A shadow is a dark shape that forms when something blocks light. A shadow has the same shape as the object that makes it. A shadow made by sunlight often has a slight bluish tint. Light usually travels in a straight line.

We draw light's straight lines as rays. A shadow forms where an opaque object blocks light from the Sun or another source. Things that do not let any light pass through are called opaque, and they make shadows. Things that let light through so you can see clearly through them are called transparent.

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, and shadows can be very long. Around midday, the Sun is high in the sky, and shadows are short. The Sun is highest in the sky when shadows are shortest. People can find that moment by looking for the shortest shadow.

A sundial is a tool that uses the Sun to tell the time of day. The part that casts 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 and casts its shadow on Earth. A shadow of the Moon falling on Earth is a solar eclipse. A shadow of Earth falling on the Moon is a lunar eclipse. In a lunar eclipse, Earth's shadow covers up the full Moon.

The dark center of an eclipse shadow is called the umbra. Inside it, the Sun's light is completely blocked. 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.

Words to know

• opaque: no light through
• gnomon: shadow caster
• umbra: dark center
• rotates: spins around

The short version

A shadow is a dark shape that forms when something blocks light. Light usually travels in a straight line, so a shadow has the same shape as the object that makes it. The Sun makes shadows that change during the day, and the Moon casts a shadow on Earth during a solar eclipse.

Quick facts

• A shadow forms when an opaque object blocks light.
• Shadows are short around midday and can be very long near sunrise and sunset.
• A sundial uses the Sun's shadow to tell the time of day.
• A solar eclipse is the Moon's shadow falling on Earth.
• A lunar eclipse is Earth's shadow falling on the Moon.