Time : a struggle for precision / William Henry Watkins

TIME i A STHJGGLE FOB PRECISION
By
William Henry Watkine

November 16 f 1939.

Summary

The subject is one of general interest from an historical standpoint,
yet "becomes of engineering importance also as methods of controlling
modern timepieces become more and more accurate.

This thesis de .Is first with the historical background of the
subject, bringing out the Tory early date at viiich the fundamental
concepts were brought forward. Secondly, the matter Df maintaining a
continuous standard of reference, and the method of checking this
against the ultimate standard of celestial moTement is sketched in
some detail.

Hespect fully submitted in preparation for entrance into Maryland
Beta, Tan Beta Pi.

W. H. Watkine

Bibliography

Brearley, Harry C. "Time Selling Thru The Ages"

Proceedings Institute Radio Engineers -&y« 1928

Proceedings Institute Radio Engineers February, 1929

Proceedings Institute 3adio Sngineers July, 192,9

Proceedings Institute Radio iingineers January, 1931

Consultants

L$Du tenant Paige, Time Service Division, Naval Observatory

Dr. B. B. Hall, Eadio Division, Bureau of Standards

Mr. Gomer L. Davles, Ohief Engineer, ".Vashington Institute of Technology

Mr. G. H, '.Vinterraute, Senior Engineer, .vashington Institute of Technology

-5-

It was a moonless night in Mo Man's Land. A man in uniform stood
silently waiting in a frontline trench. In the darkness, his eyes
were drawn, fascinated, to the luminous figures on the watch-dial at his
wrist. A splinter of pale light, which he knew to he the hour-hand,
rested upon the figure eleven. A somewhat longer splinter crept steadily
away from the figure twelve.

'Its gone past eleven," he muttered to himself. "There's less than
twenty minutes now. "

To the right and to the left of him, he now and then could see his
waiting oomrades in the blackness of the trench, their outlines vaguely
appearing and di sappearing with the intermittent flares of distiint star-
shells. He knew that they, too, were intent upon similar tiny figures
in small luminous circles and upon the steady, relentless progress of
gleaming minute-hands which moved in absolute unison with the one upon
his own wrist.' He knew, also, that far in the rear, clustered about
their guns, were other comrades tensely counting off the passing minutes.

At twenty minutes past eleven, the artillery bombardment would
begin and would continue until exactly midnight. Then would come a barrage,
protecting curtain of exploding shells behind which the uniformed figure
and his companions would advance upon the enemy's trenches, perhaps also
upon eternity.

It seemed strangely silent after the crashing chaos of the daylight
hours. There were moments when the rumble of distant guns almost died
away, and he could hear the faint ticking of his timepiece or a whispered
word out of the darkness near at hand. He knew, though, that the silence
was misleading, but the lull before a storm.

_6-

Five minutes ticked away.

In another fifteen menutes, the fury of bombardment would begin,
doubtless to draw equally furious fire from the enemy guns.

At twelve-ten plus forty-five seconds, he and his comrades were
"going over", Into the inferno of No ilan 1 s Land. That was the instant set
for advance - when the barrage would lift and move forward, a protecting
wall of fire to keep enemy pinned to the ground.

The slender hand on the glowing dial stole steadily onward. It was
ten minutes after now. Ten minutes after eleven, an hour and forty-five
seconds of eternity. His thoughts flew back to his home In the gren.t
city behind the lines.

Ten minutes after eleven. How plainly he could picture the familiar
scenes of rushing, gust ling life back there. How the millions of his
native city and of other cities and towns, and even of the country districts,
all moved upon schedule. Clocks and watches told them when to get up,
when to eat their breakfasts, when to catch their trains, reach their
work, eat their lunches, and return to their homes, newspapers came out
at certain hours; mails were delivered at definite moments* stores and
mills and factories and schools all began their work at specified times.

„hat a tremendous activity there was back there, and how smoothly it
all ran — smooth as clockwork. Indeed, one might almost say it ran by
clockwork. The millions of watches in millions of pockets, the millions
of clocks on millions of walls, all running steadily together - these
were what kept che complicated machinery of modern life from becoming hope-
lessly entangled and confused.

-7-

Yes; but what did people do before they had such timepieces? For
back in the very beginning, before they had invented or manufactured anything,
far back in the days of the caveman, even those people must have had some method
of telling time,

A bright star drew above the shadowy outline of a hill. At first the man
in uniform thought that it might be a distant star-shell* but no, it was
too steady and too still. Ah yes, the stars ware there, even in the very
beginning, and the moon and the sun, they were as regular then as now;
perhaps these were the timepieces of his earliest ancestors.

A slight rustle of anticipation stirred through the waiting line
and his thoughts flashed back to the present. His eyes fixed themselves
again on the ghostly splinters of light at his wrist. Ilhe long hand had

almost reached the figure four the moment when the bombardment of

Paris would begin again.

He and his comrades braced themselves, and the night was shattered
by the crash of artillery.

