A shadow is a dark shape that forms when something blocks light. Light usually travels in a straight line. We draw these lines as rays. A shadow forms where an opaque object blocks light from the Sun or another source. Things that let no light pass through are called opaque.

Sunlight takes about 8 minutes to reach Earth. The Sun makes shadows that change during the day. Each day, the Sun seems to move across the sky. It rises in the east and sets in the west. It seems to move because Earth rotates on its axis.

Near sunrise and sunset, the Sun is low in the sky. Shadows can be very long then. When the Sun is high, shadows are short. The Sun is highest when shadows are shortest. People can find that moment. They look for the shortest shadow.

A sundial is a tool that uses the Sun to tell the time of day. The part that makes the shadow is the gnomon. A sundial works because the Sun seems to move across the sky. So its shadow lands in different places as the day goes on.

Sometimes the Moon gets in the way of the Sun's light. It puts its shadow on Earth. That is a solar eclipse. In a lunar eclipse, Earth's shadow covers up the full Moon. Never look straight at the Sun without special eye protection, even during an eclipse. Sunglasses do not make it safe.

Words to know

• rays: lines of light
• opaque: no light through
• rotates: spins around
• eclipse: a sky shadow