42 KINEMATICS. [81. 



CHAPTER II. 



KINEMATICS. 



I. Time. 



81. Before introducing the idea of time into the study of 

 motion, a word must be said on the measurement of time. 



It is the province of astronomy to devise methods for measur- 

 ing time ; the usual method consists in transit observations. 

 Thus the fundamental unit of time in astronomy, or the sidereal 

 day, is the interval between two successive upper transits of the' 

 true vernal equinox over the same meridian. 



82. For the purposes of every-day life, it is more convenient 

 to make the measurement of time depend on the apparent revo- 

 lution of the sun. But the interval between two successive 

 upper transits of the sun over the same meridian, which is the 

 true, or apparent solar day, is not constant throughout the year, 

 owing to the inclination of the earth's axis to the plane of its 

 orbit and to the ellipticity of this orbit. The true solar day is 

 thus not well adapted to serve as a unit of time. 



Astronomers imagine, therefore, a so-called first mean sun 

 moving uniformly in the ecliptic so as to pass the perigee simul- 

 taneously with the real sun ; and a second mean sun moving 

 uniformly in the equator so as to pass the vernal equinox simul- 

 taneously with the first mean sun. The interval between two 

 successive passages of the second mean sun over the same 

 meridian is called the mean solar day. This may be regarded 

 -as the standard on which all time-determinations in mechanics 

 are based. 



The mean solar day is subdivided into 24 hours = 1440 