-8-

'Jhe story of the watch upon your wrist, the clock upon your wall, and
Big Ben of London all began countless centuries ago, and is as long as
the history of the human race. Yihen our earliest ancestors, living in
caves, noted the regular succession of day and night, and saw how the
shadows changed regularly in length and direction as day grew on toward
night, then was the first faint and feeble germ of the beginning of
time -reckoning and time measurement.

Ho timepieces were available, but the great timepiece of nature,
the sun, and a shadow falling upon a certain stone were all that the
primitive cave-dweller needed in making and keeping an appoint;nent.

The next step in planning ahead required that appointments should

be made by a timepiece which could indicate more than a single day,

since the daily position of lights and shadows was not sufficient.

to
Next a the sun, the moon is the most conspicuous of the heavenly bodies,

particurlarly arresting because of its peculiar property of changing

shape. It was noticed that these phases of the moon occured in regular

sequence, over and over again In the same way, so as a natural outgrowth

of this the moon became added to the sun to serve as a timepiece.

Centuries later comes the date 4000 B.C., the beginning of recorded
history, uesopotaraia was the cradle of one of the most ancient civiliza-
tions, and the home of a people who were learned in the movements of the
stars* Without telescopes or other instruments, it is marvelous to see
how many astronomical laws they deduced by observation alone.

First, they observed that the sun slowly changed the points at which
it rose and set, luring certain months, the place of sunrise traveled
northward, and at the same time the sun rose higher in &e sky, and at

noon was more nearly overhead.

-9-

At this time the days were longer "because the sun was above the horizon
more of the time, and it was summer. During certain other months, the sun
traveled south again, and all these conditions were reversed; the days
grew shorter and it was winter. The Babylonian priests of Mesopotamia
were the first to study these phenomena and make something of them. They
were also the first to record their deductions, j^rom the time which
was consumed in the motion from furthest north to furthest south and return,
they worked out their year.

In order to calculate time, they devised the zodiac, a sort of
bait encircling the heavens and showing the course of the sun and the
location of twelve constellations through which it would be seen to
pass if its light did not blot out theirs. They divided the region of
these twelve constellations intu the same number of e-ual parts; conse-
quently, the sun passing from any given point around the heavens to
the same point, occupied in so doing an amount of time that was arbitrarily
divided into twelfths.

They also di vised another twelve part division of the year. They
noticed that the moon went through her phases in about thirty days.
So one moon, or one month, corresponded with the passage of the sun
through one sign of the zodiac, our word "month" rai$it well be written
"raoonth", since that is really its derivation. This gave them a year
of twelve months, each month having thirty days, or three hundred and
sixty days in all.

Then from the seven heavenly bodies which they had identified with
seven great gods they got the idea of a week of seven days, one day
for special worship of each god and named for him.

In similar manner, they dividea the day and the night each into twelve

-10-

hours ; and the hour into sixty minutes and these again into sixty seconds.
The choice of "sixty" was not an accident, for no lower number can be
divided by so many other numbers and still give an integer.

The early scientists who developed all this .ilso devised a complex
method by which they pretended to foretell future events and the destinies
of men. This division of chicanery is now known as astrology, but does
not detract from the very real contributions of the ancient wise men
who practiced it.

Their year of three hundred and sixty days was five days too short,
as they eventually discovered. In six years, however, the difference
would amount to thirty days, which was exactly the length of one of
their months. So, correction was easily made by doubling the month "Adar"
once in six years. This "leap-year" principle, which we still use with
refinements, originated then with the ancients.

The Babylonian calendar remained practically the same up to the
time of Julius Caesar, onl;, a few years before the Christian ,-:poch.
The names of the months had naturally been changed into toe Latin; and
instead of doubling a whole month, the Romans had decider to add the
extra five days to several months, one day to each. That is the reason
for some of our months having thirty-one days.

T ,Yhen Caesar was Dictator of Home it had become known that the year
of exactly 365 days was still a little too short. It should have bean
365 l/4. So Caesar in reforming the calendar provided that the first,
third, fifth, seventh, ninth, and eleventh months should be given thirty -
one days each, and that the others should have thirty days, except in
the case of February which should have its thirtieth d.y only once in

four years. A little later, his successor, the dmperor Augustus after

-11-

whom the month of August is named, decided that his month must be as long
as July, which was Julius Caesar's month, therefore, he stole a day from
February and added one to August; then he changed the following months by
■making September and November thirty day months and giving thirty-one
days to October and December.

The Julian calendar, with the changes by Augustus, remained in use until
A.D. 1582, when it was learned that the average year of 365 l/4 days was
still not exactly right according to the motion of the earth around the
sun. The exact time is 365 days, 5 hours, 48 minutes, and 46 seconds. This
is 11 minutes and 14 seconds less than 365 l/4 days. Therefore, whan
we add a day to the year every four years, as Caesar commanded, we are
really adding too much. This excess was corrected by Pope Gregory XEI in
1582 when he changed the calendar so that the last year of a century
should be a leap year only when its number could be divided evenly by
400. 'Thus, 1700, 1800, and 1S00 were not leap years, but the year 2000
will be, 'This new calendar, which is the one now generally in use in
most of the world, is known as tbe Gregorian calendar.

