Lana Bastianutti, age 12, of Kingston, Ontario, Canada, for her question:

WHY DOES THE MOON HAVE DIFFERENT SHAPES?

Each month the moon seems to change from a thin crescent to a full circle and back again. These apparent changes in the moon's shape and size are actually different conditions of lighting called phases.

The moon seems to change shape because we see different parts of its sunlit surface as it orbits the earth. Half the moon is always lighted by the sun's rays except during eclipses. Sometimes the far side is in full sunlight even though it is out of view. The moon is then between the earth and the sun and we have a darkened phase called the new moon.

When the earth is between the moon and the sun we can see the entire sunlit side and we call this a full moon. For two weeks after a full moon we say it is waning as it seems to get smaller. Then for two weeks, as it seems to grow in size, we call it gibbous.