Craig Lyon, age 9, of Gastonia, North Carolina, for his question:
What makes the earth turn around?
They used to say that love makes the world go around. We certainly know that loving kindness makes the world more pleasant. When we all behave this way, everything goes along smoothly. But, of course, this does not make our lovely old earth spin around like a top. That happens because it must obey a great traffic law of the universe.
We know that the earth turns around its axis, which is a straight line through the middle from pole to pole. It makes one complete turn every calendar day, which is about 24 hours long and it never pauses for breath. This never ending spin is called rotation. As the earth rotates, it turns first one side then another side to face the sun. When our side faces the sun, we are in daylight. Then we rotate around to face the dark night sky.
As the earth turns, it sweeps day and night around the globe. But this does not explain how it turns. This is ruled by great traffic laws that control all the moving planets and starry suns in the whole universe. When our earth was born, perhaps something gave it a push to start it spinning. If this happened, it might go on spinning forever. But this is not the whole story of what keeps it turning, turning day and night.
It is pushed around by the mighty, invisible force of gravity. We are used to the earth's gravity. It hugs us to the solid ground and makes us tumble when we stumble. It pulls down falling stones and raindrops. This mighty force reaches out from down there at the very center of the globe. It reaches up above the ground. But it grows weaker as it reaches farther from the earth.
The sun, the planets and the moons also have built in gravity. The sun's is strongest because the sun is so massive. The moon's gravity is weaker than ours. because the earth is much heavier than the moon. But the rules are the same. The gravity of the sun, earth and moon reaches out and grows weaker as it goes.
Our gravity tugs at the sun and the moon and their gravity tugs at the earth. The tug between the earth and moon is very strong because they are close neighbors. The moon's gravity is strong enough to pull up our mighty ocean tides. And, it also helps to keep our old world turning around and around.
The moon's gravity helps keep the earth turning by tugging one side, then another side on and on around the globe. Sometimes the moon gets in line with the sun. Then the faraway gravity from the sun helps the moon to turn the earth and together they pull up our highest tides. The moon happens to be just the right distance to keep our world turning around once every 24 hours. If it were farther away, its gravity would be weaker. Then the earth would rotate more slowly and our days would be longer.