So, the main plan and principle of the modern calendar has remained
unchanged for 6000 ye^rs.

Astronomers today can figure out in advance what is to happen in
the heavens with an exactness which would have seemed magical not many
centuries ago, and is astonishing even yet. The accuracy is made possible
by improved scientific instruments, more flexible and improved m. thematic s,
and greater accuracy in the measurement of time.

Early humanity was dependent upon the clocks of nature, whoee course
controlled their existence as it does ours. ,,e still depend upon these
great primeval timepieces, although many times unconsciously, for it is

-13-

hard to remember that our master clocks must still tie set by the motion of
the heavenly bodies. This will be discussed l^ter.

Although the calendar which indicates the large divisions of time
was developed at an early uate, there has been only a slow evolution of
methods for making hour and minute measurements.

The first and most primitive method has already been mentioned; namely
the sun dial. At first it was only a stick placed uprl$it in the ground,
and the shadow was a measure of the hour, The first new development to
come was that of making the shadow move over a hollow space such as a
walled courtyard, going down one side, across, and up the other side as
the sun went up, across and down the sky. If the cuurtyard be covered
over with a dome and a narrow slit be cut therein, a. shaft of light vail
mark the time instead of a shadow. The first written reference to a
sun-dial of any type is to this latter design and is found in Isaiah,
Chapter thirty-eight, where reference is made to the Dial of Ahaz in
telling of a miracle.

The next improvement came when it was realized that the sun's
shadow could be accurately cast upon a flat surface at all times of
the year only if the pointer was pointing to the north. Then it was
found that the angles of division which represent the hours must be
recomputed for each latitude, as there is appreciable error here also.
This was first noted in Rome, where a captured dial from another latitude
was discovered appreciably incorrect after being installed for one hundred
years. The sun-dial marks apparent time, which differs from mean time
(which we use) by plus or minus sixteen minutes at the greatest variation
during the year, and is equal to mean time at four tiroas during the year.

-13-

A considerable time after development of the calendar, the Egyptians
devised a water clock or Clepsydra. This device comes much closer
to being a machine than a stick in the sand, and therefore represents
a fruther step in the evolution of time -pieces.

r fhe original idea was very simple. At first, it w-s merely a
vessel of water having a small hole in the bottom through which the
liquid dripped out drop by drop. As the level within the jar wr.s lowered,
it showed the time upon a scale. ]?hus, if the hole were so small
and the vessel were so/large that it would require twenty-four hours
for the water to drip away at an absolutely steady rate, the side of
the vessel could be marked with twenty-four divisions to indicate the
hours. One advantage of this device was that the water would naturally
drip as rapidly at night or in shadow as in sunlight. Ifterefore, the
clepsydra could be used indoors, whereas the sun-dial could not.

However, it had to be regularly refilled, the water had to be
clean, and the orfice had to be cleaned out periodically. Another
disadvantage was that a difference in barometric pressure due to a change
in altitude, or day to day variations with weather would cause error.
Also, as the level of the water in the vessel was lowered, the effective
press Lure forcing water through the or dice would change, necessitating
a non-linet-r scale.

An interesting fact about the clepsedra's development is that it in-
volved a new concept in the marking of time. It was not so much a
question of "when" as of 'Thow long," for the scale could not, for example,
tell the observer when it was noon, but rather indicated how long since
it had last been filled.

-14-

Ctesi"bus of Alexandra improved upon the contrivance by first
supplying a double vessel to keep the scale linear, and second by
attaching pulley mechanism to a float which rotated a clock hand. This
happened about 140 B.C., a time vtfien Roman culture was flourishing and
captive peoples bent their arts toward increasing Imperial Greatness.

An interesting use of clepsydrae arose when Pompey commanded their
use in law courts to limit length of arguments. Of course, there
was always the possibility of bribing someone to put muddy water in
the clock, but ordinarily the idea worked out quite well.

Developed simultaneously with the water clock was the hour-glass.
The principle is the same, but sand replaced the liquid medium.
This solid media flowed through the opening at an approximately even
rata, no matter how great the wei$it above it, so it was really a
technical improvement over the clepsydra. The hour-glass has largely
degenerated to the egg-timer stage, and is no longer important except to
Schiaparelli and Pather Time.

See Figure (1) for examples of early time -pieces. The sundial
was designed and placed in Oransbury Park, England, by Sir Isaac Newton.
The Hourglass on left was designed for Pulpit use, approximate sermon
length two hours. The glass on ri*£it is an example of French handiwork
in the seventeenth century.

A medieval practice was to determine the length of time for
a candle to burn, and then, having calibrated its lenght, make another
of alternate colored rigns. Although only approximate, it served
quite well as a marker for the passage of hours in the rough age when
it was used*

Our debt to the Ancients in the matter of recording time is typical
of that in many others.

