavens. These ascensions, perp-
endicular to, or at right angles with, the horizon of an observer at the Equator, are
Bight Mcensunu of the heavenly bodies , (See Fig. 1 . ) and their ascensions at any

BETWEEN THE NORTH AND EAST
AT THE EQUATOR,

With Arcs of Ascension of the heavenly bodies at right angles with it«

MGHT ASCENSION.

T^Z

HOB-ISOW^

BETWEEN THE HOKTH AMD EA«T|

ftt 45° North Latitude,
With Arcs of Ajceniisn nt Oblique Aisles with it.

other horizon than one on the Equator will be ohl^i. See Fig. 2. in which N the
North Pole, is in the centre ofthe circle of Perpetual Appariton,- (See circle ftc)
The angle, or difference, between the celestial meridian of a body and that put of
the Equinoctial which rises with the body is the aeeentional differenee. Thus , in
Kg. 3., P H being the same circle of Perpetual Apparition as in Kg. 2., Jtf E the
celestial meridian of the Star S; £ £ Q the EquinoctiB], and U S the Horixon, tiis
the point L on the Equinoctial has risen with the Star 8, and £ £ is the ascensional
difference.

But the term BJght Ascension has a much more extended eignificadon than that
iucloded in the above explanation of the pherumtenm which the term espresses.
Technically the R. A. or A. R, (Right Ascension,) of a heavenly body is said to be
an arc of die Equinoctial included between the First Point <^ Aries and the celestisl

meridian of the body. Thus if Q, Fig. 3, be the First Point of Aries, sad N E the
celestial meridian of the Star S, Q E will be the Star's Right Ascension, or titat
ar^jular cUstance of the Star fron the First Point of Aries seen at any place on the
globe, and without refweaee to tlie fact of the Star having made a Right or ao
Oblique Ascension from that place ; the lUght Ascension having been made, as al-
ready observed, at the Bquator.

For the purpose of measuring the apparent distances of celestial meridiana frpm
the First Point of Aries, the Equinoctial is divided into 24 parts, ta hoUra, by 24
horary circles, all meeting at the South and North Poles of the heayens, as ip Kg-'|.
which with its explanation will be found in the svtcceeding page.

BIGHT ASC9BNSI0N.

• In this diagram S is ike South Pole, and the circle described from it is the Equi-
noctial.. The Sun.i^ i:epresented to be in R. A. 0^ 0" 0', {m distance from the First
Point of Aries, and consequently an that Point) : 5 represents the position of the
Planet M^cnry in.R. A.'23^ 1™ 37", (as near as the size of the figure will permit) ;
% Jupiter in ». A. 29" 6°^ 22% Tj Saturn in ». A. 20^ 28«» 42% g Vesta , in R.A.
19^20°K)», and ^ Ceres, in R. A. 15^>30'"0% all. these "Right Ascensions" being
the appjorent distances of the celestial meridians of these bodies. Easterly, from the
First Point of Aries, for the 20th. of March 1844 ; their distances from the Equi-
noctial being their declination. On the day mentioned, the Moon and the Planets
Venus, Mars, Juno and Pallas will have North declination and are, therefore, exclu-
ded from this diagram, which only represents the horary semicircles of the Southern
celestial hemisphere* If the line from 23 to 1 1 in the diagram be taken to represent
the Southern Horizon of any place on the Equator, on that day, then the Sun is I''
of R. A, from the Horizon, in the East, as he will be one hour before sunrise,
whilst Jupiter and Mercury are just skimming the Horizon, and Saturn is seen to
have risen 2^^ hours, Vesta 3 hours 20 minutes and Ceres 7\ hours previously.

Right Ascension is onie of the most important elements in Astronomy ; it divides
the entire sphere of the heavens into 24 parts; it connects space in the heavens with
time on the Earth. As we know that die ,Sun is in R. A. 0^ 0°^ 0', or on the First
Point of Aries, about the 21st. of March, and that l^e passes over the great qrde of
R. A.' m twelve months, we can form an instant concepticm of the Sun's place
(nearly,) in the heavens at any time by allowing 2^ of R. A. for the Sun's apparent
motion, from West to East among the Stars, for every month elapsed sinc^ the 21 st.
of March. For example , on the 22 nd. of April 1844, the Sun's R. A. will be 2^,
on the 23 rd of May 4^, and on the 21st of June 6^. When we learn that the Sun
is in R. A. 6^ and the Moon in R« A. 18^, (that is, 12^ distant,) we can immediate-
ly infer that the two luminaries are in opposition, and that, about the time when

MN. SAT. SEA. - 41

i» 0«i*>

the Sun is rising in tlie East, the Moon is setting in the West ; and if we leam that
the hours of R. A. of the Sun and Moon are equal we are at the same time informed
that the Sun and Moon are in conjunction, that the Moon is just completing or
commencing a lunation, and that she will rise and set with the Sun. In the N. A.
(Nautical Almanac,) the R. A. of the Sun is given for every day, with the differ-
ence for every homr ; and the R. A. of the Moon for every hour of every day, with
the difference for every ten minutes.

Ring of Saturn. See solar system. -

Sagittarius, (the archer). The sign of the Zodiac which the Sun mters about
the 23rd. of October and leaves about the 21st. of November.

Satellites. Attendant, or secondary planets, as our Moon and the Moons, or
Satellites, of Jupiter. See solar system.

Saturn. One of the Planets. See solar system.

Seasons. The phenomena of the Seasons are caused by the poles of the Earth
bang always directed towards the same points of the heavens, and the axis of the
Earth being inclined 23^ degrees from a line perpendicular to the plane of the orbit,
as in the first diagram, in which X is a line perpendicular to the plane of the Earth's
orbit, p a pole of the Earth, and P the direction of a celestial pole. The celestial
pole, if placed in conformity with the size of the orbit in the diagram, should be sev-
eral feet from P, in the direction jp P, so that in whatever part of its orbit the
Earth may be conceived, the angle between a line from a pole of the Earth to a pole
of the heavens and a line perpendicular to the plane of the orbit may only slightly

*'•-... ^'

«
«

vary. There is in reality a very small variation at different periods, for an account
of which see nutation and precession.

The second diagram represents the Earth in the four positions it occupies , with
respect to the Sun, about the 21st. March, 21st. June, 23rd. Sept. and 22nd. Dec.
If we imagine the Earth, in each position in this diagram, to lie revolving on its
axis, it will be seen that, about 21st. March, the Earth will present, in the course
of its diurnal revolution, the entire circle of the Equator directly towards the Sun.
In this position, the Earth having the Sun vertical (perpendicular,) to the Equator,
and at an equal distance from its two Poles, equal days and nights are over the
Earth the Poles being in the boundaries of light and darkness. On the 21st. June
the Earth, having progressed over one quarter of its orbit, in its daily revolution,
turns its Northern hemisphere towards the Sun. The Sun then shines without
setting on the North polar circle, the inhabitants of the Northern hemisphere are

:TBB jgnUUBONS.

■^r

J/hftf^

u-..

^^<£0M^

,.i|ff^«>l^'rci'

Thju

Svd^iuica <)i«

gs'^S^e/U^

J^ffT^^

4''

c.— ••*•

sec; . mar .sol. 43^

theh'^y tll^8ttn!Mer^86Ml0e;f^tt]|<l'tlK>te of^^ Soitth have th«ar/^l%iler; ike
S6\iW|Kfflli^c&ti[^ bdiBg 'iisr diirkti^si. Oil tkil 2Sti. of September ih» Eaitb has
reiU;%er'a'ipoifit'lRMeK'ib»''E Bgiiiitiiniri^difeedjr towaids tiie Sun , and'

e^ikaPdiy^ aii^^n^S'arA'o^ir th^ EarA; Thk is the second Sunnier at the
Etftmsfr, tH&'Atmt^taA IS^sp^^mx ^t' th« Kortii, imd tlie Vernal^ or -Spraig, Eqwnoix
a^'tte Sbttm? The EM&'tkentMissesoiittfthfcfciifrti^ positiM ddineated in die
di^rt^; wfeSdi ooMM abotit the* 22nd. Decenber, ^en the Ettdi pvesents its son*
tl^ faifi&S^{«iiT« tl^ the gl^t Itin^ It is then the Summer 'sdstiee at
thb^ S^Ch'taidthe' ^Winter sobtiee at the Nivth $ the North polsr ciieie^heinKin*
dMOimt ItS^^Ptkb^l^ 8eei» thittMMrls only one day and night at' the fblea
tUtoiighefvtrth^y^iu!^ Ute SM's rqrs^oldy r6aeh W from that pokt^if the Eatth
wfxere titti SubI iai viltiea]^ alid'odfy'illuMttat^ hatf tlie ivoildat^ one time ; and aa
the Poles are 90® from the Equator, it is nig^tf«f the N«»th Pole when the Sun is
South of the Equator, ^which continnDil 'diftiiig half the year,) and day when the
Sah ii Nbite' oft6i&1B^t6rVth% lte¥mft oNhk o^SOHfing at the Sooth Pble; Ti»
orbit of the Earth is ellii^cal, and that jtet'of the t»rbit in whieh the Earth is sku-
ated during the Winter of the Northtttt h^iieiS|)here is 3 millions of miles nearer
^e" ]^BAh^ tUhif 'tli aj^diti^' paH. Thi^ elll^feity is searoely peiveptible in so
itiiWf iP f^sm' as' tluf diigMh ;it attiomi^ to a dMefre^ee of only a parts in 190,

l&cuLAR TiWW, TSiorioyi&A'KctEttiiA^io^* An indefinite time, motion or
acc^eleration. Thl? Secular weeterdtum of tlUMa&iif^ Me&n mibtioH is a slbw and' con-
tiiiuai increase of the' speed of the l^ooii over the Earth, eomjfared with her motion
oi sbmVcehinneis pfeVibus, dtused by her gradually approaching the Earth. If con-
tinued, for an immensity of ages, the Moon would at Ust comie in contact with the
Earth, but it is theoretically shown tUat thd acceleration has its bounds, although
mdefiniie,

