Larry Lair, age 13, of Des Moines, Iowa, for his question:
What exactly causes the high tides?
The ocean tides were observed by the shore dwellers and by the daring seafarers of early history. They soon saw that one of the regular daily high tides tallies with the passing of the moon overhead and they concluded that all the rising and falling tides are controlled by the moon.
Our knowledge of the tides has progressed a long way since our early ancestors discovered their relationship to the moon. They assumed that the purpose of this heavenly body was to cause high and low tides to wash up and down upon the beaches of the Earth. In most places, two high and two low tides occur in a 24-hour day and during a lunar month their heights vary somewhat with the changing phases of the moon.
The explanation of all this had to wait for the thoughts of a brilliant young man named Isaac Newton. As a student in his early 20s, Newton figured out the laws of gravitation and in some 300 years no one has corrected his figures. We had to grasp his Explanation of the gravitational attraction between heavenly bodies before the space age could begin. His laws of gravitation are used every time our computers figure the trajectory of a spacecraft.
And Newton made it possible for us to understand how and why the moon governs our ocean tides. The force of gravitation reaches out beyond a heavenly body, diminishing as it goes. All the satellites of the Solar System are held in orbit by the gravity of their planets and their lesser gravity exerts a pulling attraction on their parents. The Earth tugs at the moon and our sizeable satellite exerts a sizeable tug back at the Earth.
The spinning Earth turns first one side then another toward the moon. When the moon is overhead, it pulls up a gigantic bulge of our oceans' waters. A trough of lower water is left on each side of this high tide and another bulge of high water falls back and piles up on the opposite side of the globe. As the moon rides over the sky, these two high and two low tides chase each other around the globe.
Our heaving ocean tides are modified by other factors near and far, but most of their energy is the pulling gravitation of the moon. Our orbiting satellite rises and sets some 50 minutes later each night, and its pulling power takes a little while to work. Below it, the high tide lags a little behind the passage of the moon overhead and the other tides follow the leader. Every 24 hours, most shores get a high and a low tide, then another high and low tide and the cycle starts about 50 minutes later each day.
The massive sun is much farther away, but it also exerts some tidal pull upon our oceans. Twice in the lunar month the Earth is in a line with the sun and the moon. Then the sun and moon tug at us from the same direction and their united pull causes the highest high tides of the month. Then the orbiting satellite passes to one side and exerts its pull from another direction. The sun and moon tug from different directions and cancel out some of their pull at the Earth. Then we get the lowest high tides of the month.