Ellen Bostick, age 12, of Brooklyn, N.Y., for her question:
Why do the tides change?
The tides of the earth are pulled to and fro by the force of gravity. The earth feels the tug of gravity from her close neighbors, the sun and the mo,~. The larger the heavenly body, the greater its pulling power. Hence, we woul. expect the earth to feel more pull from the giant sun than from the little moon. But this is not so. The mighty power of the sun is strong enough to hold the earth in its orbit, But when it comes to the pull which causes the ocean tides, the gravity of the moon is almost twice as strong as the sun.
This is because the power of gravity weakens with distance. The big sun,over 93million miles away, is more than a million times bigger than the earth.The little moon is only one fourth the size of the earth. But its average distance from us is about 235,000 miles ‑ some 370 times closer than the sun.The gravity of the sun and moon work together to fern, our tides, but two thirds of the pulling power is exerted by the moon.
The gravity of the moon pulls at the earth in a straight line. The tug i4 strongest ran the side of the earth which happens to be directly under the moon. And, as everyone knows, the moon is always moving to a different plac in the sky. It rises and sets. This is because the earth turns on its axis once every 24 hours. Each side turns to face the moon and then goes ar‑;und until it is facing the opposite direction.
This means that first one side of the earth, then another feels the pulling power of the moons gravity. The ocean below is pulled up several feet into a high tide which follows the moon around the earth. High tides, however, wash up on the beaches twice in every day and night period ‑ and the moon passes overhead but once, This is because the moon s pull effects the opposite side of the earth. The high tides under the moon fall away into troughs which are the low tides. And masses of water pile up on the opposite side of the earth. This forms two high tides, on opposite sides of. the earth with troughs of low tides between them, As the earth turns, facing the moon with first one side then another, the two high tides and the two low tides follow each other around and around the world.
The moon also makes a circle around the: world, once every month. The new moon is in a line between us and the; sun. At full moon, we are in a line between the sun and the moon. At such times, the sun and the mc,on are pulling at the earth from the same direction. This united effort gives us the spring tides, the highest high tides of the month. Between new moon and full moon, the moon is more or less at right angles to the earth rind the sun. Then it is pulling at the earth in a different direction from the sun, Some of the gravity which causes our tides is cancelled out. Then we get the neap tides, the lowest high tides of the month.
Day and night, the high tides chase each other round and around the earth, High tide washes up on your favorite beach twice every day and night. This endless parade of high and low water is caused by the earth turning on its axis. Once every 24 hours each side turns to face the moon and so feel the pull of gravity which heaps up the ocean in high tides.