BsuttiAMinUR. JSjBit a diameter. By the term "senudiameter^' of the Sun or
Moon it ^erally understood half the horizonial diameter of
tfther body slipaitated by a'Uiie drawn perpendicukr to the eye
of die sj ^ ta tOi vMiu ^^ amiexed figare. The semidiametm
of the' Sun aifd M6otf are eontinnany undeigoing a change of
n^ypAietitaiie 6n aoeocbft eff t^eir vuying distanees from the
EAtii. The dilidtdiKDieter of the Stmiagivenin the Nantical
itimanad foi^ every dtty, and the seinidiameter of the Moon for
ereiy nd9n and Audn%ht# To the senaidiameter of the Moon, thus given, a correc-
tion has to be applied for augmmiaHon. [See augmsmtatiom and viamstkr.}

E(ii8M]^i8; (Oie dctii^ioiij. ¥be si^ o^the Zodiac vHifeh the Sun enters about
ltier23^<$. ^b^W, tend MM tiibout Oie 21st. of Noyembef.

8iDEiipAi«, SiD^RAL, SiDEREAN. Starry. l^Sidtreal Time of the Sun's Semi-
dianjieter facing the Meridian is the sidereal time occupied by a semidiameter of
Ihe^un, on any particular ttay^ in passing a meridian point, lliis time is variable
ap^rding to tne greater or smaller distance of the Sun frokd the Earth and the po-
sition of the Earth in its elliptical orBi^ \See time, semidiameter and meri-
dian.)

Soi«AR. Relating to the Sun.

30LAR DAY. See TIME.

44 THE SOLAS SYSTEM.

Solar Systsm. The Solar S^stfunis Omt purt of the UmTone.wliich has the
Smi for its centre^ and includes the Eerth and.oUier Planets which reyolve anmnd
the great luminary ; the Comets heing periodicslaiidoceaaionalTisitoi^s of theSystem.
The diagram which represents the .Orbits of each Planet does not. truly depict the
sixe of each body. To approximate towards a correct representation would, require
a drele of more than a milq and a half in diameter, on the droumferepee of which
Uranus or the Geor^an would be .of the size of a cheery or. small plum, Saturn a
«nall orange on a drde four fifths jof n mile in diameter, Jupiter a moderate sized
Qrange on acirde nearly halfa mile across* A globe 2 feet ip diameter, placed in
the centre would represent the Sun, Metciuy would only be* of the ease of a grain of
mustard seed, 81 feet ftom the Sun, Venus* a pea, 141 feet, th^ Earth, also a pea,
214 feet. Mars , a large pin's head, 326 feet, Juno, Ceres, Vesta and Pal]as,.grains
of sand, 500 to 600 feet ftom the Sun»

THB SUN

revokes on liis axis in 25|- days. His diameter is 882,000 miks, fi^ii he fills a
space twice as huge as the orbit of the Moon*

MBRCtJRYrf

Nearest the Sun revolTes the Planet Mercury,, a small stai: wi^ a Teiy bri^i
bluish li^t. He is distant from the Sun 37 mUUcns (Smiles, but^rom the Earth
he appears so near the solar luminary as to.be only pocasioiially visilj^le, a littlf be-
fore sunrise or a little after sunset. He never appears more than 20^. from the Sun,
or one ninth part of the visible sphere from East to West. During a Solar Eclipse
he is sometimes seen ; and he is also observed to pass between the Sim and the
Earth, traversing the Sun's disc. His diameter is about 3100 miles, and he revolves
arpund the Sun in 88 days*.

VENUS

is the most beautiftd Star in the heavens, and is* called the Evening and the Mom«
ing Star. When towards the West of the Sun she rises in our horizon before the
Sun in the Morning, and when towards iheEast she shines in the Evening aft»
the Sun has set. She never appears more than 29^ (or one taxHh part of the 'visi-
ble sphere) from the Sun. She is to the East and West of the Sun 9 or 10
montiiis alternately. When a morning Star she is sometimes called Phosphoms or
Lucifer, and when an evening Star Hesperus, or Vesper. ' She has phases, like the
Moon, sometimes homed and sometimes gibbous, and like Mercury is seen to trav*
erse tlie Sun's disc, which proves that her orbit, as well as that of Mercmy, .is be>
tween the Sun and the Earth. Distant from the Sun 69 millions of miles, and
travels round the Sun in 225 days, at the rate of 76,000 miles an hour. Diameter,
7,900 miles. Revolves on her own axis in 23 hours.

Venus and Mercury are called the inferior Planets, their orbits being .within, or
inferior, to that of the Earth. The other Planets, whose orbits are exterior to, or
larger than that of the Earth, are called the superior Planets.

THll EARTtt

is distant firom the Sun 97 millions of miles, and travels round its orbit in 365^
days. Mean diameter, 7,900 miles ; its diameter from Pole to Pole being 26 miles
less than at the Equator. Circumference 24,850 miles. Revolves on its axis from
West to East in 24 hoursi and, like all the other Planets, (with the exception of the
Satellites of Uranus,) pursues its orbital course round the Sun from West to East.
Velocity in its orbital revolution 58,150 miles an hour, or 150 times swifter than a
cannon ball.

The Moon and the Satellites, or Moons, of Jupiter, Saturn and Herschell are
called secondary Planets to the primary Planets around which they revolre.

JU f *'

THEvgOLAR 8TSTEM>

■U«

jtok>^- ^ ^"^

\ojiiiS^^-^s^£^i

the' ^tXft St^TfiM.'

TBZ HOOK

is « secondu; FUnet to her primary the Earth, tnm which she ia distant only
236,847 mileB. Diameter, 2,160 miles. She makes one rerolutioii on her siis ia
27^ days, and rerolveB around the Earth in nearly the ssaie period of time, invaria-
bl; turning the same hemisphere towards oa.

MARS

gives a duller light than Venna, and ia of a dniky red colour. Distant from the
Sun 145 milHona of nules. Travels ronnd the Sun in 687 days, and revolves on
bis own axis in 25 hours. Diameter half that of the Earth, 4190 niles.

pSRKS, PALIfAS, JtJNO Sad TKBTA,

are fonr Planets generally called Asteroids, only lately diswvered, and never viable
to the naked eye. Kameter of Pallas 2,090 miles, that of Ceres 1,600 miles, of
Jnno, 1,425 miles, that of Vesta is not known. Ceres is 260 millions of miles from
the Sun, Juno, 300 millioni, Vesta, 226 millions of miles from the Sun. The orbit
of Pallas is very excentric and crosses the orbits of the other Asteroids.

JTJPITKB,

thelai^est and most splendid of the Planets, ia diatant from the Sun 493 millions
of miles, travels round the Sun in 11 years 314 days, at the rate of 25,000 miles aa
hour, and revolves on his own axis in 10 hours. Hisdiameter is 90,000 miles j his
bulk 1300 times greater than the Earth, and he has 4 Satellites, each larger than our
Moon. These Satellites are of conuderable importance in Astronomy, aa they re-
rolve so rapidly around their primaiy that their transits over tlie disc of Jupiter,
or an eclipse of one of them, is almost a daily occurrence, and the exact Greenwich
times when these pheugmena are to take place are foretold in the Nautical Alma-
nac. By a comparisoQ of tlie time at Greenwich when a transit or eclipse ia to be
seen, with the time when it is observed at any other place, the longitude of that
place from Greenwich is immediately Inferred without any fruiher calculation.
For instance, if an eclipse be foretold to occur at Greenwich at twelve at Noon, and
it is actnally observed at some other jiace at oAe F. H, that place will be distant
from Greenwich the twenty f^iHth part of the drcumference of the Earth easterly
from Greenwich, or in longitude .15°£. The observers at this place and at Greenwich
will both be witnessing, the phenomena at the same minute, but it will only be twelve
o'clock at Greenwich, when it is qae o'clock at the place 1^° easterly. Jupiter is re-
markable for an appearance of l^^ts over his disc, snppoaed to be caused by his
atmosphere being lesa dense ,at hi^^ Bquator than at his Poles, and, consequently,
varying the luminous appearance of the planet.

SATVHN WITH BIS RINGS.

Sfttnm, when licwed by the naked eye, is of a dull leaden lustre, and is hardly

T ^ SOlAR ^YSTEM, ^\ 7

22,000 mOes an hour, and rev6^^6^'o8PE^^t)^ k^^iPldi^dm^'tlki fb^KKM^f

the Bings does pot exceed^lM unless hut .iheir.4»r^l3i 4joggdOf»T «| j^^jt^ ^es ;

:th$. ^te]^o;i4ianictter jo(:th^ extmof ,^ipgis i76^0pP| t^mte betveen^tb^la-

net and interior Ring beipg, jPj^QpO^l^s, wfd the intoryS*^ betw^n tliie^ tw 'fijxxgi

IfBDO miles.-.^Thfi Bing does i^iot^.f^nrays present the appearance asi in^the^oure :

.,when the thin ease is turned to us it appears only as a very fine sTrai^rht line. The

^ Nautieeu Almanaq contains predictions^ of the position, magnitude ana appearance of

HERSCHELL. VRANUS, OR THE GEORGIAN, .

but
meter is about '35.000 jpiiles, ipi^ its quli 80 limeslthat oTthelBar^.^it is'cer^iinly

:U ._„.._.".,. ■"

round its orbit in 84 jedrs.

