94 SOLAR TIME. [SECT. xn. 



equinox or solstice, have been universally adopted as the 

 measure of our civil days and years. The solar or astro- 

 nomical day is the time that elapses between two consecu- 

 tive noons or midnights. It is consequently longer than the 

 sidereal day, on account of the proper motion of the sun 

 during a revolution of the celestial sphere. But, as the sun 

 moves with greater rapidity at the winter than at the summer 

 solstice, the astronomical day is more nearly equal to the 

 sidereal day in summer than in winter. The obliquity of 

 the ecliptic also affects its duration ; for near the equinoxes 

 the arc of the equator is less than the corresponding arc of 

 the ecliptic, and in the solstices it is greater (N. 145). The 

 astronomical day is therefore diminished in the first case, 

 and increased in the second. If the sun moved uniformly in 

 the equator at the rate of 59' 8"'33 every day, the solar days 

 would be all equal. The time therefore which is reckoned 

 by the arrival of an imaginary sun at the meridian, or of one 

 which is supposed to move uniformly in the equator, is 

 denominated mean solar time, such as is given by clocks and 

 watches in common life. When it is reckoned by the arrival 

 of the real sun at the meridian, it is apparent time, such as 

 is given by dials. The difference between the time shown 

 by a clock and a dial is the equation of time given in the 

 Nautical Almanac, sometimes amounting to as much as six- 

 teen minutes. The apparent and mean time coincide four 

 times in the year ; when the sun's daily motion in right 

 ascension is equal to 59' 8"'33 in a mean solar day, which 

 happens about the 16th of April, the 16th of June, the 1st 

 of September, and the 25th of December. 



The astronomical day begins at noon, but in common 

 reckoning the day begins at midnight. In England it is 

 divided into twenty-four hours, which are counted by twelve 

 and twelve ; but in France astronomers, adopting the decimal 

 division, divide the day into ten hours, the hour into one 

 hundred minutes, and the minute into a hundred seconds, 

 because of the facility in computation, and in conformity with 



