Light usually travels in a straight line. When something blocks the light, a shadow forms. A shadow is a dark shape.

Some things do not let any light pass through. They are called opaque, and they make shadows. Other things let light through. You can see through them. They are called transparent.

The Sun seems to move across the sky each day. It rises in the east. It sets in the west.

The Sun makes shadows that change during the day. Near sunrise and sunset, the Sun is low. Shadows can be very long. Around midday, the Sun is high. Shadows are short.

A sundial is a tool. It uses the Sun to tell the time of day. The part that casts the shadow is the gnomon. The Sun seems to move across the sky. So the shadow lands in new places as the day goes on.

Sometimes the Moon gets in the way of the Sun. It casts its shadow on Earth. This is called a solar eclipse.

There is another kind of eclipse. In a lunar eclipse, Earth's shadow covers up the full Moon.

A sundial and an eclipse are alike. A shadow forms when something blocks light.

Words to know

• opaque: blocks all light
• gnomon: shadow caster
• eclipse: a sky shadow
• transparent: see-through