On the Moon, the sky is black, even in the daytime. On Earth, the daytime sky is blue. What is different? The answer has to do with air.

The Moon has only a very thin exosphere and no air to scatter sunlight. Scatter means to send light off in many directions.

Sunlight looks white, but it is really made up of all the colors of the rainbow. A glass prism can split white light into those colors.

Light moves in waves. Red light has the longest waves, and blue and violet light have much shorter waves.

Earth's air is full of tiny bits called molecules. Most of them are nitrogen and oxygen. They are too small to see, and they bounce sunlight around the sky.

Blue light gets bounced around the sky more than the other colors, so the sky looks blue. Look near the horizon, though, and the sky fades to a lighter blue or white.

Clouds look white for a different reason. The tiny drops in them bounce all the colors of light about equally.

When the Sun is low in the sky, its light has to go through more air to reach your eyes. At sunset the sky turns orange and red.

Never look straight at the Sun. Even dark sunglasses do not make it safe for your eyes.

Words to know

• scatter: bounce about
• molecules: tiny air bits
• prism: light splitter
• horizon: the far edge