SIDEREAL DAY. 



by two or more observers observing the same transit at the same 

 place. 



28. If the transit of the same star be observed for two or more 

 successive nights, the interval which elapses between any two- 

 successive transits can thus be determined. Now it has been 

 found that this interval is absolutely the same, not only for all 

 stars whatever, but also that it is the same at whatever part of 

 the earth the observation may be made. By comparing the 

 results of ancient with modern observations, it has also been 

 found that this interval has not undergone the least change. 



It is well known that this apparent diurnal rotation of the 

 heavens, by which a common motion is thus imparted to all 

 celestial objects, is the optical effect produced by the rotation of 

 the earth upon its axis, and the time of that rotation is conse- 

 quently the interval which elapses between two successive 

 meridional transits of any fixed star. 



Such is the constant and invariable character of this motion r 

 and its absolute uniformity, that Laplace has shown, independently 

 of all theory, that, as a matter of fact, the time of this apparent 

 rotation of the heavens cannot have suffered any change amounting 

 to so much as the hundredth part of a second since the time of 

 Hipparchus, being an interval of twenty centuries. 

 This interval is called a SIDEREAL DAY. 



The sidereal day is subdivided into hours, minutes, and seconds, 

 in the manner already explained. 



The circumference of the celestial sphere being supposed to be 

 divided into 360, through which it revolves in 24 hours, it 

 follows that it turns through 15 per hour, 15' per minute, and 

 15" per second. 



It is perhaps to be regretted that the terms minutes and 

 seconds have been used in two different senses, the more especially, 

 as their application in both these senses is constantly necessary in 

 all astronomical works. As applied to the arcs of circles, or to 

 angular measurement, a MINUTE signifies the sixtieth part of a 

 degree, and a SECOND the sixtieth part of a minute. As applied 

 to time a MINUTE signifies the sixtieth part of an hour, and a 

 SECOND the sixtieth part of a minute. 



The confusion which might arise in calculations in which 

 both time and angular measures are involved, is prevented by the 

 adoption of the letters m and s , to express minutes and seconds 

 of time, and the signs ' and " to express angular minutes and 

 seconds. Thus 8 h 30 m 25-6" 



expresses an interval of time consisting of 8 hours, 30 minutes, 

 25 seconds, and 6-tenths of a second ; while 

 8 30' 25-6" 



127 