The Ephemeris of the Planets occupies 164 pages of the Nautical Almanac, and

contains for each Planet, for Greenwich Mean Nooff'df'^i^^'dl)^ id^the'^l^^ the

apparent Geocentric Bight Ascension Aid U^KftdftV»r$^t^etkiis^'Of Ait^Mertiian

Paaaage ; and the Logarithm of the true distance ftsmi Ae Earths th^: Heli^omtric

Longitude and Latitude, and the Logarithm of the Radius Fee tor t^ It also contains

for each Planet (with the exception of the Asteroicls,) at its transit over the M^rid*

ian of Qreenwich, the apparent Bight Ascension and' Declination, and fhe variation

. of B^t ABoenaion 8nd DddiintiDn lb¥ OB^ttour^f L6ri^tud4> tfi» SSderealtSQi»« of

the Semidiameter passing the Mmffi£^» A^togtd$p['VieBMimA&ai:cfi^

.. eter. and the Horizontal ParaUax* Vpr an. explanation of each of these terms in

Itais's^ fhS rfespeAive wordts; ' ' " ''' ^'^ ''-' ' - ^'^^ "' ^'- ^ — ^

M The nature of Cometa (hair^ Stars) hvKkt known. . It is cerUdn, howerer, that
. they' are occibiotial Tisitiira^f the 'Sobt^ Systefn^ ! .(See^Cfhanm)*^.'fTlaUKkWBM)et
also is not detennined,' and milj three httvefaad thrilr penodicalv^titmft^dc^^
first of these {Halle^js Comet t>t the Ck^m^t of ir&9*>^ppe4riiigi«¥eiyn76£ /^,
the second^ (Ekcke's Comet) ey^ry thi^ee years and H tiUid^ Md tiber:tlM«( (Bi^
I C6ntet) every six y^ars and thrte quartfelm.:. Tbe-nuHMs^^s^ASom^ is ttte^ototial
pdnt, or b6dy, liround wbi6h tibtere is anebulouiS' a|^earanee'eailed*tilMtiitei|yv«r.^e
Mdeus'and hair fbgethfer form the hifodi^' tki.'Coiket. - Te^'tlub^id' ^eiiettll^,'r'btit
not alwayd, atisadied a luminous tram called tHe tail,' fieme>Ckntetir iutveHad
aevisral'(one of them as miny as sixdistsnctyiaibi and the'diramrions of thetegas-
, eons 6r electiric appendages hai^ odeasioiiaUy beelr'abtdluildte NaiitioiKlAitna-

nae fat the yiears wheii these' Coiiiefir lire ei^eeted^ IsoMifastHBi Ephe»iSfis'4f their
last appearance, edqiprising the geocentxicBa^t Ascension ^attd}DeeUnalioii %atii:the
variations in one hour, the time of the Comef s daily passage -of* th^'Mimidbntmd
the logatitfasis of its Sisttoeed from AelEkuf aiidldBaMii *. - r «u«r T^:r» . t^l

' • - tv *4> • ', «» J>

48 SUP. tAU, TIP.

i, ii — - — ' ** ' — : — *^

Solstices. Those points of the Ecliptic the most elevated above, or at tfie
greatest distance from^ the Equinoctial ; so called because the Sun, when arrived at
Qkemajipeara to stay or stop a short time. . See colures.

SuppLXMXMTf (in geometry). Such an addition to the arc of a circle as will
suffice to complete 180^ or a semicircle. In Astronomy it is the number of d^e^
required to oomplete 180^ Irpn^ North to South ^c. * ,

Sox«8TiOB8> SoXfBTirrAt Points. 8t& colitrbs and sxasoks,
SovTBiKO. A term given to the approach of the Moon to the southern iute of
the celestial meridian of a spectator. iSf«e culmination. . t

SraERis. .The Celestial Sphere is flie concave or arched fece of the heavens,
as seen from the Earth. A right Sphere of the Sun or a Star is one in which either
body rises or siets in a perpendicular line to the horizon, while an oblique Sphere is
one in which it continues parallel to the horizon. The Celestial Sphere at the Eqnk-
tor is always a right Sphere for the Fixed Stars, as they all rise and set at right
angles with the horizon. At the Poles the Sphere is always a parallel one ; the Sti^
never rising or settiiig, but those visible describing complete drcles, parallel to the
horizon. ' At all other parts of the world the Sphere is more or less oblique for the
Fixed Stars, but some parallels of latitude near the Equator have ar^ht Sphere Jor
the Sun and Planets at some seasons of the year luscordin^ to the periodical changes
of those bodies. See. also bight ascensiqn.

Stars, See vixed stars,

Stbllail Starry ; relating to the Stan.

SVN. See BOLAR STSTBM*

Stnodical MoNtB. See moon.
Syzygy. See conjunction.

Taurus, (the bull). 1%e sign of the Zodiac which the Sun enters, aboat the
20th. of April, and leaves about the 20th. of May. '

Tides. The rotation of the Eai^ on ifs axis, combined with the attractfoti of
the Sun and Moon exerted on the fluid portion of the globe, occasions the pheno-
mena of the Tides. If the Earth were at'irest between the Sun and Moon the
waters immediaA^y under the two luminaries would be raised hy their attraction
and remain permanently elevated; hut as die relative positions of all three bodies
are continually nndergoing a change, so the attraction on thewaters is continually
shifliiig in position ; the attraction of the Siln and Moon being exerted in the same
direction when the Moon is New and Full, (or in conjunction widi and opposition
to the Son,) forming the high or spring tides, and the attraction bdng divided in
amount between that due to the Sun and that due to the Moon when the Moon b in
her, first and third quarters .; the tides being then neap or low. The devation of the
tide due to the Sun is 2 feet and to the Moon 5 ^t ; when^ iheivfbre, Jboth the at-
tractions are exerted together the tldid wave near the Equinoctial line, where the
attractioii is greatest, is 7 ft. high, and when the Sun and Moon are in quadnittge,
(one four& of a great celestial drcle apart,) the height is about 3 ft;^ heSng-the dev-
atidn due to the Moon lessened by the attraction of the waters, in anangnlar dbee^
tion, duie to. the Sun. f

Tina explanation (to a certain extent only,) contains the theory of what is. ealled
tibe Chreat Tidal Waves^ which flow daily from East to West under the Sun and

THE TIDES. 49

Moon ; that of the Sun bemg sometimes superposed on, or coincident with, that of
the Moon and sometimes being independent. But there is this difficulty immedi-
ately occurring in connection with the phenomena, that there is not merely one
high tide per diem (each day,) but two ; or, in other words, the waters are not only
elcTated under the Moon but at the same time are heaped up on the opposite side
of the globe. To receive an explanation of this difficulty it must first be conceived
that the tendency of the solar and lunar attractions is to bring the waters of the
globe towards the Equinoctial line, thus causing a protuberance over the Equatorial
rfigions and a consequent diminution of convexity, or height of the waters, at the
Poles. Then, with reference to the following diagram, A c B d represent the
waters, P a Pole of the Earth, and M the Moon. Let the effect of the Moon's
attraction be conceived to be exerting itself on the waters immediately under that
Satellite^ causing high water at A, and we shall immediately perceive that the influx
of water at A must cause a great flux of water from c and d, which flux, or flow, of
water, if continued long enough, would equally lessen the waters at B ; but, before
this latter effect can take place, the attraction has shifted its position by the Earth
revolving from A to d, and the waters at d, being then under the Moon, receive part
of the waters which flow from A and B, another part falling towards c. The great
body of water at A, when no longer under the perpendicular attractive influence of
the Moon, falls nearly as much by the force of gravitation to c as it does by the
oollateralj or sideways, attraction of the Moon to d.

Again : let A represent high water of the superior tide, (the tide next ensuing
after the transit of the Moon,) B high water of the inferior tide, (the tide opposite
the superior) ; c and d low water and M the Moon. Conceive the Earth revolving
from D to A : thjen, when d is opposite the Moon, A and B will become low wa-
ter, c and D high water, d being the superior and c the inferior tide.

To view the subject clearly two forces must be conceived constantly' in action ;
the force of attraction which raises the waters towards the Moon or Sun, and the
force of gravitation which exerts a constant effort to equalise the height of water,
but with only partial effect on account of the superior force of attraction.

But this outline of the cause of the GretU Tidal Waves delineated in the diagram
has to be considerably modified to bring about any conception of the phenomena
witnessed at different places on the globe^ The actual occurrence of high water at
differ^it places would indeed agree with the theory were the land only formed into
two continents. North and South, and the whole of the Equatorial regions covered
with water, allowing an uninterrupted flow of the element, from East to West, to
follow the afyparent da£[y course of the Moon ; but as this is not the case , as Africa
and America, running North and South, are barriers, interrupting the free progress

50 THE TIDESL

of the wave, and as some great seas are in unfavorable situations for reeeiving an
influx of water from the great oceans^ or for parting with any, the whole theory is
modified at every locality. In the Mediterranean, for instance, there is scarcely any
tide, the entrance at the Straits of Gibraltar being small, and the Baltic Sea, which
is still more remote and separated from the ocean, has no perceptible tide. At An-
napolis, in the Bay of Fundy, the tide is said to rise sometimes upwards of 100 feet,
and at Bristol the difference between high and low water is 50 feet. This division
of the subject is so diversified that every part of the world, every bay, inlet and
river, might justly demand a separate treatise to account for the local variation of
height, and time elapsing, between the transit of the Moon and the rise of the wa-
ters : in this place, therefore, it cannot be attempted.