Sun Dial Designed and

Placed by Sir Isaac Newton

in Cranbury Park, Winchester,

England

I ulptt Two-Hour Glass ,-, . Tf ™

F . rrench Hour Olass

American, 1700-sO „. , , ,-,

. „ t ■ Eighteenth Century

in the iissex Institute, , -, ..

c 1 iut- m e Metropolitan

Jal till j a VI 3. 5 S .. "\ t

Museum

Types or the Earliest Time Telling Devices

The sun dial is the first ancestor of ad time tellers, and the sa?id
glass was probably the first portable time telling device.

FiV I

-15-

We owe to them our whole fundamental system and conception of It ;
from the astronomy by which we measure our years and our seasons down
to the arithmetic of our minutes and seconds and the names of our
months and days.

However, in the modern application and practical use of their
legacy, we owe nothing. Aristotle never made a clock or watch or any
device more than ornamental today, although the general plan was so
well worked out that in has never been bettered.

It seems difficult to understand the instinct which led man to
keep their learning like a secret among the initiated of a my s tic
fraternity, feeling no impulse to make known that which they knew. The
great men of bygone days thought and did tremendous things which are
now everyone's common property. But the common people of that tine
lived in a fashion terribly primitive by comparison and in an ignoranoe
which certainly was weakness and yet might somehow have been bliss.

The ancient world is gone - both the body and the spirit of it.
But there remains along with their art and philosophy the hour-^lass to
symbolize the relentless flight of time which they feared but could not
stop; and the sun-dial in front of Student-Center, a memory to the
worldly-wise philosophy which counts only the shining hours.

The typical modern clock is powered from the pull of a weight or
the pressure of a. spring, unless it is operated by electricity or compressed
air. The regulator of mechanical clocks is known as the escapement, the
recording device generally consists of hands, dial, anl
a gear train, and there may perhaps be a striking mechanism.

The first direct forbear of Jig 3en appeared in the eleventh century,
coincidental with the appearence of ;illiam the Conqueror in England,
Bxis was the monastery clock, which struck the hour and told all
people within hearing when it was time for prayers.

-16-

An example is the elaborate Clock of Strasbourg Cathedral in Lorraine,
which was built in 1352. See Figure (£). This clock represents the
extreme in medieval craftsmanship. It is three stories high and stands
against the wall someyrtiat in the shape of a great altar with three towers.
Among its movements are a celestial globe showing the positions of the
sun, moon, and stars; a perpetual calendar; a device for predicting
eclipses; and a procession of figures representing the pagan gods from
whom the days of the week are named. There are also devices for showing
the age and phases of the moon and other astronomical events* The hours
are struck by a succession of automatic figures, and at the stroke of
noon a cock percked upon the topmost tower flaps his wings and crows.
Although is keeps no better time than a $.79 alarm clock, it is a mechan-
ical monstrosity reminiscent of the age in which it was created, and
beautiful in the eyes of all cook's Tour guides.

The first pendulum clock was made about 1665 by Christian Huyghens,
a Dutch astronomer and mathematician who discovered the rings of Saturn.

From that time on, the important improvements of clockwork were
chiefly made in two directions - those of mechanical perfection of the
escapement and the compensation for changes of temperature.

The ultimate in perfection of modern clocks used for maintainence
of time standards uses one more refinement, a "slave clock". This is
simply a device which removes the friction of the escapement at the
end of er-.ch pendulum swing. Clocks of this type were developed around the
turn of the century by Hiefler and by Shortt, three each of which
are sealed in the vaults of the Naval Observatory in .vashington, D.C.

See illustrations for examples of jeweled watches, developed at a

A Time Piece of the Middle Ages

The huge and elaborate Clock of Strasbourg Cathedral, in
Lorraine, was built in 1352 and is an example of the first clocks.

Fi\. X

London
about 1600

Octagonal Rock
Crystal Watch
French, 156090

Square
French Watch
Late Sixteenth Century

Oval

French Watch
I590

Shell Shaped

Rock Crystal Watch

French

Cross Shaped

Rock Crystal Watch

French

Book Shaped

Swiss Watch

1560-1600

When Watches Were Jewels

Watches of the Sixteenth Century, with but one hand, and
pierced metal or rock crystal cases. In the collections of the
Metropolitan Museum.

F«'v 3

Limoges Enamel

Watch
English 161025

English Repeater
about 1650

Silver

Skull Watch, French.
Intended to remind the
wearer that each second

brought death nearer

Gold Enamel
Watch — -French

French Watch
intended for the
head of a cane,
1 645-70

Agate Case
French

Seventeenth Century Watches

Grew more elaborate and ornamental, but scarcely more useful,
In the collections of the Metropolitan Museum.

'V

-17-

time when beauty was everything and accuracy didn' t make much difference.
Figure three shows typical watches of the fifteenth and early sixteenth
centuries, and figure four watches of the late sixteenth century*

The evolution of timepieces having been briefly reviewed, it becomes
important to consider in detail the methods of establishing the accuracy
and precision of modern clocks, watches, and noon whistles.

