Nancy Preston, age 9, of Albany, New York, for her question:
Why isn't the moon always full?
The ball shaped moon turns around and around to give all its sides a chance to face the sun. But somehow it manages to keep the same side facing the earth, so we never see the other half of the big ball. What's more, most of the time we see only part of our half. We see only the part that is bathed in golden sunshine. Once each month the sun shines all over our half and we see the full moon. But it doesn't last because the moon moves on and the sunshine creeps around to the other side.
Notice how the morning sun shines on the east side of a house. Then the west side casts a shadow. In the evening, the west side gets the sunshine and the east side gets the shade. Now stretch your mind way up to the moon. The beaming sun is up there in the sky also 93 million miles away in space. Its golden sunbeams fan out in all directions. If you turn to face it, they fall on your face and your shadow falls behind you. Sunbeams also fall on half the moon and the other half is in shadowy darkness.
If the moon stayed exactly in a certain position, the sun would keep shining all over the side that faces the earth. Then we would see a full moon all the time. But up in the sky, nothing stays still. The earth and the moon swing around like a couple of dizzy dancers and never rest for a moment. Each year they travel together in a huge circle around the sun. As they go, the moon keeps swinging in circles around the earth. Each of these moon orbits takes about a month. It gives the sun a chance to shine on all sides of the round moon but we see only one side. And as daylight and darkness creep across our side of the moon, we see only the sunny part.
Once each month, the orbiting moon passes between us and the sun. Then daylight covers the half we never see and our side is hidden in shadowy darkness. But not for long. The moon moves on its way, a thin sliver of sunshine peeps around one side ¬and we welcome the New Moon. As the moon moves on, the golden sliver grows bigger. In about two weeks, the moon is around the opposite side of its orbit. Now the earth is between the moon and the sun. The sun shines all over the side that faces us and we see the Full Moon.
But not for long. The orbiting moon moves on its way and gradually turns its other side to face the sun. The daylight creeps around to the side we never see. And as this happens, the darkness creeps across our side of the moon. Shadows begin to hide one side of the Full Moon. Each night they hide a little more and the round pumpkin moon shrinks to half and quarter size. At last the moon passes between the earth and the sun once again. And one again we see the Old Moon disappear, just before the next New Moon appears.
The earth spins around on its axis once every 24 hours. So daylight and darkness move around our globe every calander day. But the moon takes almost a month to spin on its axis. Each lunar day lasts about two weeks, and so does each lunar night. Meantime the moon makes an orbit around the earth, keeping the same side turned towards us. This is why we see the sunlit areas change and why the round pumpkin moon appears only once each month.