The interval between successive transits of the Sun across the observer’s meridian, that is, the rotation of the Earth with respect to the Sun. Strictly this is the apparent solar day, which varies slightly during the year because the Earth does not move at a uniform rate in its elliptical orbit about the Sun (see equation of time). Its average length, the mean solar day, is 24 hours or 86 400 s. Because of the Earth’s orbital motion around the Sun, the solar day is about 4 min longer than the sidereal day, a discrepancy that adds up to one whole day in the course of a year.