A shadow is a dark shape that forms when something blocks light, and the Sun makes shadows. Light usually travels in a straight line, drawn as rays, and a shadow forms where an opaque object blocks light from the Sun or another source. 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, and the Sun is highest when the shadow is shortest.

The Sun makes shadows that change during the day. The Sun rises in the east and sets in the west, and it seems to move across the sky each day because Earth rotates on its axis. A sundial is a tool that uses the Sun to tell the time of day. Its gnomon is the part that casts the shadow. A sundial works because the Sun seems to move across the sky, so its shadow lands in different places as the day goes on.

The Moon can get in the way of the Sun's light and cast its shadow on Earth, and that is a solar eclipse. An eclipse casts two shadows, and the umbra is the dark center. Only people in the center of the Moon's shadow see a total solar eclipse. In an annular eclipse the Moon is too far away to cover the whole Sun, so a bright ring of Sun shows around the Moon. There is not a solar eclipse every month, because the Moon's path around Earth is tilted. Never look straight at the Sun without special eye protection.

A shadow of Earth falling on the Moon is a lunar eclipse. A lunar eclipse happens at a full moon, when Earth is between the Sun and the Moon, and Earth's shadow covers up the full Moon. The Moon's phases are not caused by Earth's shadow falling on the Moon.

The Moon is always half lit by the Sun, and 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

• gnomon: shadow caster
• annular: ring-shaped
• umbra: dark center
• terminator: day-night line