In the Nautical Almanac there are directions for ascertaining the height of high
water at a great number of Ports, but the method exhibited only approximates to
correctness* To predict the times of high water, and height of the tides, with the
greatest accuracy which our present knowledge of the subject will permit, requires a
series of tables depending on the following data, and constructed expressly for the
latitude and local peculiarities of a Port.

I. The Time of the Transit of the Moon.

II* The Moon's Horizontal Parallax.

III. The Declination of the Moon.

iv. The Sun's Horizontal Parallax.

V. The Sun*s Declination.

The necessary explanations to this part of the subject will be given in the regular
order of the above numerals.

I. The time of the transit of the Moon is necessary to help us to an approxim-
ate, or nearly accurate, time of high water at any place, which we arrive at from
previous recorded observations. Let us take, for instance, the day of the New
Moon« when we know that she is passing the meridian with the Sun at noon. A se-
ries of observations will show us that the tide in the open sea continues to rise 3
hours after the Moon has passed, and that at any particular l^ort the lunitidal in-
ervaly or time elapsing between the transit of the Moon and the high water next fol-
lowing, is always the same on this day of the lunar month, wind and other atmos-
pheric changes permitting ; and the time when this lunitidal intertal expires is
called the Establishment of the Port.

Let us suppose, for example, the ^'Establishment of the Port'^ to be 12 hours 45
minutes» and that we have a table before us containing the time of the transit of
the Moon for every day in the month. Add 12 hours 45 minutes to the time of
transit at New Moon, namely noon, and we learn that the approximate time of flood,
or high water, next ensuing atler the Moon passes, is 45 minutes past 12 at night.
After this the ebb tide, or flowing out of the water, commences.

Between two floods, or two ebbs, about 12 hours 15 minutes elapse at New
and Full Moon, and 12 hours 30 minutes at the quarters. These effects are
called the Priming and Lagging of the tides, and are produced by the gremt
tidal wave being the result of two operations, or , in other words, they are to be
accounted for by the theoretical existence of two waves, one solar and One lunar^
which when they are superposed (brought one over the other) effect the/}ni»tng,
and when separated produce the lagging of the tide, because their jmnt height has
its maximum (greatest height) at some point immediately between than, and that

THE TIDES. 51

connecting point becomes, in fact, the apex (top) of the two wares when considered

as one.

From a previous explanation it ^11 be seen that the highest or spring tides occur
once a fortnight. The highest spring tides are due to the Moon being in perigee
(nearest the Earth), and the smallest spring tides to the Moon being in apogee (far-
thest from the Earth).

II. The first correction of the approximate time is for the Moon's horizontal
parallax. {See paralu^x.) The Moon's parallax increases in amount as she ap-
proaches the Earth to be in perigee, and this correction is, in feet, one for the in-
crease or decrease of attraction caused by the Moon advancing towards, or receding

from, the Earth.

IH. The second correction is for the Declination of the Moon, (5^^ declina-
tion), which latter, when compared with the latitude of the Port, gives us the cor-
rection due to the Moon southing near or remote from the zenith. The nearer the
Moon is to the zenith of the Port the greater will be the attraction she will exercise
on the waters, and the tide will arrive quicker and be higher in consequence.

IV. The third correction is for the Sun's Horizontal Parallax, the explanation to
which is similar to that given at ii, and the same may be said of : -

V. The fourth correction, for the Sun's Dedination, the explanation to which b

similar to that given at iii.

All these corrections, tabulated expressly for the latitude of the Port, enter into
the calculations, both for height and time, but the tables for some latitudes scarcely
need embrace the corrections of the height for the Sun's Horizontal Pandlax and
Declination, these corrections being too trifling in amount for any practical ad-
vantage.

Having thus ^ven an outline of the most scientific method, at present in use, of
calculating the time and height of high water, it may be only necessary, in fcondu-
sion, to add that the predictions, when accurately calculated, are sufficiently near
for every practical purpose, although separate calculators, adopting the same theory,
may, by the construction of their tables, deviate from each other several minutes.
The author of this work having constructed a series of tables precisely according to
the formulce adopted by the Admiralty for their annual publication "Tide tables for
the English and Irish Ports " was, at first, surprised to find a difference between
the results from his tables and those issued by the Admiralty, and on comparing the
tide tables published in Liverpool he found a similar difference between each of
them, in some instances amounting to 6 and 7 minutes in the predicted time of
high water. The cause, however, soon appeared to be in the number of corrections
required, which, if estimated in minutes only and not in fractional parts of a minute,
would easily permit a number of small differences to creep in, accumulating to sev-
eral minutes. The following is a specimen of the calciilations, from four hands,
of the time of high water at Liverpool.

ADMIRALTY. HENDERSON. HOLDEN. ^ PAYNE.

p.M P.M. P.M. P.M.

H, M. H. M. H. M. H. M,

19 24 19 17

56 11 55 54

1 32 1 38 1 31 1 30

As the theoretical eonstmction of thi^tables may be presumed tg be similar in all
lour instances these results may be consideied curious*

62 TIME.

Time. Time is divided into periods, cjcles, years, months, weeks, days, hours,
minutes, seconds and fractions of a second. A Grand Celestial Period, or Platonic
Day, consists of about 25,800 years, during which takes place the Precession of the
£quinoxes. (See precession). A cycle of the Sun, or Solar cycle, consists of 28
years, at the conclusion of which (with an exception on account of Leap year, see
bissextile), the days of the months return in the same order to the same days of
the week as in the previous cycle. A cycle of the Moon, pr Lunar cycle, (called also
the Golden Number^ as deserving to be written in letters of gold), .consists of 19
years, after which the New and Full Moons return on the same days of the months
as during the previous cycle, but 1 hour and 28 minutes i^ooner. There is an ex-
ception to this rule respecting centesimal years, (years completing a century). The
two last mentioned measurements of Time are among the six cycles in the Nautical
Almanac. The other four are, 1st., the Epact, 2nd. the Pominical letter, 3rd. the
Roman Indiction, and 4th. the Julian period.

I. Epacts of years and months are terms applied to th& liifpon's age on the first
day of the year and the first day of the month. The Epact of a year is a number
expressing the excess of time in the Solar above the Limar year. The Solar year,
or time the Ea^h occupies in making pAC complete revolution over its orbit round
the Sun, is 1 1 days longer than the- Lunax year, or time occupied by the Moon in
making twelve complete revolutions round the Earth. On the first year pf the Lunar
cyde the Solar and Lunar years commen,ce together, (thatis^ the first day of the
year is also the first-day of the Moon's age), and the epact is called 0. On the se-
cond year of this cyde the Lunar year has. commenced IT davs previous to the
Solar year, (that is, the Moon is 1 1 days old on the 1st. JT^i^ry)^ .$nd the epact is
called 1 1 . The third year the epact is twice 1 1 =: 22 ; the fourth year, 3 X 1 1 = 33,
but, (deducting 30 for one montjii .completed), the true epi^ct is ^caU,ed 3^ and so
on to the end of the «ycle. The^paqt of a montn expi^.e^ses ^the ^oq^'s age on the
first day of the month. The. epact of the month being IsQoynij the M9on*s age on
any particular day of the mon^th can be easily aacextained.

II, The Dominical Letter is a. letter answering to the ^st I)oj{iinus, pr Lord's day
[Sunday] of the year. The seven first letters. of the alphabet, bging applied to the
seven first days of the year, the letter answering to the first Sunday is the dpmini-
,cal letter of the year. If Sunday be the first 4ay of the y^r the dominical letter
for that year wiU be A ; if Monday be the fir^t^day, the (femii^cal letter will be G.
Qut in Leap year there are two dominical lettws ^ed,.pai9.ely;» pne letter found as
above, to be used only during January^md Eeljyuiarj;, jmd the ^ext j^revipus letter to
be U3ed during the remainder of the year. The first.day .Qf the year 1844 (a Leap
year) happens on a :Mond4iy and the doioinical letters ,are G.; F. '

XII. The Bpman Indiction is a cycle of 15 j^eps hy -^ich ihjs^ ^mp^i^rprs of Rome
ordered public acts to be .d^ted. The Poxiiitf j^jPQjpe) ^conti^^ic^ th(^,use pifit.

IV. The JuUan Period is a cycle of 7,980 ye^rs,'^ made Jby,^rftfpl^g the cycles
of the Sun, Moon and Bpinan Indiction together, 28xl9xlS =7^966/ 'fins
period is found by* adding 4^713 to the year of the Christian era.'

The three last articles aire not used in modem Astronomy ; but bdna; found in the
Nautical Almanac an explanation maybe desirable, to remove any expe^tion of their
utility to the nautical student. We now proceed Ifp explain Astr<^omical, Civil
and Mean Time, and Hour Angles.

