Popular Science Monthly/Volume 56/December 1899/How Standard Time Is Obtained


ALMOST everybody knows that observatories are the places from which standard time is sent out and corrected daily or hourly. But comparatively few have more than the vaguest idea of the means used at the observatories for obtaining it.

Probably the majority of people suppose that the observatories obtain the correct time from the sun. When the average man wishes to give his watch the highest praise he says, "It regulates the sun," not being aware that a watch which would keep with the sun around the year would have to be nearly as bad as Sam Weller's. The farmer may safely decide when to go in to dinner by the sun, but if the mariner was as confident that the sun marked always the correct time as the farmer is he would be sure to be at times two or three hundred miles from where he thought he was. In other words, the sun—that is, a sundial—is only correct on a few days in each year, and during the intervening times gets as far as a whole quarter hour fast or slow. These variations of the sun from uniform time caused no end of trouble between the astronomers and the fine clockmakers before it was discovered that sun time is subject to such irregularities. The better the clock, the worse it often seemed to go.

But as the variations in sun time are now accurately known, correct time might be obtained from the sun by making proper allowance, were it not for the difficulty of observing its position, with sufficient exactness. The large disk of the sun can not be located so perfectly as can the single point which a star makes. For this reason astronomers depend almost wholly upon the stars for obtaining accurate time. It is the method of doing this which we propose to describe.

There are several hundred stars whose positions have been established with the greatest accuracy by the most careful observations at a number of the principal observatories of the world. If a star's exact position is known, it can readily be calculated when it will pass the meridian of any given place—that is, the instant it will cross a north-and-south line through the place. The data regarding these stars are all published in the nautical almanacs, which are got out by several different observatories for the use of navigators and all others who have uses for them. These stars are known as "clock stars."

Every observatory is provided with at least one, or, better, several clocks that are very accurate indeed. Every appliance and precaution which science can suggest is resorted to to make these clocks accurate. The workmanship is, of course, very fine. What is known as the "retaining click" prevents their losing a single beat while being wound. The small variations in the length of the pendulum which changes of temperature would cause are offset by compensation. The rise of the mercury in the pendulum bob, if the weather grows warmer, shortens the pendulum precisely as much as the expansion of its rod lengthens it, and conversely if it becomes colder. Such clocks, too, are set on stone piers built up from below the surface of the ground and wholly independent of the building itself. Often the clocks are made with air-tight cases, and sometimes are placed in tightly closed chambers, only to be entered when absolutely necessary. Some fine clocks even have appliances for offsetting barometric changes, but those affect such clocks less than other influences or imperfections which can not be accounted for, and thus they are seldom provided against.

The astronomer's principal clock—the one he uses in all his calculations—marks what is known as sidereal, not ordinary, time. The revolution of the earth in its orbit sets the sun back in its place in the heavens at the rate of about four minutes a day, or one whole day in a year, so that this clock, indicating star time, gains this amount and is only with ordinary clocks once a year. After it is once adjusted, no attempt is made to regulate it exactly, as the astronomer would better calculate its differences than disturb its regulation, always provided its rate is very uniform and accurately known.

One or more of the other clocks, however, are made to show ordinary time, and corrected by observations taken every few days. It is from this clock that the standard time is sent out.

It is possible to connect any of these clocks telegraphically with an instrument in the observatory, known as a chronograph. It consists of a cylinder with a sheet of paper around it, on which

PSM V56 D0229 Section from the paper band of a chronograph cylinder.png

A Short Section from the Paper Band of the Chronograph Cylinder, showing Tracings of Pen connected with Clocks: 1, seconds of sidereal clock; 2. both sidereal and common clocks; 3-10, the tracings of the mean-time clock fall steadily behind the other; 11. sidereal only; 12, connected with observer's key. The extra teeth show when a star passed each of the five spider lines. At the extreme right is a "rattle," put in to show where the observation is on the cylinder.

rests a pen connected with the telegraphic instrument which follows the beats of the clock. The cylinder is turned slowly by clockwork, and the pen, carried slowly along by a screw, describes a spiral on the paper with jogs or teeth in it about a quarter of an inch apart, caused by the beats of the clock. In this way the astronomer secures a visible record of the beating of his clock, or rather of the movements of his telegraphic recorder. Thus, if he has another key on the same circuit with the clock, connected with his chronograph recorder, and should touch it between the beats of his clock, it would put in an extra jog or tooth on his record, and it will show, what he could not have told in any other way, in just what part of the second he touched this key, whether in the first or last part of the second, and precisely how far from either end—that is, he can determine fractions of a second with great nicety.

