Why is it light, and then dark? The Earth spins. It spins 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. One day and one night together take about 24 hours.

The Sun is a star. It is the only star in our solar system, and it is the biggest object in it. The Sun is very far away, about 93 million miles (150 million kilometers) from Earth. Its light still reaches us and makes our days.

Many people think the Sun moves across the sky. It only looks that way. The Earth is what turns, and it turns toward the east. The Sun seems to rise in the east in the morning and set in the west in the evening.

Twilight is the glow in the sky after sunset and before sunrise. The Sun is out of sight, but the air still scatters its light. At sunset, the sky and clouds can glow orange and red.

Some people think the tilt of the Earth makes day and night. It does not. Day and night come from Earth's spin. The 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.

Other planets have days too. 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.

Some days are very odd. Mercury has almost no air to hold the heat, so it is very hot by day and very cold at night. Its day-night cycle, from sunrise to sunrise, lasts about 176 Earth days. Venus spins in the opposite direction from most planets, so on Venus the Sun would rise in the west.

Uranus is tipped on its side. Its poles get decades of sunlight, followed by decades of darkness. Compare that with Earth, where the Sun comes and goes about every 24 hours.

Living things follow day and night too. Animals that are active in the daytime are called diurnal. Humans, squirrels and songbirds are diurnal. Many animals, like owls, are awake at night and rest in the day.

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. People sleep best at night, when it is dark.

Words to know

• circadian: 24-hour body clock
• diurnal: active by day
• twilight: evening glow
• axis: spin line

The short version

Day and night come from Earth's spin. The Earth turns around once in about a day. Daytime is when sunlight reaches the ground, and the side facing away from the Sun is dark. One day and one night together take about 24 hours. Earth's tilt does not make day and night. It gives us the seasons.

Quick facts

• The Earth spins around once in about a day.
• One day and one night together take about 24 hours.
• The Sun only looks like it moves. It is really the Earth that turns, toward the east.
• Earth's axis is tilted about 23.4 degrees, and that tilt gives us the seasons.
• Most living things have body clocks called circadian rhythms that follow a roughly 24-hour cycle.