Earth spins around once in about a day, and one day and one night together take about 24 hours. The speed of Earth's rotation is not constant, though, so the day does not stay exactly the same length.

Even the word day has more than one meaning, because Earth turns once relative to the Sun in about 24 hours, which is a solar day, while relative to distant stars one turn takes 23 hours, 56 minutes and 4 seconds, which is a sidereal day. The sidereal day is about 3 minutes 56 seconds shorter than the mean solar day.

Why would the Moon slow down a whole planet, when it is so much smaller? Earth spins faster than the Moon orbits, so the tidal bulge the Moon raises runs ahead of it. The Moon's gravity pulls on that bulge, and the bulge tugs the Moon forward. That gives the Moon energy, and the Moon spirals away while Earth's spin slows. This is why it moves away from Earth about an inch (3.8 centimeters) a year.

The Moon's pull raises tides, and the tidal effects the Moon has on Earth's rotation are making Earth's days longer, because tides raised by the Moon and Sun add about 1.7 milliseconds to the length of a day each century, which is a change too small for anyone to feel.

How can anyone know how fast the Moon is moving away? Scientists bounce laser beams off the Moon, and changes in the beam's round-trip time reveal how fast the Moon is moving away. Ancient days can be measured too, because scientists can read ancient day length from growth bands in corals and from layers in ancient rocks. Rock and fossil records suggest the day was only about 21 to 22 hours long some 600 million years ago. Days now are a few hundredths of a second longer than they were around 1200 B.C.

Other planets turn at very different speeds. One turn of Mars takes about 24.6 hours, close to a day on Earth. Jupiter spins fastest of all the planets, so a day on Jupiter is under 10 hours. A day on Uranus is only about 17 hours 14 minutes, but the planet is tipped on its side, with a tilt of 97.77 degrees.

Venus spins in the opposite direction from most planets, so on Venus the Sun would rise in the west. One spin takes 243 Earth days, but a solar day there, from one noon to the next, lasts about 117 Earth days, because Venus spins backward and the Sun's motion adds to the turn.

One day-night cycle on Mercury, from sunrise to sunrise, lasts about 176 Earth days, because Mercury turns three times for every two trips around the Sun. From some parts of its surface the morning Sun appears to rise briefly, set and rise again. Mercury is very hot by day and very cold at night, because it has almost no air to hold the heat.

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 about 23.4 degrees compared with its path around the Sun, and that tilt gives us the seasons. In March and September the Sun is directly over the equator at the equinox, and day and night are about equal, but a hemisphere's summer solstice is its longest day and its winter solstice is its shortest day.

On Earth, the day does not end in a snap. 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. At sunset, the sky and clouds can glow orange and red.

Daylight also sets a clock inside you. Most living things, including animals, plants and microorganisms, have body clocks called circadian rhythms 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 it and drives how much melatonin the SCN makes. Melatonin is a hormone that makes people sleepy, and it rises at night and falls in the day.

Humans need natural darkness for good sleep and health, and even dim light at night can disturb sleep and melatonin. Light at night from electronic devices can confuse the body clock and cut melatonin. Bright lights at night can also confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians.

Words to know

• sidereal: measured by stars
• bulge: swollen part
• circadian: about a day
• melatonin: sleepy hormone
• hormone: body chemical