Day, the period of the earth's rotation on its axis. The distance of the first fixed stars is so great that their apparent positions are not affected by the motion of the earth in its orbit; hence the time between the successive passages of a fixed star or any other equally fixed point of the heavens over a meridian is uniform, and is called a sidereal day. The point of the heavens usually employed is the vernal equinox, which, though not absolutely fixed, does not vary apparently during a single day. This day is only used by astronomers. In common life the day is measured by the return of the sun to the meridian, called the solar day. It varies from several causes, its average length being about four minutes greater than that of the sidereal day. The principal causes of variation are the motion of the earth in its orbit and its varying distance from the sun. The day according to which clocks and watches are regulated is the average of all the days in the year, and is called the mean solar day. The time of this day is called mean solar time, while the actual time by the sun on any given day is called apparent time. The difference between the two is the equation of time. They coincide four times in a year. Their maximum difference, occurring twice in a year, is about 16 minutes.

Astronomers in measuring mean solar time suppose a fictitious sun moving uniformly in the equator, and coming to the vernal equinox at the same instant with another fictitious sun moving uniformly in the ecliptic, and coming to the perigee at the same instant with the real sun. The sidereal day is 23h. 56m. 4.09s. of mean solar time. Astronomers begin the day at noon, and count the hours from 1 to 24. In most countries the civil day begins at midnight, and the hours are counted from 1 to 12 at noon, and thence from 1 to 12 at midnight. - The word day is also used in opposition to night, to denote the time during which the sun is above the horizon. This varies with the latitude and the season of the year. As we go north it increases in summer and decreases in winter. At the equator" the day is always a little more, and the night a little less than 12 hours. At the poles the day is a little more, the night a little less than six months.