The ultimate standards of reference are the celestial bodies, which,
although they may slowly change position, nevertheless do so at rates
which may be allowed for with great exactitude* This checking is carried
on at a great many observatories throughout the world, but as the method
is almost exactly similar everywhere, the arrangements at the Javal
Observatory, „ashington, 3.C. will be considered as typical, A factor
in the choice of this Observatory additional to its proximity is that
work there is continued on a twenty-four hour basis, and the time measure-
ments resulting are broadcast throughout the world, being considered
the ultimate standard for at least the ,Ve stern Hemisphere*

The standard time transmissions are available to anyone who either
uses a radio or desires to run a wire to the observatory proper. No
charges are trade, the work being carried on gratuitously by government
funds under auspices of the Navy department*

At the present time transmissions go to the Time Central, i*avy
Department; Postal Telegraph; American Telephone And Telegraph Co.;
Western Union; and the District Fire Alarm lines. ,e stern Union uses
the transmissions to keep their numerous clocks on leased service correct,
while the Fire Department uses the service to insure that their employees
do not work more than eleven thousandths of a second overtime. The
Navy Department is responsible for sending the transmissions over the
air via its official station ITAA.

-IB-

Transmission there is on several frequencies and many times throughout
the day.

The growth and usefulness of the Naval Observatory after 1842 resulted
in the purchase of its present site of seventy- two acres on Massachusetts
Avenue in northwest '.Vashington, 'Ihe location is circular in shape, having
a radius of exactly one thousand feet with the standard clock vault at
the center.

The Observatory is the United States' source of data required for
navagational astronomy as well as its official time standard.

In 1844 a time ball was dropped from the staff of the building every
day. This has long since been discontinued.

In 1865 the Western Union service to its telegraph offices was
begun, and has since been gradually expanded.

In 1904 the -"aval radio station 1TAA, located in Arlington, Virginia
sent out the first signal ever broadcast.

In 1905 the "IAA regular time transmissions were begun and have
been continue i - ever since.

In 1931 the broadcasts were increased from one per day to six per
day.

In 1934 twenty daily broadcasts v,ere initiated, and automatic
transmission to the transmitter was begun through Time Central, ^avy
Department.

The directors of the Observatory are justly proud that the average
error of all transmissions during the. last year was less than 0.011
seconds, a fact which will be further enlarged upon a bit later.

One of two instruments must be used in making accurate time deter-
minations. At the Observatory is a six inch Transit Circle, or Meridian

-19-

Transit telescope, made in 1897, which is still in continuous use for
observing sun, moon, and planets and selected stars. 1216 purpose of the
Observations is to determine their fundamental position by noting their
position at time of transit over the meridian. Having determined their
position, the time of transit across the meridian may be very accurately
computed from other data. About seventy-five hundred observations are
made e^ch year.

The other instrument which may be used in a Photographic Zenith
Tube. The instrument at the Observatory was built in 1911, has an aperture
of eight inches, and a focal length of seventeen feet. It is mounted
vertically, and is immovable. It is used to photograph the stars
as they pass near the zenith in order to determine the variation of
latitude due to the earth wobbling on its axis. This variation of
latitude is used in the correction of all observations for declinations.

In addition these photographic observations are used to determine
time to the nearest 1000th of a second. The Naval Observatory is the
only one to use the method. Since human error is eliminateu, accuracy is
greatly increased.

The accepted basis of time measurement is the earth's rotation on
its axis. The rate of this rotation is usually considered uniform,
although It is doubtless affected by both progressiva and other variations.
This rotation causes the sun and stars to appear to cross the slcy from
east to west. If a person located on the earth's equator measured the
time interval between trae successive passages overhead of a very
distant star, he would thereby measure the period of the earth's rotation.
If he then made similar measurements on the sun instead of a star, he
woula obtain a result about four minutes longer than before. This

-^0-

difference is due to the earth's motion around the sun, which continu-
ously changes the apparent place of the sun among the stars. The effect
is the same as that noted when traveling in an automobile; the near
objects appear to move backward when judged by the more distant ones.
Thus during the course of a day the sun appears to move a little to the
east among the stars, so that the earth must rotate on its own axis more
than 360° in order to bring the sun overhead again.

