Why is it light, and then dark? The Earth spins. It spins around once in about a day.

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

The Sun is the biggest object in our solar system. Nothing else near us is as big.

The Sun is very far away. It is about 93 million miles (150 million kilometers) from Earth.

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. That is how long it takes the Earth to spin once.

You may have heard that the Sun moves. In fact, it only looks that way. The Sun looks like it moves across the sky. It is really the Earth that turns.

The Earth turns toward the east. The Sun seems to rise in the east in the morning. It seems to set in the west in the evening.

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. A day on Jupiter is under 10 hours.

Animals that are active in the daytime are called diurnal. Humans, squirrels and songbirds are diurnal.

Many animals, like owls, are awake at night. They rest in the day. People sleep best at night, when it is dark.

Words to know

• diurnal: active by day
• spins: turns around
• star: a far sun