The tides are the result of forces between the earth, the moon and the sun. These three stellar bodies act as two systems, the earth - moon system, and the sun - earth system. To understand the tides it is sensible to start with one of these; we select the earth-moon system.
It is commonly said that the moon rotates around the earth. A more accurate statement is that moon and earth rotate around their common centre of gravity. Because the earth has so much more mass than the moon, this common centre of gravity is actually inside the earth, but it is still true that the moon makes the earth rotate around this centre of gravity, just as the earth makes the moon rotate around the same centre of gravity. (In reality the common centre of gravity is much closer to the centre of the earth than where it is shown in the diagram, which is not drawn to scale anyway.)
(The same principle applies to the sun-earth system. The large body in that system is the sun, and the centre of gravity of the sun-earth system is situated inside the sun.)
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© 2000 M. Tomczak