An Horary Circle is a gircle ofany conea3rabkjSi8se,.bfti[i9g:;ei^h^r.Qf tJkQ ^P^lesof
the Equator for its centre. When we:make.the South £ote.^jD^MKi9f:ttbilimcIe

TIME. 53

we read the hours from left to right as on a watch dial, but when we have the
North Pole for the centre the hours are reversed and we read from right to left.
To prevent confusion of the terms Horary Circle and the circle of Bight Ascension
(both being marked by hours, minutes and seconds)^ the student must be careful to
identify them as entirely distinct ; Bight Ascension being measured by the division
of the Celestial Equator into 360° or 24 hours, commencing with the first point of
Aries, (which in this place we may consider as an immovable point in the heavens),
and being reckoned from" West to East, or the contraiy way to the apparent daily
course of the Snn, whilst the Horary circle is moveable and answers to the 24
hours of a Solar day, the point on it marking noon of any place being precisely on the
meridian of that place, and the numerals marking the hours rising from the East
and falling towards the West like the apparent course of the Sun. An Hour Angle
or Horary Angle, called also meridian distance, is an angle contained between a Pole
of the Equinoctial, (the North or South Pole), the meridian of the place of observa-
tion, and the celestial meridian of the objeet to which the angle is referred. The

accompanying diagram represents an Hour Angle from the North Pole, where the
hours are read from right to left, (the West being' to the left). The Stars A and B
have no hour angle, their celestial meridian being represented as coincident with the
meridian of the spectator, but the meridian of the Star c makes an angle with the
meridian of the spectator of 6^ W. (6 hours West) and has culminated (See culmi-
nation) 6 hours previous to the Stars A B. The line, or radius, from the Pole,
or centre, of the Horary Circle to the circumference, passing through a Star, ia the
celestial meridian of that Star.

For the explanation of Sidereal, Solar and Lunar Time, we use the accompanying
large diagram, representing an Astronomical clock of a novel character, the circular
dial of which, having the principal Fixed Stars of the Southern Hemisphere repre-
sented on it, must be supposed to revolve once from East to West in 23 hours 56
minutes 4 seconds of mean Solar, or clock time. This revolution will truly describe
Stdereal time, or that apparent daily revolution of all the Fixed Stars which is owing
to the daily revolution of the Earth on its axis. The dial has represented on it the
two great circles of the Ecliptic and Equinoctial, the months and (is supposed to
have) the days of the year. On the EcHptic is marked the twelve signs of the Zodi-
ac, and on the Equinoctial the degrees and hours of Bight Ascension. (S^ee right
a^scension). There are also two hands proceeding from the Pole, or centre of the

TIME. 65

Eeliptic, to describe the motions of the Sun and Moon on and near the Ecliptic, the
Sun being always on that drde, and the Moon never distant from it mudi more
than 26^. The Solar hand, S , which makes a daily revolution with the dial (or
sphere of the heavens) in the direction of the hours surrounding the dial, will de-
scribe a Solar day, or the apparent daily course of the Sun over the Earth, from
East to West ; accompanying the Fixed Stars to a certain extent, but having an inde-
pendent daily motion fh>m West to East, (in the direction January, February,
' March, &c.,) which will cause it to occupy about four minutes longer in revolving
firom noon to noon than the time occupied by the Stars in performing the same re-
volution. This small independent motion, accumulating day after day, will cause the
Sun to advance in the Ecliptic at the rate of one twelfth part of the circle of the
Ecliptic in a month, or entirely over the circle in a year. When the Sun has arrived
at precisely the same point on the dial froin whence at the commencement of the
year he set out, 'the Sidereal year of 365 days, 5^ 48™ 10* is completed. The diff-
erence between the Sidereal and the Solar, or Tropical year, (the latter a year of
four complete seasons), is only 20°* 20", the Tropical year being 366 days, 5^ 48"
50". When the Sun arrives at either an Equinoctial or Solstitial point he finish-
es a Tropical year, and when he arrives at the same Fixed Star from which the year
previous he set out, he finishes a Sidereal year. The AnomalisHe year is 365 days,
5h > 13m 49«, The Tropical varies from the Sidereal year by 20" 20», a quantity
equal to the recession of the Equinoctial point, (See precession) and the Anomal-
istic year has a still smaller variation of 4" 39% being the difference between the
completion of a Sidereal year, and the arrival of the Earth at its perihelion (nearest
point to the Sun) from which the previous year it set out. All these data are

in Mean Solar Time.

The Sun appears to make 365 revolutions over the Earth while the sphere makes
366 , but at the conclusion of the 365 Solar days, [say at noon,] the Sun has not
arrived at the exact point of the heavens whence he set out at noon the 365th. day
previous, but will occupy about 6 hours more to accomplish it. We therefore say
there are 365 j- days in the year, and nearly every fourth year (See bissextile) we
add one day to the calender, called an intercalary day, so that our calculation of a
Tropical year may be kept correct. If this regulation were not adopted we should
advance about 25 days every century in reckoning our seasons, and in 700
years Spring would not commence until September, nor Summer until Decem-
ber.

It will be observed that that part of the Solar hand of the clock which represents
the Sun will always be on the Ecliptic, and twice a year also on the Equator, the
yearly motion being oblique to the Equator. The Sun will be North of the Equator
from the 21st. March, or the Vernal Equinox, to the 21st. September, and South of
the Equator from September to March. (See seasons). It wiU also be observed
that the hours are marked from to 24^ an Astronomical day being measured from
noon to noon through 24 hours. Those points where the Ecliptic and Equinoctial
intersect each other are the Equinoctial points. The point of intersection in March
is the first point of Aries, from which the Eight Ascension is reckonedon the Equi-
noctial, and the Celestial Longitude on the Ecliptic. The Solar hand in the dii^ram
shows the month to be December and the time of the day noon ; that the Sun is in
Yf Capricorn in R. A. 18^ or 270°. A Solar day is not performed in the same in-
variable time, like a Sidereal day, [the apparent daily revolution of the Fixed Stars,
which is perfectly uniform] but the length of the Solar days throughout the year is

56 " TIME.

t w » i h ^ * I 9 i 1 * 1" T V ^ g ' " • » t. ' . a ;; '■ : * ' ."; '■ ) ■* !■■ p ' m « *■ j i n m

v^miB^Mtli owiiie^tDtiii^dliptkai fond of the aiibU of tlie Earth (which ouiMi
the Eai^th to vary in its distance .fiom the 81m 3 inil]ioii8.df Bules)^ the obliqwtjr of
the£diptie» {Sh fiE^AOKtf) and the variaticm \m the obhquity of the Eeliptic. [See
onutaiiltt:.) A calculation for the meqnaHty in the length of solar dajs isins^ted
in thb Nai|tk»l Ahnanacnader the title of the Equation ^ Time, The caleidation
ia pf ft '4jiQBiaiitf which added to^ or subtracted from. Apparent Time makes Apparent
h^ Mean Timiitt €i Mean into Apparent Time. Apparent Time is reckoned from
the> tramit of the Son .oTer the meridian of any plaee, and b the unecpial time of a
a4K)laf dffjT* -A Son dial describes Apparent or Solar Time« To fiuaUtate the men-
tion of Mean Tixne in certain cases Astronomers imagine a ** Mean Sun " ttaveisiiig
the heavens, and describing Meaiv or average^ Solar Time. What is.meant» for in-
stance, by the instant of the Mean Sun being on the first paint of Aries is, the is*
stant that the True Sun would be on that point if the Sun kept eipaJ time. Mean
l^ime, then, is an average time deduced from all the unequal lengths of the selar
days in the year, and is ike standard of the measorement of Time by dock work.
Examples in the Equation of Time:- . . .

I. ,0n the 1st. February .1844 the Sun will pass the meridian of Oreenwiph .13°^
49' ;76 (13 minutes 49 seconds and. 7^ hundredths of a second) after the ^^ Mean
Sun ", or irfter the mean noon as shown by a well regulated dock. The time of tcan-
sit of the meridian by the True Sun is the apparent noon, and the Equation of Time
(i3°* 49* *76 according to the precept in the Nautical Almanae) has to be added
to this noon to convert it into mean noon.

Ex. II. The transit of the Sun over the meridian of Greenwich, on the Ist. No-
vember 1844 will take place \6^ 17" -23 previous to the "Mean Sun '*, and 16°»
17' *23 will have to be subtracted from the Apparent Time to convert it into
M^nTime.

The second hand of the dock depicted in the diagram is to describe the revolu-
tion of the Moon, as seen from the Earth. Like the other hand, this is carried
round from East to West by the daily revolution of the dial, and it also has an in-
dependent movement urging it over the dial in a contrary way to that of the diaTs
revolution, or froin West to East, by which it passes round near the Ecliptic in 27
days, 7^ 43™ 5% which is called 2k periodical month, but as. during this time the Sun
has also l^en advancing in the same direction, (West to East,) the Moon occupies
29 days, 12'»44™ 3" in performing a revolution from one of her conjunctions with
the Sun (the time of NewMoon^ to the next conjunction^ This latter ^period b
called a synbdicdl month. The Lunar hand in the diagram shows the Moon to be
in or near II Gemini, and in Bight Ascension about 3^, or 55^.

