Day

Solar days not being of uniform length, and therefore not a convenient unit for estimating duration of time, the day is understood to be a mean day; i.e., the length is the mean of that of the solar days throughout the year, 24 hours. On April 15, June 14, Aug. 31, and Dec. 24, the mean and apparent day practically coincide. The greatest deviations are Feb. 10, when mean noon occurs 14m. 29s. before the sun reaches the meridian, and Nov. 2, mean noon occurring 16m. 20s. after the sun's meridian pa age. The civil day begins at midnight, the astronomical day at noon. The sidereal day is the interval between two consecutive meridian pa ages of the Vernal Equinox, and is 23 h., 56 m., and 4 s. in length.