Day, in chronology, period of time required for one rotation of a celestial body, especially the earth, on its axis. This period is shorter or longer depending on whether the sun or another star is used as a reference point; thus, the sidereal day—the time it takes for the earth to rotate once relative to a star not the sun—is 4 min shorter than the mean solar day. The solar day, measured by the interval between meridian passages of the sun, varies in length because of the variation in speed of the earth in its orbit. In consequence, the length of the solar day is averaged over the period of a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes. Each type of day is divided into exactly 24 hr that vary in length proportionately to the respective type of day.
The civil day now begins at midnight, local time. In ancient times, the Babylonian day began with sunrise and with sunset among the Athenians and Jews. The day is still often regarded as starting with sunset in ecclesiastical (particularly Jewish ecclesiastical) usage; until recently, the astronomical day started at noon, and the Julian day still starts at noon.
In common usage day, as distinct from night, is the period of natural light between dawn and dusk. The period of daylight, most nearly constant near the equator, varies with the latitude and the season, reaching a maximum of 24 hr in the polar zones in summer, a phenomenon known as the midnight sun.