Day

A day is a unit of time arising from the rotation of the Earth on its axis. The term has two meanings: one refers to the complete period of 24 hours, the other to the period of daylight that precedes the night. By standard calculation, a day is 86,400 seconds.

The day forms the basis of the calendar for months and years. Throughout history, people have divided the day into hours, minutes, and seconds, with the divisions varying according to custom: some cultures begin the day at sunset, such as Islamic and Jewish calendars, while others begin at midnight.

Scientific basis

The Earth completes one rotation on its axis in a period slightly less than 24 hours when measured against distant stars; this is called the sidereal day, approximately 23 hours and 56 minutes. The solar day is somewhat longer, because as the Earth rotates on its axis around the sun, it must rotate slightly further for the sun to return to the same position in the sky.

Light and darkness

The distribution of light and darkness depends on Earth's axial tilt. At the equator, daylight and night are nearly equal throughout the year. Moving toward the polar regions, the difference grows larger: in summer daylight lengthens, while in winter it shortens. At the poles there are periods when the sun does not set or does not rise.

Measurement and adjustment

Earth's rotation is not perfectly uniform; it gradually slows, and small variations occur within it. For this reason, Coordinated Universal Time (UTC) occasionally has a leap second added to it to adjust for the difference between atomic clock time and Earth's rotation.