It innst be wdl borne in mind by the student that the movements of this clock
^ ouly represent the apparent revolutions of the Sun and Stars. He must remember
' that the Stars and the Sun must be considered as having fixed places in the heavens,
that their daily transit over the Earth, from East to West, is owing to the revolution
of the Earth on its axis, and that the yearly p^issage of the Sun in a contrary direc-
tion, from West to East, over the Ecliptic is due to the revolution of the Earth in its
orbit round the Sun. The Moon, however, is actually passing over the Earth, but
in a direction contrary to her apparent motion from East to West, tn reality she is
proiceeding swiftly from West to East, her apparent daily transit from East td West,
like the apparent daily transit of the Sun and Fixed Stars, being the effect of the
Earth's rotation on its axis. A Lunar day is the tune elapsed from the instant the
Moon leaves the meridian of any place to her return to the same point tfgain, the

TIME. 57

I ^1 !!■■ ^i»<— 4»..*— iM^— ■— ^ — ■Ill I I . I -I II m Will — mMM—

next day. This iime is more variable than that measnred by the apparent passage
of the Son over the Eatth, but averages 28^ 48°" 46* of Mean Solar Time.

Greenwich Time> or Greenwieh Date, is tihedate, or day of the year, and the
exact Mean Time at Greenwich when placed in comparison with the exact Mean
Time at some other place.

The Astronomical year commences on the 1st. Jannary at ^oon^ twelve hours
-aUber the commencement of the Civil year, which is reckoned from midnight. During
a voyage ronnd the world, or only over half the world, a curious effect of Time has
to be allowed for in the calculations of a navigator. As soon as he passes longitude
180^, (precisely halfway round the world from Greenwich,^ bis apparent noon will
be simultaneous with the midnight of Greenwich, and his Astronomical day will
have to change its term, according to his having proceeded to this longitude from
the East or.from the West. If he sailed easterly, or the contrary way to the appa-
rent daily pro^ss of the Sun over the Earth, Lis time will be twelve hours in
iulMnee of the Greenwich time, and if he sailed westerly his time will be twelve
hours later than the Greenwich time, and he will have to change his date (the day
of the year according to his log book,) by adding to or subtracting from his previous
date twelve hours, so that tiie d^te of his noon may be of the same term as the date
at Greenwich. Suppose, for instance, a navigator has sailed easterly from Green-
wich, and on the 1st. January, (according to his log book,) he passes from 1 79^ £.
lon^tude to 179^ W. longitude. His noon will then be 12^ 4°* in advance of
Greenwich Mean Time, and both his astronomical and civil date wiU be different to
that at Greenwich. For whilst his log-book has reckoned his time as noon of
the 1st. January of a new year, the time at Greenwich wants 4 minutes to the previ«
pus midnight, tiie completion of the old year. Civil Time ; or 12^ 4™ to the comple-
tion of the old year. Astronomical Time, and the date at Greenwich will be 31st. of
December. On the contrary, if the navigator sail westerly, and pass from 179^ W.
to 179° East lon^tude, he will have lost upwards of 12 hours on Greenwidi Mean
Time, and the midnight of his 31st. December will actually be the noon of the Ist.^
January at Greenwich. Without care, therefore^ in altering his date after passing
the 180th. degree of longitude the calculations a navigator would take out of the
Nautical Almanac would be erroneous. Navigators making a voyage round the
world will either lose or gain an entire day from the time of their setting out, accor-
ding as they proceed easterly or westerly. And as an entire circuit of the globe,
or tiie complete circle of longitude, 360^ from any one meridian to the same again
is equal to 24 hours of Mean Solar Time, so every degree lying due East and West
is equal to 4 minutes. It is noon at Greenwich 11°* 55" before it is noon at Liver-
pool (Longitude from Greenwich 2° 58' 55" W.) ; 25°» 22'» before it is noon at Dub-
lin (Long. 6° 20' SO^W.) ; and 4^ 56"» H" previous to the noon of New York,
(Long. 74° 3' 31" W.). It is noon at Greenwich 19" 53» later than the noon ot
Amsterdam (Long. 4° 53' 15" E.) ; 2^ 1°* 10* later than the noon of St. Peters-
burg (Long. 30° 17' 33" E.) j and 7^ 32» 56» later than the noon of Canton (Long.
113°14'0"E.)

Mean Equinoctial Time is the Mean Solar Time elapsed since the Mean Solar
Time of the Yemal Equinox. [See seasons]. The instant wh»i the ''Mean Sun'*
arrives at the Yemal Equinox it wiUbe on some particular meridian, and the Equi-
noctial Time at places under that meridian will correspond with the Solar Time
throughout the ensuing Equinoetiid year, while the Equinoctial Time at all other
places will be quantities oorresponding to their longitudes frron the meridian at

as TBO. VIR. URA.

iR^ueb i|i0> Meaa \iisroti EqpNnoK iook ]^\9Mk B^^f^iim^ SinnV fit the ^emmfm
of the nei^t VtBnti Skpism ift ^ith^ meridifla tbmt 89? fttrftiar tor* i^: ws(fiwr4*
tfaefmiinuil vev^latioiia of ibe. .Simi iiviing tiie Eipuno^liBl jr^ar hemg ,3(*& 4^&
•2412217 (3^d: 4qrt u4 249217 frftctiotw ^ «. iii9}i«^» Tfap fifi^iieoa^) 'Sim
is then reckoned from that meridian. Examples :- The ^'UmASmf^ BktiM T^tfmw
Umh 22nA. 1841 is <mt]i« tomOiw 4^ 34! W 'StStW^ of <^^mm^ mik.ibfi
fniotM»n of the ]SqK»oqtkl Tiaae^^ Oraeufri^ f^ ei^^ihgr-t^nfimmfyt t|K^^«|it-
siiii^ Bqmooetid^yfW, '809580^ At th^liqim^JIbetiih '']Mt4|i

Sob" i^i on tba meniim 10^ ^ 4![ 'h& WtH-(£ (^nemnAm^lMimti^^
£qi|iaootial Tiaie fyp Qn^tm'vk^htwBin^ 'J»^7^m^ of'U%%\7 Im tlM^ ttoi of tin?
Pluvious 3mir« At, t^e JBqimosrof Maidi 21Mr. 1843 ibf &M]«fti.ift 'iW^
('J242217 1^ dum 4ie:pr^('kHiSrfTaclion9>tlie itmw^ ^m^i D^fMsbii^ 7^

48f 7^ *9S iSul <rf^<Sf>e^wiKih| Aod'en Mnreb 21^. 1844 mtm6Mi9t ^08282^^
(-24221 7i^^aiii).tlia m«fidte of tiie Vmial Eqsuii^s^bmg !>" SS^ 20^' '4ft)fi^
Aftw th4i }re«r tbe Vtfimit^'^glm^i^ ^]Mri4i«A/ti^4iiul^ 9W<Nf;£lf»^r

iri^h^' Theutiiify of£qw»»ti4ll^)N^ As««9i^

ngi^Sk but it is.]^ xftiMbiiiMdi

T&iMsnv Fusioigorer; or tauten Wken^oetestisl^bedf app^aM>to'crMB4lie
diseof.tbB8im;or>«nir'Of tbe^tittttiy 80t*Aft< sr^VfiM))' or^ to^ ereib

.the metidittB,! or the rptitxie -v^KJioalj it ia> ^idMi & tnmsK.^ (fi^^ : AfistM^ara^ G«i.iif «•
HAjTUMf mnd^ MifikittfAM;): ^b* IMWMff 4^fM^jfir9tfpoiHi i^Jriea is a tienn^ a^^pWS
to tfaaiBSlant when tlMttpncdtft^pdlttt of tbeh6arre(B8'Whete.tii^ i!<il(>iie^8nd'£4»i*

^Aiiaotmt die eateulstfen^f tbt^ tM&ste is f^ tb^ m^tidiaa 4>f :>G)MsiMriebL liife
^&teKff>(/kt (Sbft#/fifi^iis<»t«^)«i9ageoTeii^-lt9.priuaiyPlSiMt^ i|S^Meft^irom4il»'Sftiili^
as Jjht drsAsfitst of^Japker^ft ISbiM^M oi^-tbe diso^if Oi^ilw^

• ¥tN0Si • S^'e^ B&Uktt fiird^EsM.

Vl AGO, ' (l^e >vvt^j . tW s^ ^f tiie IkfBttc which #te Sim ^t^s abM^ the
2drd. of An^fia^dteai^aboatthe 22ndi of Septcfmber, IHhefoBbwiMgfiAerbe
tjomsiitted to membrf th67i]ifff-{>mre oonVe^efnf for cafiing to- mind thddi^s 'of
'^ months on whidi the Sun estters i^e signs of the Zodite. 1^ nkontfas ^mKy
^aeiily be-^und by remembering tSiilt (Aries answers io Mftrch^ antil; the otSieriSgHi
^ liieJo^ev^moAthsinsaceesstmi^ftiDm Ariesaad Mardi* (iSte page5,'inwMdH;he
isi^8> are pkeed' iieco»Inig to ' thar 4M^der ft^

Niiieteen days forethe number for Pisces,
Tw^tpifor^nitim^ and AquariuAy -

Tnbilil; Oefiinii Oawcr and Ams ...

