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
