Look out the window at noon, and then look again at midnight. The light is very different. The reason is the Earth, which spins around once in about a day. Daytime is when sunlight reaches the ground. The side of the Earth facing 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.

Watch the sky from morning to evening, and the Sun seems to travel. The Sun seems to rise in the east in the morning and to set in the west in the evening. It looks as if the Sun moves across the sky. In fact, it is really the Earth that turns, and it turns toward the east.

The Sun is a star. It is the only star in our solar system. 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.

Two ideas are easy to mix up here. Some people think the tilt of the Earth makes day and night, but it does not. Day and night come from Earth's spin. 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. The spin and the trip around the Sun are two different motions. The 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. A hemisphere's summer solstice is its longest day. Its winter solstice is its shortest day. When it is summer in one hemisphere, it is winter in the other.

Other planets have days too. One turn of Mars takes about 24.6 hours, which is close to a day on Earth. Venus spins in the opposite direction from most planets. On Venus, the Sun would rise in the west. Mercury is very hot by day, up to about 430 degrees Celsius, and very cold at night, because it has almost no air to hold the heat. One day-night cycle there, from sunrise to sunrise, lasts about 176 Earth days. Uranus is tipped on its side. Its poles get decades of sunlight followed by decades of darkness.

Animals that are active in the daytime are called diurnal. Humans, squirrels and songbirds are diurnal. Some animals are active at night to escape the heat of the day, and some hunt, find mates or hide from predators at night. Most living things have a roughly 24-hour body clock.

Most living things, including animals, plants and microorganisms, have body clocks called circadian rhythms. These follow a roughly 24-hour cycle. In humans the master clock is in the brain, in a spot called the SCN. In the evening the brain's master clock tells the body to make more melatonin, a hormone that makes people sleepy. Melatonin rises at night and falls in the day.

Time zones are areas that share one standard time, and their borders follow local laws rather than straight lines. Standard time zones began in the United States on Sunday, November 18, 1883, when railroads started using them. Traveling across time zones causes jet lag. The reason is that the body clock no longer matches the local day and night.

Bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians. Light at night, even from electronic devices, can confuse the body clock and cut melatonin.

Words to know

• terminator: day-night line
• equinox: equal-day time
• melatonin: sleepy hormone
• hemisphere: half of Earth
• circadian: 24-hour body clock