And.Cfpiicorn^Sf em^ b«va tMrnfifitwo;
And to the others twenty three are due.

l^KoaRAw Y^ TbAt •d^p%r(;«ie^b of tb& • sci^aoe ,of > Aatomosq^. wH^ teiicbe^
tb^,frrtngwepMf^H9?4i|^'W4^ tbo oidUienieiiaia ^M^d&.of irodciiig A^t/r^
i^mca),.p9oble«9&w

WmB&. The siOija^D&lte •kdl'iir^MMlOHtt^^
dqp««d^ in a gsdaAiiMBffm, ofi^Ae^iiAiefSMslift^Md MiddnMliatt^tli^^ifajK

THdB^ WIBTDS. 'SB

'tm^^mi^^^mmmmmm^mtm^mmmm—mam^i^^^^tt^i^^^it^m

Arid bf Ae netion «iid'pBHial absenee of the •riarhq^. »^VV3i«a'» pwrtkia of tbe^at-

atempbere beDottes lirefied by tbe-tolar heat to as* to atvixd, i or^sJdfteed'faj oold

'90 tat i»'dnem^i a eoimier conent take* plaoe in another part. HHifln thr ivnnd

Mwa H is m^M^ tofiU up a partial vacancy 0tyiaiamuttiaiai.i^sAtakt port of the •at-

ii»apkttQei<«Qd(a»i^e4liiiiis at 6i]e period of diB yaar.NoiUL o£ihe:Eqiiat(lr,»ancl at

• aaottifir Aoalli ttf it, hetia thd immediate caase«f jthe:£eriodioal Mm^^mhixAi'mmy

b^'tbasiiefin^t'diiibg tiie montls 6f May^ ^tee^ Jviy^ AaigRat:Biid-8e]M:embcr,

) [titer Ster being Naettk'bfilK&Silwitoi;] the wind- hkNnra' firom^tllct ftmtb, (vfaem.tbe

!jdtiaH*OM | > MtttiM^y coiaden8ed by tbetibaenoe^rf ; add

dnr^B^rtfieieiMindtor of tte jreai; the Sim being! «onth/o£iherfiqttator, the Mirien-

«ed ai^o£4be^NcM;lL^U4iiiia towaiftla/the melied atiDMphere'iaf ^tbe Scptiu s&&me

- dajFk ^M(MPeranAal»»'lilie«baii0e> happeningr«bent :tbe[ 21tt.*«f IiiiaBcheand.2dlh^«f

•6e|ileihberplte»v6|iftn9iiearrtbefE4a^ to.eiifaas)TldaUedNiyiMildt^

tapttitienBt SwOvoMk

Vnteift^planiitienafiiMdatfily fte^^fintf^itaal'Wftvi'of ABf«8tronanMl(caanecof
'4hflF!t«(iestBal!irifcids, anilUt i«ew )ida taWiuBB^dialriytiiMiidfedoby tke
v«Mm'tfaift4feifiaith'isconiaQ0niidly re^ .irinis

aroaiai'iMtaaHy«{ho.TQ«iU of^tthe raie&ctiiiia of -the ^atmdtpheie byr^tiieiacui^.hvt a
^[iiBath^''tMbBAj^yiio& a/8eitth<^astad7

'«Mbat tbetSanth ; )the ^ianiifilBg» &mm, Abo-fiim iMowiog his: coniaef from ^^aat to
flMMMifed^themi]3tt:iii60&nanrin^ that dinedioni to^lfilLiap ifbe pKttial tacanoy voau-
wL'l^ tiie difllnd saMfinttoii*

•rShe «^<Benoe'ofiiaf%8toQ»ian8tniil»9B».thAttheifr^^
(■JBiir0niine'a|p|ttoach the Sqnatfnr,^ and/moie 'lalaaUeras tiveirc^dedeliQm^ tonaards
die Foka. In certain parts of the gloteaaiConstentaoe<th0rtiBdsi;^hAtvth€^are
i/m»edLibf^^per»Mnent or trade .winds from Uowiog. always ^)early,iii'.the,^an^ direc-
rtion In tl|.e Atlantic and Padfic oqean?, ^^^^er a region extending to 28° on each
side the ^q uator^ the wind is' almost always easterly. Farther to the northward the
wi^dapnncipally vaiy between the North and East, and farther to the southward
the winds.gradually become more soatherly as we ascend towards the South Pole.

In the ten^perateaoiies, where the Sun, has, not such an imn^ediate influence as at

ibejl^natonal ^r^ons, tho'iwds are so Tariable and capricious as to render expla-

oiMaoyi totally, iijcipoasible^. and the connee^on between the solar influence and the at-

yaoapheie fBijpp^rs'iO:he lost* 'Connected with the subject of the* winds may be in-

>t90^iiQad the theory ovt which' that TslaaUef instrument the barometer is constructed.

vAi>biionlflir^a' laaehinetfcr testing Ae density and elasticity [or rarefaction] of
lAMi4tDi08phMi^ 'In other' words* it is/ainadune by which we measure the prepare
Mofdveair^onHie 'Earth. ^Uteiair.wn^hnHithe, like water or any other fluid, is of a
<>4ertilin.spedfic;gia?it7»or'Weig^9»and it ^ia found that a column of it, an inch
square and reacktog'ftodi tIterEarthivp tothe highest donceivable point of the sky,
^,atmo9pber^ haB^ictuaQ^a specifiq gnmty of fiteeuipoMnd^ saad a pressure to
,ihat extent v^ themefor^, .contmually exerted on-evaiy aqnare inch of the .Esrthfs
. 4B«rfiiQe. This ^ped^ gravity is equal to that of a coUmui of water an inch square
and 33 feet high» or of a oolunm of mercury, of the save diameter, 29i inches .in
heigjit* A tube, therefore, to contpdn.a quantity of water which would be svpport^
by the atmospi^ere pressing on its lower extremity 'would have, to lie.neair4Gft. h^,
4Hid a tabe for the common barometer oontainhig mercury is- 34 inches inJMjgfat,

In cafan weather the^atmosphereia denser, >or heavier, tlian at any othaF.tiia^.b&-
canse Ae particles eomposmg the air, being thenin annDdistArbed states lie ^j^r

ZEN.

ZOD.

and are in a greater quantity in any given space, than in wet and etormy weadier,
when the air being a^tated is spread over a larger space than that it otherwise
occupies, and the pressure on the Barth's surface is consequently not so great. In
cahn weather, then, the atmosphere is capable of counterpoising or supporting the
greatest quantity of mercury, or other fluid, placed in a machine so as to twAvb the
atmospheric pressure, and when there is rain or wind it will support the least quaa*
tity ; it is upon these £EUSts the utility of the barometer is founded, as a weather
guageii As the atmospheric pressure increases the mercury is forced upwards in the
tube, and as the pressure decreases the mercury falls. But a plainer explanation
may be afforded by the common ghssfountatM, used for bird-cages, in which we
see a quantity of water supported in a vessel by the pressure of the air on the app-
erture at its base precisely in the same manner as the support of the mercury is eff-
ected in the barometric tub^. The range of the barometer, in England, is from 28
to 31 inches ; in other words, the atmosphere is sometimes so dense or heavy that
it will balance or support a colmnn of mercury 31 inches high, and at other times
it is so expanded or hght as to be only capable of supportii^ 28 inches : the mean
of this variation, namely 29i inches, is marked "^^dumgeable" on the barometric
scale. The range is least at the Equator, where the marks on the scale ''stormy"
nnd ''very dif' are of no utility, because the mercury never fiiUs so low as to reach
the first of these terms and never rises so high as to point to the second. For those
climates where the utility of the mercurial barometer is affected, and indeed in all
situations (such as on ship-board) in which a great nicety in testing the state of
the atmosphere is advantageous, an oleagmous (oily) instrument called the sympie*
someter is preferred, the most modem and improved form of the instrument being
that invented by Mr. Cummins of London,

Zenith. That point of the celestial sphere immediately ovet-head at any place.

The Nadir is that point of the sphere immedi-
ately opposite the Zenith. Zenith distance is
the apparent distance of a celestial body from
the zenith of an observer, and is measured on
an arc of his meridian from the zenith to the
horizon. When the zenith is North of the bo-
dy the zenith distance is called North, and
when the zenith is South the zenith distance is
called South. The * |, in the diagram, is 45^
South of the zenith, yet its zenith distance is
called 45^North, the zenith being to the North
of the star. The * 2 is 22^" North of the xe-
nith, yet its zenith distance is 22i^°.Soath, the
zenith being South of the ^tar.

Zodiac. A broad belt, or space, in the heavens, 8° on each side of the Edipdc^
and, consequently, 16° in breadth. There are 12 constellations of Fixed Stars in
this belt which give their names to the 12 divisions or Signs of the Zodiac, the first
6 of which, r Aries, Taurus, n Gemini, S5 Cancer, iXLeo and ig? Virgo are termed
boreal or northern, because the Sun when in those signs is at the North of the
Equinoctial, and the last 6, £i Libra, in.Scorpio, ^Sagittarius, YfCapricomus, rr
Aquarius and K Pisces, the austral or southern signs, as they are South ef the
Equinoctial. All the Planets (with the exception of the Asteroides, or telescopic
Planets^^ make their excursions within the Zodiac. "

aWB^>

NXWTOir's HBAD.

NAVIGATION WAREHOUSE,

39 South Castle Street^ LiverpooL

E8TABLI8HBD 1770.

MELLIN6 and PAYNE,

LATE DAWSON aXld MBLLINO,
SVCCKSSORS to BTWATBS & DAWSQM, and SGKKTOIf amTB,

OPTICIANS, CHRONOMETER MAKERS, NAUTICAL
and MATHEMATICAL INSTRUMENT BiAKERS,

A6BMT8 FOB THB 8AI.B OF THB ADMIBALTY CHABT8,

have constantly in store^ for wholesale and retail demand^
every article connected with the ahove branches of trade.
Chronometers Repaired and Rated by a Transit Instnunent.
Second-hand Chronometers, with good rates.

