Day and night come from Earth's spin. The Earth turns around once in about a day. Daytime is when sunlight reaches the ground. The side of the Earth that faces away from the Sun is dark. The line between daylight and darkness on Earth is called the terminator. One day and one night together take about 24 hours. Daytime is a little longer than night: it averages slightly more than half of the 24-hour day.

You may have heard that day and night are always exactly 12 hours each. In fact, near the equinoxes the day at the equator is about 12 hours and six and a half minutes. In March and September the Sun is directly over the equator at the equinox, and day and night are about equal.

The Sun seems to rise in the east and set in the west, so it looks as if it moves across the sky. It is really the Earth that turns, and it turns toward the east. Nicolaus Copernicus determined that the Earth rotates daily on its axis. He published his system in De Revolutionibus in 1543.

Earth's axis is tilted about 23.4 degrees compared with its path around the Sun. That path takes 365.25 days. Some people think the tilt makes day and night, but it does not. Day and night come from Earth's spin, while seasons come from the tilt. Earth's tilt points the same way throughout the year. A hemisphere's summer solstice is its longest day, and its winter solstice is its shortest day.

Other worlds have days of their own. They can be very short or very long. One turn of Mars takes about 24.6 hours, close to a day on Earth. Venus spins in the opposite direction from most planets, and its spin takes 243 Earth days. But a solar day on Venus, from one noon to the next, lasts about 117 Earth days. That is much shorter than its 243-day spin, because Venus spins backward and the Sun's motion adds to the turn.

Mercury is stranger still. Its day is odd in more than one way. It 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. One day-night cycle there, from sunrise to sunrise, lasts about 176 Earth days.

Uranus has a day of only about 17 hours 14 minutes. But the planet is tipped on its side: its equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees. Its poles get decades of sunlight followed by decades of darkness.

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. Standard time zones began in the United States 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.

Most living things have roughly 24-hour body clocks, called circadian rhythms. Species, both nocturnal and diurnal, evolved with natural cycles of dark and light. In humans the master clock is in the brain, in a spot called the SCN. In the evening it tells the body to make more melatonin, and melatonin rises at night and falls in the day. Traveling across time zones causes jet lag, because the body clock no longer matches the local day and night. Humans need natural darkness for good sleep and health, and even dim light at night can disturb sleep and melatonin.

Light at night, even 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. Animals that are active in the daytime are called diurnal, and many animals, like owls, are awake at night and rest in the day. Both kinds of animals evolved with natural cycles of dark and light.

Words to know

• meridian: north-south line
• circadian: 24-hour body clock
• nocturnal: active at night
• terminator: day-night line
• equinox: equal-day time