Even if the earth did not rotate at all on its own axis, the sun
would rise and set once during the year t because of the earth's journey
around the sun. The stars, however, are not within the earth's orbit.
Since they are generally more than a million times as distant as the sun,
their apparent positions are only very slightly affected by the earth's orbit-
al motion* The apparent positions of the stars in the sky are commonly
reckoned with reference to an immaginary point called the vernal equinox,
which is the intersection of the celestial equator and the ecliptic.
The sun is at tne vernal equinox at the beginning of spring, when it
passes over the earth's equator on its journey northward. The period of
the earth's rotation measured with respect to the vernal equinox is called
a sidereal day, although it might better be named an apparent equinoctial
day. 'The period with respect to the sun is called an apparent solar day.
Unfortunately, both the sun and the equinox move at variable r;*tes among the
st. rs, so consequently the apparent solar and sidereal days are of var-
iable length. In order to overcome this irregularity, mean time has been
devised, ^ean solar time, which is generally used in ordinary life, is
sometimes ahead Of and sometimes behind apparent solar time, but on the avera-
ge it is the same. Ihe difference between these two different kinds of
time is called the equation of time. Its maximum value is a little over

-21-

sixteen minutes. The difference between apparent equinoctial (sidereal}
time and mean equinoctial (sometimes called uniform sidereal) time is
due to the mutation or nodding; of the earth's axis. Its greatest value
is only a little over a second, and its greatest daily change is a little
over a hundredth of a second. 3ecau.se this difference is so small, sidereal
time has generally "been used oy astronomers.

In recent years, a few observatories, including the Naval "ibserv^tory ,
have begun to employ mean equinoctial time in computing the rates of
precision clocks.

Since the sun does not rise and set in different parts of the world
simultaneously, it is evident that the various parts of the worla have
different solar times. In order to reduce confusion, standard time zones haie
been adopted. All the points in each zone use one uniform time which is
generally different from their local times by not much over half an hour* and
in some places, of course, is exactly the same.

Tn general, these time zones differ from Greenwich, or zero meridian
time, by some whole number of hours. In the continental United States,
there are four time zones, "astern Standard Time is the local time of the
seventy-fifth meridian, and five hours less advanced than Greenwich time.
Central Standard Time is the local time of the ninetyfch meridian, and
six hoars less advanced than Greenwich time, fountain Standard for the
one -hundred fifth, and Pacific Standard for the one -hundred twenty th meridian

The Naval Observatory is thus able to furnish one time signal which
will provide time for all zones.

Since the object of the time zones is mainly one of convenience, the
boundaries between the zones have been placed where they will be the
sou 2*0 e of least inconvenience, rather than along the exaftt meridians.

For Instance, if the lines wer^ straight, the people in one part of a
small town might be using different time from their neighbors, and the
railroads might have to make time changes at inconvenient points rather
than at terminals. The Interstate Commerce Commission holds hearings
concerning the placing of these divisions.

In order to determine time with high precision, it is necessary to
observe stars, or other celestial objects, with a telescope. Such
observations are most conveniently and accurately made when the stars are pass-
ing over the meridian at the place of observation. The meridian is an irnmaginary
line in the sky, passing through the zenith and the north and sDuth
points. The most commonly used instruments for time determination at fixed
stations are called meridian transit telescopes. 3aey are pivoted so that
they may be pointed anywhere along the meridian, but not elsewhere. The
observations are usually made visually, and the times of transit are
recorded by a mechanism which is either operated or regulated by the observer.

Recently, a new type of instrument has been put into service for
work at the Uaval Observatory. The telescope, called a Photographic 2enith
Tube as already described, is rigidly fixed in a vertical position and
therefore cannot photograph any objects except those which pass very
near the zenith. At the lower end of the tube is a basin filled with
mercury. The light from a star passes through a lens at the upper
end of the instrument, continues down through the tube, is reflected from
the mercury surface, and comes to focus on a smiill photographic plate
located just under the lens. The location of the plate and tte curves
of the lens are ;uch that the lens and plate may be tilted as a unit,
through a small angle, without sensibly altering the position of the
image on the plate. >f the plate and lens, both are rotated through 130°
then the distance on the plate^the image of the star^ Before and after
reversal^, corresponds to twice the zenith distance of the star, V^ere it

-23-

possible to take both photographs at the same instant, when a star was

exactly at the zenith, the two images would coincide, and the time of

meridian transit for that particular st-r would be the time when the

photograph was taken. In actual practice, the plate is 4 1 *i ven from

west to east so ^-s co keep pace with the motion of the stars image, and the

clock time at which the pi te is in certain positions is automatically

recorded. The ira.ges obtained before and after reversal do not coincide,

but by measurement of the distances of the images it is possible to determine

the positions of the stars during exposure and so to deduce the times of transit.

By observing the time of meridian transit of the sun, the tinre of local appar-
ent solar noon is acertained without further calculation. SAn observations
are not so precise as those of stars, owing tD the size of the sun's disk,
and the unsteadiness of the atmosphere at midday.

It is possible to observe a number of stars during one night,
thereby increasing accuracy. To make the star observations useful, the
positions of the stars in the sky, as measured from the vernal equinox,
must be known. As the relative motions of the sun and earth fix the
position of the equinox, solar observations are a necessity. Observations
of the sun and stars regularly over a period of several years, show their
relation very accurately. r fhe results of this long series of observations
are utilized in making the calculations for the valuation of a single
night's observation.

