The interval of time between two successive transits of the sun across a meridian. This interval is about four minutes longer than the sidereal day, largely because of the sun's apparent annual motion eastward along the ecliptic (actually, the earth's “westward” motion along its orbit), which motion delays the sun's return to meridional transit. Also, this interval is inconveniently nonuniform due to systematic variations in the earth's orbital speed around the sun and the sun's changing declination. The concept of the mean solar day has been invented to circumvent these practical difficulties.