Maria AVaZ, age 10, of Williamsport, Penna., for her question:
How does the moon control the tides?
All the heavenly bodies are forever pulling at each other in a gigantic tug of war. We cannot see the rope between the moon and the earth, between the earth and the sun, between the sun and the moon, but nevertheless the mighty tug goes on day and night. The tugging power is the force of gravity. The more massive a heavenly body, the greater its force of gravity. However, gravity weakens with distance. The gravity of the big sun is much, much stronger than the gravity of the little moon. Both pull at the earth. But the sun is so much further away that it cannot pull at us as hard as the moon.
The moon's gravity pulls at the earth so hard that it lifts up the face of the ocean several feet. The moon, of course, seems to pass over the sky, passing first over New York and three hours later over San Francisco. Its pull of gravity is strongest when it is directly overhead. Where it pulls hardest, the oceans are lifted up in high tides. As the moon seems to go over the sky, the high tides follow it clear around the globe.
Actually, of course, the moon does not go around the world every day as it seenme to do. But the earth does turn on its axis once in every 24 hour day. Our globe faces the moon with first one side, then another. The side facing the moon feels the pull of its gravity and we get high tides.
Directly under the moon, a great bulge of water is pulled up in high tide. This extra water is dragged from the oceans half way around the earth. Here the water is low in a giant trough. Where there are bays and beaches, the water washes away in low tide.
This trough of low water causes another bulge to rise up on the side of the globe opposite from the moon.
As we have high tide, the people on the opposite side of the world also have high tide. In between the two high tides are low tides. As we face the moon with first one side, then another, the high tides and low tides follow each other around and around the globe. And on every beach, two high tides and two low tides follow each other up and down every 24 hours.
The moon orbits around the earth once every month. Sometimes it is between us and the sun, sometimes we are between it and the sun. Sometimes it is on one side of the earth and sometimes on the other. The gravity of the sun also tugs at the earths tides, though not as strongly as the moons gravity. Sometimes they pull at us in the same direction and sometimes in different directions.
When the sun and moon pull in the same direction we have the highest high tides of the month. This is when the moon is new and the sun, moon and earth are in a straight line. When the moon is at the side of the earth, it pulls in the opposite direction from the sun and we get the lowest high tides of the month.