Solar Day
The period during which the sun is above the horizon or shines continuously on any given portion of the earth's surface. Also called astronomical day, since the length of this day is continually varying, owing to the eccentricity of the earth's orbit and the obliquity of the ecliptic, a mean solar day is employed which is the average period of one revolution of the earth on its axis, relative to the sun considered as fixed. In astronomy and navigation the day is reckoned from noon to noon, but the civil day is reckoned from midnight to midnight. The mean solar day is uniformly equal to twenty-four hours.