84 DIVISIONS OF TIME. SECT. XII. 



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. 149). 

 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 denomi- 

 nated mean solar time, and 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 true or apparent time, and 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 sixteen 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 



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 iii common reckon- 

 ing 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 their decimal system of 

 weights and measures. This subdivision is not now used in 

 common life, nor has it been adopted in any other country ; and 

 although some scientific writers in France still employ that 

 division of time, the custom is beginning to wear out. At one 

 period during the French Bevolution, the clock in the gardens 

 of the Tuileries was regulated to show decimal time. The mean 

 length of the day, though accurately determined, is not sufficient 

 for the purposes either of astronomy or civil life. The tropical 

 or civil year of 365 d 5 h 48 m 49"' 7, which is the time elapsed 

 between the consecutive returns of the sun to the mean equinoxes 

 or solstices, including all the changes of the seasons, is a natural 



