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.

The Sun is a star. It is the only star in our solar system. It is also the biggest object in it. The Sun is very far away, about 93 million miles (150 million kilometers) from Earth.

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 and set in the west.

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

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. Some hunt, find mates or hide from predators at night.

Other planets have days too. One turn of Mars takes about 24.6 hours. That is 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. One day-night cycle there, from sunrise to sunrise, lasts about 176 Earth days.

Venus spins in the opposite direction from most planets. On Venus, the Sun would rise in the west. One spin of Venus takes 243 Earth days. That is longer than a Venus year of about 225 Earth days.

Uranus is tipped on its side. Its poles get decades of sunlight, followed by decades of darkness. Here on Earth, one day and one night together take about 24 hours.

Words to know

• diurnal: active by day
• twilight: evening glow
• scatters: bounces around