2 14 POPULAR SCIENCE MONTHLY. 



These variations of the sun from uniform time caused no end 

 of trouble between the astronomers and the fine clockmakers be- 

 fore it was discovered that sun time is subject to such irregulari- 

 ties. 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 es- 

 tablished with the greatest accuracy by the most careful observa- 

 tions 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 jiass the meridian of any given place — that is, the in- 

 stant it will cross a north-and-south line through the place. The 

 data regarding these stars are all published in the nautical alma- 

 nacs, 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 pcndidum 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 pre- 

 cisely as much as the expansion of its rod lengthens it, and con- 

 versely 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 imperfec- 

 tions 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 



