A shadow is a dark shape that forms when something blocks light. The Sun makes shadows that change during the day. Near sunrise and sunset, when the Sun is low in the sky, shadows can be very long. Around midday, when the Sun is high, shadows are short. The Sun is highest in the sky when shadows are shortest. Each day the Sun seems to move across the sky because Earth rotates on its axis.

The Moon can also get in the way of the Sun's light and cast its shadow on Earth. A shadow of the Moon falling on Earth is a solar eclipse. The umbra is the dark center of an eclipse shadow. Only people in the center of the Moon's shadow see a total solar eclipse. Never look straight at the Sun without special eye protection, even during an eclipse. Sunglasses do not make it safe.

There is not a solar eclipse every month, because the Moon's path around Earth is tilted. A shadow of Earth falling on the Moon is a lunar eclipse. It happens at a full moon, when Earth is between the Sun and the Moon. During a lunar eclipse, Earth's shadow covers up the full Moon.

The Moon is always half lit by the Sun. How much of that lit half we can see changes as the Moon moves around Earth. The Moon has a day side and a night side, and the line between the lit and dark halves is called the lunar terminator. On the Moon, when the Sun is low, shadows of craters and mountains get long, like shadows on Earth when the Sun is low.

Words to know

• eclipse: a sky shadow
• umbra: dark center
• terminator: day-night line
• rotates: spins around