PAYNE'S PATENT MAGNETIC BINNACI^,

FOR IRON CRAFT AND STEAM VE8SEI<8. •

M ^ P are now prepared to execute orders for the above,
and warrant iheir accuracy in compensating the magnetic in-
fluence of Iron craft on the compass needle.

The appearance of this Binnacle is octagonal, sormounted
by a handsome brass skylight containing a himp. The size in
diameter is 2 ft., and in height 2^ ft. Price, complete in ev-
ery respect. Twenty Guineas.

Agent for the Binnacle, in London, Mr. J, C. Dennis, Opti-
cian, 118 Bishopsgate St.

6. p. PAYNE, PRINTER, 39 SOUTH CASTLE ST. LIYERPOOL.

CUMHINS'S PATENT MINERAL STMnESOHETER.

The Patent Mineral Sympiesometer has now been in use
in all climates, for a considerable tirne^ and in no single
instance has it been known to fail. It gives to the Mariner
perfect indications of the slightest change taking place in
wind or weather : not like the old sympiesometer, continually
causing^ doubts in the mind of the observer by false rising
and falling without causey it will* in all temperatures, give the
true barometric state of the atmosphere with the great adviin-
tage of showing the variations much sooner, while the instru.
ment is not above one third the siie, does not require to swing
in gimbals, is not the least affected by the motion of the ship,
requires neither cork nor stopper to prevent the fluid from
coming out when carried^ will keep in repair a length of tune
and the fluid will not evaporate or congeal.

Made and Sold by Charles Cummins, Chronometer Maker, .
148 Leadenhall St. London.

Agents for Liverpool, Mellmg ^ Payne, South Castle ^t.

•^

MASSEY'S PATENT SOUNDING MACHINE^
AND PATENT PERPETUAL LOG.

MANUFACTURED BY EDWARD MASSEY, CHRONOMETER &c MAKER,

2B, King Street^ ClerkenweUf London.

¥tma CAPTAIN BEAUFORT, Hydrosimhtf to the AmiralU.— 1831.

^ Ton bare done me the nonour oi aaking my opinion orvDor Patent Loff j I Iibtotio hadtation in xvpljing
tihat it is a ntoat Talnable iniitnmiflnt, and if lued along with Irofeaeor Barloirs correction for the local attraction
of tkkB CompaaBy a ihip's reckoning would approach yerf nearly to the troth ; a large proporlion of the rappoeed
ennenta in the ocean wonld diaappear, and thoee that do exist wonld be deteinuned with predaion.'*

CAPTAIN BAtFIBLD,R.N.,F.R.AA 1887. '

**D«ep as the water is to the northward of this dangerous reef, (Green Idaad ReeO^here ii no other guide, ina
thickfoff, wban the light cannot be seen, but the ■oondingi; yet it wUl nerer do-to loae command of the TeaMl
by ronndiBff to, in the ramd ebb tide, (which lefai np<m the rea at the rate of five knote) for the pmpoae of getting
bottom in tlio Usual way by the common Deep Sea Lead. Here, then, it is that MA86ET'S Patent SODNDINO
Machink becomes ofinraluable service to the seaman, enaUinghmi to obtain correct soundings despite of the
rapid tide« and without interfering wUh the course and rate of the Teaael,— ^unwy qfthe Biwr St. Lawmen,'

Printed fbr the H^drographer'a Offict.

Copy of a letter from CAPT. BASIL HALL. R.N., Extracted from the Tunes, March 23, 1S41.

To THE Editor of the Times.— &, I feel very desirous of calling the attention of shipowners, shipmastersL
nnderwritera. and all other persons interested in the seeurxty of navigation to an instrument of long-tried and
well-eiitablisbed utility, but which, in the merchant senrice at least, ii Tery rarely used, and searoely at all
known, exeept to the most educated and intelligent comnumdeis.

I aUode to Maiaey'a Patent Sounditt: Machine, which I do not hesitate to say ii one of the most usefial instra-
menta ever cmtployed on board ship. jNothing, indeed, seems more unaccountable than the feet that any ship
shonld be allowea to proceed to sea widiout bdng fnnuahed with so cheap, so ample, and so infallible a means
of aaocntaanini^ one of^the most important elements in the practice of nsyigatlon, an element, too, which by any
other method it is generally difficult, and often quite impossible, to obtain with the required degree of accuracy.

Ev^ery one can understand that the depth of water on such dangerous coasts as Uiose of Great Britain and Ire*
land is of much value in the navigation of a ship, especially in dark winter nights, or foggy weather. But it
may not bave occurred to persons who have not ccmsidered the sntject praeticalfy, wat the ** aoundinjgs," as they
are technically called, owe the whole of their value to two material dreumstaaces, only one of which is taken into
aceonnt. Tae first is, a oorfeet knowledge, from actual survey, of the depUi of water on all the different parts
of the eoasts; and seomdly, to the fiMuity and accuracy with which the navigator can measure the deptti of
water firom on board his ship.

It is qnita obvions, that unless we possess charti on whidi the soundings are correctly laid down, it is of no use
to detennine the depth of water when making a voyage ; but it surely is as obvious, or ought to be so, that the
best charts in the world are uselesi, unless the navigator has the means of measuring at any moment dT the day
ixt night, and in all weathen, the true depth of water in which his ship is sailing, ft may be said that ineocrect
soundinn with correct charts are worse than ussIms, inasmuch as tney must tend to mislead, and thus nmch
of the rtaentific labour and expense which are bestowed on our great national surveys are wasted.

It is perfectly well known to every practical Maman, that when a ship is going through the water even at a
moderate rate, particularly if the nignt be dark and rainy, and the water not very shallow, it is the most difficult
thing possible to get a thoroughly correct ** cast of the lead;" and if tfie depta of water be considerable, and
the wind blowing hard, with a high sweU, it is always a mattOT of great uncertainty, even when the ship is hove
to, and her way as much deadened as is p<Msible. But there arise continually cases in which, thoush it be highly
important to ascertain the true depth of water, it is so inconvenient or dsngerous, or both, to heave the sh^> to, that
eitaer no soundings are taken at ul, or a most incorrect, and therefore a mischievous, determinatioa is come to.

Maasey*8 sounding Machine (as I can testify from having used it in every part of the world, in all weathers, and
at all depths) nvss an exact measure of the mpth of water, evenOuragh the ship's wa^ be considerable, the night
as dark as pitch, and the swdl very high. With moderate care the results are not only infellible, but the treble
of using the instrument is not one whit grMter, and vcvy often much less, than that of heaving the lead in the
ordinaoy way. In bad weather, when, be it remembered, we are generally most anxious about our soundings, it
takes a longer timeMUid employs a greater number of hands, to heave the ship to, in order to get a good cast of
the deep-sea-lead. This, in fruit, is so serious a consideradon in a merchant vessel^ that soundings, which ought
to be actually measured, are not unfrequently taken for granted, to the imminent nsk of the ship. But with Mas-
aey's machine two or tluee hands, with a proper length of Ium, not only accomplish the work, but insure its being
done much more expeditiously and much more oorrectly than if all hands were turned out of their hammocks
and drenched to the skin to obtain a result which could not be depended on.

surveys,

department w ,

wilh any officer if, when on his trial for the loss of his ship, suppomng the s&pwredc to have been caused' by an
error faibis soundings, it shonld be established that he had neglected to avail himself of fliis admirable method of
ascertaining his depth of water.

The Thames steamer, on her voyage from Dublin to London <m the <th of Jaauaiy last, having lost her reckoning
in a gale of wind, was carried on the rocks of Scilly and perished, frmn having misotken St. Asnes Lighthouse for
that of the Loncnhip, near the Land's-end, a mistake into which they could hardly have frillen had they have been
provided with Mas8ey|s Machine, for without any difficulty they might have ascertained that no such soondings ex-
isted near the LongsnipB lights as those which they must nave nad when near the Cribawethan rock, on which they
struck.

The same observation might be made with no less troth Tespectin|; many of the shipwrecks on which pave our
beadles every winter, and therefore, I anxiously appeal to tiie shippmg interests of our country (as I have repeatedly
done before ), to disregard the petty cost of an instrument as usefiil m its way as either the sextant or the chronometer,
and frff more easily handled. If 1 had not already occupied so much of your valuaUe space, I diould be tempted to
recommend also the use of the patent log devised by the same ingenious and public-spinted pers<m. It is an instra-
ment rimilar in principle, and of tiie greatest use m navigation, but perhaps, not of sndi primary importance as
the sounding machine, which itis a positive crime in any navigator to oe witnont

I remain, your most obedient servant, BASIL HALL, Captain, R.N.

« North 8SA^— The quality of the bottom, was noted at every fifth cast of the lead ; 12,000 of thespedmens
have already been preserved, and the greater part are laid out in dietr correct positions on a large chart, delineated
on the floor of a store-house in Deptfoid dock-yard. The bottom consists of sand, fine sand ana blade specks, and
occasftonally broken shells, yet too irregularly disposed to enable the navigator to profit by them. But the banks
tluuBisd ves, as strikinsly illustrated in the several sections here given, from excellent points of depsrtnre. so that by
the aid ofthischart,tne mariner in the thickest fog, may boldly cross from England to the coast of Holland, if he
willbutattendb>hi8lead,ande8pedally ifhehave on board uiat invaluable instrument Massey's Lead, without
whidino ship shoold navigate the North Sea." — iVaa/tcoJ Magtuin^t April, 1842.

AgentifoT Liverpool^ MeUing and Payne, No. 39» 8auth Castle Street.

» V

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