As a matter of fact, he has such a key at the telescope which he uses to make his observations in taking time, so that when he wishes to record the precise instant in which anything takes place which he is viewing through his telescope he has but to press the key in his hand and an extra tooth will be put into the record which the clock is making, somewhere among the regular teeth put in by the beating of the clock. Later, when he takes out the sheet he can see just where the tooth came, and so at what instant it was. If, now, he knows exactly what the instant was according to the true time as given in his almanacs—that is, what his clock ought to have shown at that instant—he can tell how nearly right his clock is. Once knowing how this clock is, it is a simple calculation to find how the clock which sends the signals is running, and to alter it if needed in a manner we shall describe later.

The observations the astronomer makes use of to determine these instants of time are upon the "clock stars." He uses a rather small telescope, known as a transit. It is placed with the nicest accuracy in a north-and-south line. It can turn over vertically, but can not move sideways out of its line. Its alignment is kept perfect by occasionally sighting some small mark a few rods from the observatory, either north or south.

If the astronomer points this transit, say, halfway up the southern heavens and sees a star pass across the center of its field he knows that that instant gives, as it were, the "noon mark" of that star. If it is one of the "clock stars," he knows by his tables what that instant of time is—should be—by his clock.

We have seen what his means are of comparing his clock and his observations. But observe, now, how much pains he takes to get the most exact observations.

To begin with, he must have calculated to a nicety his location. The director of an observatory always knows where he is located in a sense that few other men do. The accuracy of a large part of his observations of any kind depends on his first having determined the latitude and longitude of his observatory within a very few feet. Then the data given by his tables are all modified, and adapted to conform to his locality.

There are stretched across in the eyepiece of his transit five spider lines. The central one is on the central line of the field of his instrument. In observing a star for time the astronomer watches it as it is carried by the rotation of the earth past each of these spider lines, and presses his key—that is, makes a record—as it crosses each line. Taking the average of these five observations, he makes the possible error very small. But, in addition to this, he also usually makes observations on at least four clock stars, which gives him twenty observations to average up and determine by. As he inspects the record of these observations which has gone upon the chronograph sheet along with the clock beats he is able to determine, after proper calculations, how his clock stands.

Such observations are made every three or four evenings, and thus the clocks are not given time to get far out of the way. It is not usual for a good clock to show a variation of more than half a second. If the astronomer finds that his clock which is sending the time is running a fraction of a second slow, he goes to it and lays on the top of the pendulum bob a minute clipping of metal, which is equivalent to shortening the pendulum an infinitesimal amount. When he takes his next observation he discovers how his clock has been affected, and again treats it accordingly. Thus the time that is sent out automatically by the clock is kept always correct within a small fraction of a second. Those who receive the time sometimes arrange electro-magnets near the pendulums of their clocks, which act with the beats of the observatory clock, and their attraction is enough to hold or accelerate the pendulums as needed to make them synchronize with the observatory clock.

It will be seen that the means of obtaining exact time involve a very considerable outlay, and that the services of highly trained men are needed. The public is thus greatly indebted to the railroads, telephone companies, and other corporations which usually bear the expense of securing standard time. It is probable, however, that from motives of scientific pride no observatory would undertake to charge for this anything like what would be exacted for such rare service in any department of the commercial world.

It is worth while to note that even with such perfect clocks and favorable conditions it is still impossible to secure perfect timekeeping. Add to this the fact that it is not usual for those who send out the time, after it has been received from the observatory, to pay much heed to variations, even of several seconds, in their master clocks, and we see why it is a disheartening task to keep the best watch as near the second as the owner would fain have it. In the first place, the watch could hardly be made to keep such time if kept still in an unchanging temperature; secondly, it is still less capable of it when subjected to the jolting and changes of temperature it encounters when carried; and, thirdly, the means of obtaining time with sufficient exactitude are rarely available to the general public.