By use of data contained in Naval publication #Ehe American 3phemeri s

-24-

and Nautical Almanac ", the correct sidereal and mean solar times of star
transits are derived*

During the exposures with the photographic zenith tube, the clock
times at iflhich the photographic plate is in certain positions are auto-
matically recorded by an electric chronograph which records graphically
both the clock ticks and the signals from the photographic telescope.
Thus the clock time is determined within 0.001 second. By comparing these
clock times with the theoretical times, computed as already indicated, the
clock error is determined.

As the times of the star transits are most readily computed using
sidereal time, the standard clocks- are rated to run on sidereal time
also. These clocks are maintained under constant temperature and pressure.
They are specially designed and manufactured for precision purposes.

They are never disturbed, never reset, never interfered with in any way
except for repairs. The actual rates of the clocks are not so important,
provided they are nearly constant. They are checked by the astronomical
observations, and their errors predicted for any time in the near future
On the basis of past performance. The short period variations in the rates
of the clocks permit the clock rates to be predicted within a few thousand-
ths of a second per day.

Although sidereal time is convenient for star observations, it is
not suitable for the general public. The mechanism for the transmission
of time signals is therefore rated to mean solar time. The transmission
of signals begins at 55 minutes seconds of each hour and continues for
five minutes.

Signals are transmtted every second during that tima, except that
there is no signal on the twenty -ninth second, nor on certain seconds at

-45-

the ends of minutes, as shown on the following diagrams

Minute Second 50 51 54 53 54 55 56 57 58 59 60

55 X X X X X X

56 X X X X X X

57 X X X X X X

58 X X X X X X

59 X X

The "X" indicates the seconds on which signals are transmitted. The
seconds marked "60 " are the zero seconds of the following minute. All

e

seconds from zero to fifty are transmitted jcxcept the twenty -ninth, but
are not ahown for convenience. The dash on the beginning of the hour
(59-60 above) is much longer than the others. In all cases the begin-
nings of the dash indicate the beginnings of the second.

The number of dashes sounded in the group at the end of any minute
indicate the number of minutes yet to be broadcast.

During the broadcast, the si^aa.1 is recorded automatically at the
observatory, showing signal ticks in the observatory, ticks of one standard
clock, and the signal received back by radio from the sending station.

A sample of the record of this electric chronograph is appended as
Figure (5).

Line number one is "rtat signal sent by NAA. It is received at the
observatory with one receiver for each transmitted frequency, and compared to
the other clocks by reading either total time or the time difference in
corresponding groups. In all cases each dot represents 0.005 seconds

Line number two is the record of sidereal clock standard number one
in the sealed Vault.

Line number three is not used, and runs uncontrolled.

Fiy r

-26-

Line number four is the record of sidereal clock number two, also
in the sealed vault* The other four sidereal clocks are not connected
on this particular day. Those two which previous record shows will
probably be the most accurate are the only ones used at any one time.

Line number five is a record of standby transmitting clock number
one.

Line number six is a record of the primary transmitting clock, of
which more later.

Line number seven is the record of the standby transmitting clock
number two.

By comparing the number of dots or spaces beneath corresponding
columns, it is possible to determine the error.

for example, let it be desired to determine the lag of the
transmitted signal from the radio station (HAa) behind the signal put
on the wires from the Observatory. The record is taken for every second,
and to keep columns numbered for comparison they run* 0, 1, 2, 3, and 4,
The immediate problem is to finrf the time difference between lines .
one and six in the third column. Then the time after 3.000 at which
the indicator starts to record line six is the time interval per dot
multiplied by the number of spaces. This is (0.005) (15), or 0.075 second,
variation. On line nurabei? one, the transmitted signal from the observatory,
there is a full column and one space lag. Then the lag behind 3.000 is
(0.005) (21) or 0.105 second. The difference between the two readings
is then the difference between 0.105 and 0.075 or 0.030 second, the
required lag.

Knowing the lag to be expected, the transmitting clock can then be
set that amount ahead of the standard, and so make the signal transmitted

-27-
to the public exact within the limits of the primary's accuracy.

In similar fachion, corrections can be applied to the other clocks,
and then corrections may be made to the standard itself on the basis
of star observations.

Three times a week comparative corrections are published, and once
a week absolute corrections on the basis of the astronomical data are
published. The weakly repofff lags from two to three weeks behind the
time of transmission of a given signal. This allows time for the complex
computation required to determine the absolute time at time of transmission.

Star sights both of the day before and fefctit- after are used in the
absolute determinations. These computations indicate that the average
error of the time signals as sent from Arlington is about 0.02 second.

Comparison with signals emitted from fourteen different observatories
indicates further correction of 0.01 for a national average.

II o method of transmitting has been found to decrease the error. How-
ever, for persons wishing more accurate data, the above-mentioned tables
of correction may be applied, sending accuracy up to a few thousandths
of a second.

Signals were first sent out so that navigators might check and
readjust their chronometers before leaving the harbor. Now they are
also used directly by navigators at sea.

The signals are also used for Longitude determination in precise
surveying and map making.

