This was a mystery until Newton developed the first decent theory of gravity. The Moon pulls on the Earth, just like the Earth pulls on the Moon. The gravitational pull gets weaker the farther away something is from the puller. So the ocean near the Moon is pulled toward the Moon more than the average pull of stuff in the Earth toward the Moon. Likewise the part of the ocean farthest from the Moon is pulled most weakly toward it. So the distant part of the ocean and the close part each bulge out from the Earth- the distant part because it lags as the Earth accelerates toward the Moon and the close part because it leads.
During the daily spin of the Earth, some place on land goes past those bulges about twice. (Itís not exactly twice because the Moon is also orbiting around the Earth.) So the high tides occur about every 12 hours.
Although the Sun pulls on the Earth much more than the Moon does, the Sun is so far away that its pull doesnít change much from one side of the Earth to the other. The Sun tides are a bit weaker than the Moon tides. Sometimes both types of tides add together, giving extra big tides, and sometimes they partially cancel, giving weak tides.
The earth is also a little flexible. Not only does water slosh around the world with tidal motion, but the ground itself moves up and down with the tides. This effect is much smaller than the effect of water motion, but it has been noticed with sensitive equipment.
Mike W. (and a little from Tom J.)
(published on 10/22/2007)