Earth turns toward the east once in about a day. Before standard time, towns set their clocks by the Sun's path across their own meridian. Railroad travel made standard time necessary, and railroad leaders were asked to design a uniform time system.
Railway companies in Great Britain adopted Greenwich Mean Time in 1847, and in the United States standard time began on Sunday, November 18, 1883, when railroads started using time zones. In 1884 delegates at the International Meridian Conference voted to make the Greenwich Meridian the world's Prime Meridian. The conference chose a line through the telescope at the Royal Observatory in Greenwich, and that prime meridian marks 0 degrees longitude. Time zones are areas that share one standard time, and their borders follow local laws rather than straight lines.
Time zone offsets run from UTC minus 12:00 to UTC plus 14:00. Most differ by whole hours, but a few are 30 or 45 minutes off, as in India and Nepal, and Nepal moved to UTC plus 05:45 in 1986. Somewhere the calendar date has to change, and the international date line zigzags around borders and is a convention, not defined by international law.
Some countries have moved across the line. In 2011 Samoa jumped across it by skipping Friday, 30 December 2011, from its calendar, after being on the western side of the line until 1892. Kiribati changed the date for its eastern half so the whole country shares one date. As a result Friday, 30 December 1994, was followed by Sunday, 1 January 1995, and its easternmost islands have the latest time on Earth, UTC plus 14.
Moving the clock on purpose is another matter. Daylight saving time was first seriously proposed by New Zealander George Hudson in 1895 and by William Willett in Britain in 1907. Germany and Austria-Hungary were first to use it nationally, from 30 April 1916, to save coal in wartime.
Day and night come from Earth's spin, while seasons come from its tilt as it goes around the Sun, which takes 365.25 days. Earth's axis is tilted approximately 23.4 degrees compared with its path around the Sun, and the tilt points the same way throughout the year. That fixed tilt is the cause of Earth's seasons.
In March and September the Sun is directly over the equator at the equinox, and day and night are about equal. The Sun rises due east and sets due west at the equinoxes. The summer solstice is the longest day in each hemisphere, and the solstices happen in June and December. At the equator the Sun is directly overhead at noon on the two equinoxes, and near the equinoxes the day at the equator is approximately 12 hours and six and a half minutes.
Near the poles the Sun can stay up for a whole day or more. In Svalbard, Norway, the northernmost inhabited region of Europe, the Sun does not set from approximately 19 April to 23 August. Because air bends sunlight over the horizon, polar day is longer than polar night, and the region of polar day is slightly bigger. Polar night lasts from approximately 24 hours just inside the polar circles to 179 days at the poles.
Polar night is not pitch dark all day, and twilight is the glow in the sky after sunset and before sunrise, when the Sun is out of sight but the air still scatters its light. The Norwegian town of Longyearbyen, Svalbard, has nautical polar twilight, enough light that the horizon is visible for navigation, from approximately 11 November until 30 January. In mainland Norway, Tromsø, above the Arctic Circle, has midnight sun from late May to mid-July, and the midnight sun reaches from approximately 65 degrees 44 minutes latitude to the pole.
Nicolaus Copernicus said the Sun is at the center and the Earth rotates daily. Direct proof came later. In 1851 Léon Foucault hung a 28-kilogram bob on a 67-metre wire from the dome of the Panthéon in Paris, and its swing slowly turned, which was the first simple, direct proof of Earth's rotation. A spot on the equator moves at approximately 1,675 km/h (1,040 mph) as Earth turns, and the speed falls with latitude and is zero at the poles.
Other planets keep very different days. Venus spins backward, so its sidereal day of 243 Earth days is longer than its year of approximately 224.7 Earth days, and a solar day on Venus, from one noon to the next, lasts approximately 117 Earth days. Mercury's day-night cycle of 176 Earth days is just over two Mercury years, and with no atmosphere its surface swings from about minus 180 to approximately 430 degrees Celsius. Mercury turns three times for every two trips around the Sun, and from some parts of its surface the morning Sun appears to rise briefly, set and rise again.
The length of the day itself has changed, and scientists can read ancient day length from growth bands in corals and from layers in ancient rocks. Layers in a Late Cretaceous mollusc shell, 70 million years old, show 372 days in a year, so the day was approximately 23.5 hours long then. Rock and fossil records suggest the day was only approximately 21 to 22 hours long some 600 million years ago. Tides raised by the Moon and Sun add approximately 1.7 milliseconds to the length of a day each century, and days now are a few hundredths of a second longer than they were around 1200 B.C.
The body keeps a day of its own, and most living things have body clocks that follow a roughly 24-hour cycle. In humans the master clock is in the brain, in a spot called the SCN. Light, seen by the eyes, resets the body clock, and in the evening the brain's master clock tells the body to make more melatonin, a hormone that makes people sleepy. After a long flight the body clock needs about one day to adjust for each hour of time-zone change. Flying west delays the clock, while flying east advances it.
How dark are the skies where people live? As of November 2026, approximately 83% of people, including 99% of Europeans and Americans, live under light-polluted skies more than 10% brighter than natural darkness, and 80% of North Americans cannot see the Milky Way. Blue LED lighting suppresses melatonin production about five times more than orange-yellow sodium lights, and light from electronic devices at night can confuse body clocks. Humans need natural darkness for good sleep and health, and bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians. Nearly half of Earth's species are nocturnal.
Words to know
- meridian: line of longitude
- solstice: extreme day
- twilight: glow without Sun
- equinox: equal day, night
- nocturnal: active at night
- hemisphere: half of Earth
