Earth spins around once in approximately a day, and a day sounds like the simplest unit of time, yet scientists keep more than one version of it. Earth turns once relative to the Sun in approximately 24 hours, which is a solar day, and one day and one night together take approximately that long. Relative to distant stars, one turn takes 23 hours, 56 minutes and 4 seconds, which is a sidereal day, approximately 3 minutes 56 seconds shorter than the mean solar day. Astronomers keep two star-based days, and the sidereal day is measured against the slowly wobbling equinox and is 86,164.0905 seconds. The stellar day is measured against fixed distant reference points and is 86,164.0989 seconds, approximately 8 thousandths of a second longer. Earth turns approximately 366 times a year relative to the stars but only 365 times relative to the Sun, because its trip around the Sun adds one turn to the count.
Astronomers found that Earth is an uneven clock, and Simon Newcomb and others showed that its spin varies irregularly, so in 1952 the second was defined from a fraction of the sidereal year instead of from the day. UTC, today's world time standard, is based on atomic clocks that are more stable than the Earth's rotation, and it replaced Greenwich Mean Time as the international civil time standard. Because Earth's spin speeds up and slows down, leap seconds could not be scheduled far ahead, and a standard body announced each one approximately six months early to keep UTC within 0.9 seconds of Earth-based time. In 2022 the General Conference on Weights and Measures voted to let the gap between Earth-based time and UTC grow beyond 0.9 seconds in or before 2035. At its meeting of 13 to 15 October 2026 it decided the gap may reach 3,600 seconds (one hour), from 20 May 2027, as of November 2026.
Where does the unevenness come from? In 1695 Edmond Halley noticed that the Moon seemed to be speeding up compared with ancient eclipse records, and later work showed that the real change was Earth's spin slowing down, a drag called tidal braking. Earth spins faster than the Moon orbits, so the tidal bulge runs ahead of the Moon and tugs it forward. The Moon's gravity pulls on Earth's tidal bulge in return, which gives the Moon energy and lets it spiral away while Earth's spin slows. The spin Earth loses is not destroyed, because angular momentum is slowly passed to the Moon at a rate proportional to r to the power minus six, where r is the Moon's distance, so the transfer was far faster when the Moon was closer. The Moon's pull raises tides, and the tidal effects the Moon has on Earth's rotation are making Earth's days longer.
Tides raised by the Moon and Sun add approximately 1.7 milliseconds to the length of a day each century. Scientists bounce laser beams off the Moon, and changes in the beam's round-trip time reveal how fast the Moon is moving away, approximately an inch (3.8 centimeters) a year. The past can be read as well, because 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, and days now are a few hundredths of a second longer than they were around 1200 B.C. The speed of Earth's rotation is not constant, so the day does not stay exactly the same length.
Copernicus said the Sun is at the center and the Earth rotates daily, and that opposed the older view of Ptolemy that the Earth sits motionless at the center. His Sun-centred model of 1543 was no more accurate than Ptolemy's, and it appealed mostly for its elegance, and only after Galileo did a community of working astronomers accept a moving Earth. The Sun looks like it moves across the sky, but it is really the Earth that turns, toward the east. 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, the first simple, direct proof of Earth's rotation. A Foucault pendulum at the North Pole keeps swinging in one plane while Earth turns beneath it, and the twisting effect is greatest at the poles, gets weaker toward lower latitudes and disappears at the equator. 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.
A sundial and a clock rarely agree, since sundial time can run ahead by as much as 16 minutes 33 seconds, around 3 November, or behind by as much as 14 minutes 6 seconds, around 11 February. The sundial-clock gap, called the equation of time, has two causes: Earth's axis is tilted approximately 23.44 degrees, and its orbit is slightly oval (eccentricity approximately 0.0167), so the Sun's apparent daily motion is uneven. On Mars the difference between sundial time and clock time can reach 50 minutes, because Mars has a much more oval orbit. Moving the clock on purpose is a different 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. Supporters say it cuts lighting and heating needs, but its real effect on total energy use is heavily disputed, so the question is still argued.
