day, period of time time, sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.
..... Click the link for more information. for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian celestial meridian, vertical circle passing through the north celestial pole and an observer's zenith . It is an axis in the altazimuth coordinate system .
..... Click the link for more information. . The length of a solar day varies during the course of a year, so for purposes of time measurement an average, or mean, solar day is used (see solar time solar time, time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial .
..... Click the link for more information. ), equal to exactly 24 hr. The sidereal day, used by astronomers, is measured relative to the fixed stars rather than the sun (see sidereal time sidereal time (ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian .
..... Click the link for more information. ); it is about 4 min shorter than the mean solar day. The term day is also used to refer to that part of each 24-hr period during which the sun's direct rays are not blocked by the earth, this period of daylight hours extending from sunrise to sunset; the remaining portion of the 24 hr is called night. If the plane of the earth's orbit about the sun coincided with the plane of the equator, day and night would each be 12 hr long everywhere on the earth all year long. However, because of the tilt of the earth's axis of rotation, the times of sunrise and sunset vary from day to day, with the result that in the Northern Hemisphere there are long days and short nights in the summer and short days and long nights in the winter. See equinox equinox (ē`kwĭnŏks), either of two points on the celestial sphere where the ecliptic and the celestial equator intersect.
..... Click the link for more information. ; solstice solstice (sŏl`stĭs) [Lat.
..... Click the link for more information. .

day

Time required for a celestial body to turn once on its axis; especially, the period of the Earth's rotation. The sidereal day (see sidereal 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.

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