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William Henry Watkine 

November 16 f 1939. 


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 


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 


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 


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. 


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. 


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. 


'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. 

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. 


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 


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 


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 


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. 


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. 


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, 


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 


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 


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. 


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 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 

about 1600 

Octagonal Rock 
Crystal Watch 
French, 156090 

French Watch 
Late Sixteenth Century 


French Watch 

Shell Shaped 

Rock Crystal Watch 


Cross Shaped 

Rock Crystal Watch 


Book Shaped 

Swiss Watch 


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 

English 161025 

English Repeater 
about 1650 


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 

Seventeenth Century Watches 

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



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. 


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 

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

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 


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 


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 


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. 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 


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 


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 


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 


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 


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 

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 

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. 


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 


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 

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 


photographic exposure. 

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 


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.