day

(astronomy) One of various units of time equal to the period of rotation of the earth with respect to one or another direction in space; specific examples are the mean solar day and the sidereal day.

A unit of time equal to the period of rotation of Earth. Different sorts of day are distinguished, according to how the period of rotation is reckoned with respect to one or another direction in space. A day is normally defined as 86,400 SI (Système International) seconds [86,400 s/d = (60 s/min) × (60 min/h) × (24 h/d)], where SI seconds are measured by atomic processes. See also Physical measurement.

The apparent solar day is the interval between any two successive meridian transits of the Sun. It varies through the year, reaching about 24 h 30 s of ordinary clock time in December and about 23 h 59 min 39 s in September.

The mean solar day is the interval between any two successive meridian transits of an imagined point in the sky that moves along the celestial equator with a uniform motion equal to the average rate of motion of the Sun along the ecliptic. Ordinary clocks are regulated to advance 24 h during a mean solar day. Because of irregularities in the Earth's rotation, leap seconds are occasionally added to keep the day in terms of seconds of atomic time (Coordinated Universal Time, or UTC) in step with the day based on rotation (UT1).

The sidereal day is the interval between any two successive upper meridian transits of the vernal equinox. Similarly, as for the solar day, a distinction is made between the apparent sidereal day and the mean sidereal day which, however, differ at most by a small fraction of a second. The difference between sidereal time and solar time arises from the Earth's revolution around the Sun, which results in a year containing one more sidereal day than solar day. A mean sidereal day comprises 23 h 56 min 4.09053 s of a mean solar day.

The mean solar day and the sidereal day vary together in consequence of variations in the speed of rotation of Earth, which are of three sorts: seasonal, irregular, and secular. See also Earth rotation and orbital motion.