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Where the tides come from

We've known for a long time

Filename: FergAstro.htm

I'm sure I was taught, many, MANY years ago, that the tides arose from the moon pulling the water towards itself. Well, I learned recently, that may be so... but would only account for about 4" of tide, and in any case doesn't explain the high tides on the side of the earth away from the moon.

No, a friend of fifty years told me the other day. It is to do with the fact that the earth spins around the moon in the same way that the moon spins around the earth... though it must be admitted that the "wobble" of the earth is much less than the wobble of the moon.

And it turns out that whoever told me it was the moon pulling the water should have known better. (Maybe she did... and I misunderstood?)

The correct answer has been known for a long time. The following comes from the third edition of James Ferguson's Astronomy. (The first edition: 1756) (Newton's work, on which Ferguson based his work: 1687)

Image of title page, Ferguson's Astronomy, third edition, 1764
Image of Plate IX

(I owe Mr Ferguson and his engraver an apology, if you notice the slight discontinuity just to the left of where it says "Fig. II" on the diagram.

Image of page 147 from Ferguson's Astronomy, third edition, 1764
Image of page 148 from Ferguson's Astronomy, third edition, 1764
Image of page 149 from Ferguson's Astronomy, third edition, 1764

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