Day, in astronomy, the average length of time between successive noons. Noon is defined as the instant when the sun is highest in the sky. Since apparent motion of the sun is caused by the rotation of the earth, a day is therefore the length of time the earth requires to turn once upon its axis in relation to the sun. In everyday use, a day equals 24 hours. To astronomers, who use sidereal time (based on movement of the earth in respect to the stars), a day is 23 hours, 56 minutes, 4.09 seconds.
The length of day is gradually increasing as the tides slow the spinning of the earth. In 100 years, a day will be about 1/1000 of a second longer than it is now.
The word day is often used to mean the period of daylight between sunrise and sunset, as distinguished from night, when the sun is below the horizon. Near the Equator, this period of daylight is about 12 hours long throughout the year. Elsewhere, days vary in length through the year, being longer in the spring and summer, and shorter in the autumn and winter. In the Northern Hemisphere, the longest day occurs about June 21 and the shortest day about December 22.
The term day may also mean any period of 24 hours. In this sense, the Babylonian day began at sunrise, the Greek and Jewish day at sunset. Among the Egyptians and Romans, the day began at midnight. Our calendar day follows this system.