Before standard time was adopted, towns set their clocks by the Sun's path across their own meridian. Railway companies in Great Britain adopted Greenwich Mean Time in 1847, and railroad travel made standard time necessary elsewhere, with railroad leaders asked to design a uniform time system. In the United States, standard time zones began on Sunday, November 18, 1883, when railroads started using them. Time zones are areas that share one standard time, and their borders follow local laws rather than straight lines. In 1884 delegates at the International Meridian Conference voted to make the Greenwich Meridian the world's Prime Meridian, and they also recommended a global time zone system based on Greenwich Mean Time. The conference chose a line through the telescope at the Royal Observatory in Greenwich as the prime meridian. That line runs through Greenwich, England, and marks 0 degrees longitude.
In humans the master clock is in the brain, in a spot called the SCN, which sits in the hypothalamus just above the optic chiasm. Special light-sensing cells containing melanopsin in the retina send it signals, and its cells keep each other in step. Light, seen by the eyes, resets the body clock and drives how much melatonin the SCN makes. In the evening the brain's master clock tells the body to make more melatonin, a hormone that makes people sleepy. When melatonin is taken, it works opposite to light: taken in the afternoon or early evening it moves the body clock earlier, and taken in the morning it moves it later. In 2007 the World Health Organization's cancer research agency listed shift work that involves circadian disruption, a disruption of the body clock, as a probable carcinogen, and a later report said even dim light intrusion is likely to disturb sleep and suppress melatonin.
The evidence for these clocks was built slowly. Most living things, including animals, plants and microorganisms, have body clocks called circadian rhythms that follow a roughly 24-hour cycle. In 1729 Jean-Jacques d'Ortous de Mairan kept a heliotrope plant in constant darkness and saw its leaves still open and close each day. In 1823 Augustin de Candolle found that Mimosa leaves kept in constant conditions cycled every 22 to 23 hours, not 24. In 1971 Ron Konopka and Seymour Benzer found the first body-clock mutation in fruit flies and named the gene period. How do we know the SCN is the master clock? Hamsters whose SCN was destroyed lost their daily activity rhythms, and when an SCN from another hamster was transplanted in, the animal took on the donor's rhythm. Plants have body clocks too, which tell a plant what season it is and when to flower.
Blue LED lighting suppresses melatonin production approximately five times more than orange-yellow sodium lights, and light from electronic devices at night can confuse body clocks. 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, as of November 2026. Globally, light pollution increased by at least 49% from 1992 to 2017, and by 16% between 2014 and 2022, though not evenly everywhere. Nearly half of Earth's species are nocturnal, and bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians. Humans need natural darkness for good sleep and health, and many animals, like owls, are awake at night and rest in the day.
Other worlds show how strange a day can be. Venus has no moons and turns very slowly backward, and scientists explain this as a tug-of-war between the Sun's tides, which tend to lock it, and heating of its massive atmosphere, which pushes the other way. 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, because the Sun's motion adds to the turn. On Venus the Sun would rise in the west. Venus and Earth meet every 583.92 days, which equals 5.001444 Venusian solar days, so the same face of Venus points at Earth at each close approach, although the idea of a spin-orbit resonance with Earth has been discounted.
A tidally locked world need not show one face forever, because on an oval orbit the spin tends to lock to the orbital speed at the closest point to the star, which is why Mercury turns three times per two orbits. From some parts of Mercury's surface the morning Sun appears to rise briefly, set and rise again. One day-night cycle on Mercury, from sunrise to sunrise, lasts approximately 176 Earth days, and with no atmosphere its surface swings from approximately minus 180 to approximately 430 degrees Celsius. The planet Proxima Centauri b, found orbiting a nearby red dwarf in 2016, is almost certainly tidally locked, either with one side always facing its star or in a 3:2 spin-orbit pattern like Mercury's, as of November 2026. Earth, by contrast, spins around once in approximately a day, and that day is getting longer.
Words to know
- sidereal: measured by stars
- eccentricity: how oval
- meridian: line of longitude
- circadian: about a day
- melanopsin: light-sensing pigment
- carcinogen: cancer-causing thing
