Time required for a celestial body to turn once on its axis; especially, the period of the Earth's rotation. The sidereal day (seesidereal period) is the time required for the Earth to rotate once relative to the background of the stars (i.e., the time between two observed passages of a star over the same meridian of longitude). The apparent solar day is the time between two successive transits of the Sun over the same meridian. Because the orbital motion of the Earth makes the Sun seem to move slightly eastward each day relative to the stars, the solar day is about four minutes longer than the sidereal day. The mean solar day is the average value of the solar day, which changes slightly in length during the year as the Earth's speed in its orbit varies.