Divisions of Time. 5 



and the clay in the apparent motions of the sun ; 

 the hours, minutes, &c. have been introduced to 

 facilitate the estimation of the subdivisions. 



That space of time is called a day in which the 

 earth performs one rotation on its axis, and in 

 which consequently the sun appears to make an 

 entire revolution round the earth from east to 

 Avest. The natural or astronomical day is that 

 in which the sun appears to revolve from the 

 meridional line which passes through any given 

 place to the same meridian again. The astro- 

 nomical days are, however, not always of the 

 same length, while the civil day is always of 

 equal duration. This difference has given rise 

 to a distinction which is necessary to be known. 

 The former, which is usually measured by the 

 sun-dial, is called solar, and the latter, which is 

 measured by the clock, is called mean or equal 

 time. 



The difference between mean and solar time 

 depends upon three causes. 



1st, The earth (according to the third law o 

 Kepler) does not pass equal portions of its orbit 

 in equal times. It proceeds sometimes quicker, 

 and sometimes slower, and consequently the sun 

 appears to advance more or less rapidly in the 

 ecliptic. In the first case the day will be some- 

 what longer, because the earth to rejoin the sun, 

 or to present to him the same meridian, must 

 make somewhat of a larger turn upon its axis. 