Still another use in in gravity determination by means of which
minerals and oil are located, and geodetic questions investigated*

Radio monitoring stations use the signals in checking their sub-
standards, which are in turn used in checking the frequencies of trans-
mitting stations.

-28-

Seismologists use the signals to coordinate records of various
stations throughout the world.

It is now of interest^ to outline briefly the physical set-up
at the Naval observatory which accomplishes such precise determinations
and comparisons as described above.

The transmitting clock is run at a frequency of 1000 cycles. ±'o
obtain this continuous frequency, and maintain it accurately, a crystal
oscillator operating on thirty kilocycles is used. The piezo-electric
crystal which controls actually measures three inches long, one -ha If
an inch wide, and one quarter of an inch thick, the output of this
electronically operated circuit is put into a six kilocycle multivi-
brator unit which transforms the frequency to five thousand oycles,
one-sixth the original, ©lis in turn is fed into a five kilocycle
multivibrator which reduces to the desired frequency of one thousand
cycles, or one kilocycle, the output of this circuit is not very
strong, although of the correct frequency, so it is therefore amplified
until sufficiently strong to run the clock motor.

The transmitting clock itself consists of a small motor which drives
a disc about seven inches in diameter at exactly one revolution per second.
The disc is divided into one thousand divisions, and a stroboscope set
up within the case allows reading of the disc's position to within a
tenth of a diviiion, or one tan- thousandth of a second. The motor shaft
is also connected through electric contactors to a switchboard, where it
automatically controls magnets, chronograph, power, time transmitter, and
sidereal switches, in fact everything necessary to make the service
automatic.

-29-

The chronograph contains seven cylinders, each driven, or set up so
that it may be driven, by a different clock. Each cylinder has a number
of printing fingers, brought into printing position by energy from its
driving clock. The record used as Figure (5) was taken from this machine.

Standby transmitting clock number one is similar to the regularly
used transmitting clock in construction, and is available for emergency
use.

Standby number two is a pendulum clock which is not controlled by
the crystal oscillator as are the other two. In case of failure of the
oscillator, this would be used for transmission of the tine signals.
An Interesting feature of this clock is the provision made for making
small adjustments. A magnet coil is placed directly beneath the swinging
pendulum. An oscillating current of the same frequency as the pendulum's
period is supplied through a reversing switch so that the force field
of the magnet may be made either to oppose or aid the swing of the
pendulum, A means is provided for comparing the output of this clock
with that of the primary transmitting standard so that the speed- retard
control of the standby can be used to visually adjust the two clocks
within one ten- thousandth of a second. Unfortunately, the standby will
not stay in step for periods over an hour, so thin resetting is neoessary
quite frequently,

A single pen chronograph is used in the observatory proper to record
the one second ticks of the sidereal standard clock {in the vault] being
used for the days primary standard, and the time at which photographs
are taken. These two records show up on the same line by means of a
small control magnet which acts on the pen to push it off normal position
in one direction for the standard ticks, and in the other direction for

-30-

photographic exposure.

tube
The photographic zenith A has already been described with one exception.

The photographic plate must be exposed for approximately twenty seconds.
Therefore, the mercury pool which acts as a reflecting mirror in the bot-
tom of the tube, and the lens which concentrates the light, and also the
plate and plate holding mechanism are all driven by an astronomical
clock which keeps the units pointed to the same spot in the heavens as
the earth rotates.

The Btar photographed is generally not emotly at the zenith, there-
fore, the plate is examined under a Traversing Microecope. This instrument
is set up to measure small angles with extreme precision. The resulting
measured angle is applied as a correction to the single pen chronograph's
record of the time at which the photograph was taken. The corrected
time then gives the exact time of the star's transit across the meridian
and zenith as shown by the record of the standard clock down in the vaults*
(Record of second ticks of clock- as shown on chronograph) Knowing the
apparent time of the star's transit, and having already calculated the
exact time at which that particular star may be expected to cross the
zenith, the variation of the number one sidereal clock is the difference
between the two values.

The complex astronomical computations required to obtain accuracy
are performed upon a mechanical computing machine of standard design,
but modified to give a greater number of significant figures.

Power used at the observatory is taken from the regular lines, but
standby units are Installed which will take over automatically in case
of line failure.

The stages of mankind's struggle towards precision measurement of

-31-

tirae would In retrospect seem to split into four main divisions. Jhe
first is the long prehistoric era, extending back into the distant past
far beyond the memory of man. The only measures of the passage of time
during this interval were the sun and moon, and the passage of the seasons.
The second period heralded the first rude devices for less rough measure-
ment, including the sundial, water clock, and hour-glass* Setting up
of a calendar might also be included here. Next is the medieval period,
which has seen the development of a mechanical instrument which keeps
fairly accurate time. This merges into the modern period where split
second accuracy is the order of the day, and very precise astronomical
measurements are used to check the accuracy.

It seems safe to say that lodern time-pieces of one type or another
now exist ready to meet all requirements. If history be any indication,
then any future need for greater precision will as in the past in some
way be met.