Welcome to You Ask Andy

Michael B. McBride, age 11, of Wichita, Kansas, for his question:

Does the moon cause the waves?

Some people think that the moon makes the waves of the ocean. But the truth is that the earth makes its own ocean waves, with some help from the sun and the winds. However, the moon does cause a lot of upheaval in the ocean. In fact, it heaves around a lot more water than the wind blown waves do. The moon causes the great global tides that move the waters of the whole earth    and even the earth itself.

Most of the planet Earth is covered by salty seawater    and water is a restless fluid. The least little thing upsets it and sets it moving far and wide in all directions. The blowing breezes ruffle up waves on its shiny surface. But the moon does even more. With the help of the sun and the earth, it makes the tides rise and fall around the world.

The force behind the scenes is gravity. The earth's gravity pulls at the moon and the moon's gravity pulls at the earth. The sun's gravity pulls at the earth and the moon    and the earth and the moon pull back at the sun. These two way tugs go on all the time. But the moon's pull is more than twice as strong as the mighty sun's, because the sun is 400 times farther away, and gravity weakens with distance.

Out little old moon is less than a quarter of a million miles from the earth. If the massive mighty sun were at this distance, its gravitational pull on the earth would be 27 million times stronger than the moon's    and goodness knows what this would do to our tides. As it is, the moon's tidal pull heaves up great bulges of our ocean waters.

The moon's pull is strongest where it passes directly overhead. Then it pulls up a high tide on the ocean below it. It also pulls at the whole planet and when the moon is directly overhead, the earth moves a little in that direction. This leaves another bulge of water lagging on the opposite side of the globe.

Hence, the earth has two high tides, one under the moon and another on the far side of the globe. The bulging high tides leave troughs of low water between them    and these are the low tides. So at all times, around the world there are two high tides and two low tides. Now let's see why these ups and downs chase each other around and around the globe.

As the earth rotates around its axis, the moon appears in the east, rides overhead and sets in the west. During the 24 hour day, it circles the sky around the entire globe. This means that the center of the high tide bulge. also circles the globe every 24 hours. Following behind it comes a low tide, then another high tide and another lost tide.

Every 24 hours use get a high tide and a low tide, then another high tide and low tide Meantime, during the lunar month, the earth, the moon and the sun change positions. When they are in a line, the sun and the moon pull together and we get the highest tides of the month.

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