A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

# Why are there high tides twice a day when the earth rotates beneath the moon only once a day?

December 16, 1988

Dear Cecil:

Why are there high tides twice a day when the earth rotates beneath the moon only once a day? In diagrams it appears the moon's gravity causes the earth's oceans to bulge (creating a high tide) not only on the side toward the moon, but also on the side away from the moon. I've heard some unconvincing explanations for this, including: "the water on the far side is flung away from the earth" (why?); "the moon attracts the earth, and the water on the far side is left behind" (why isn't the water on the far side attracted too?); and "the earth and the moon both revolve around a common point" (I know that, but what does that have to do with the question?). Please help.

Cecil replies:

Not to discourage you, Kathleen, but this makes 22 questions from you in three months. Think quality, not quantity. This isn't a scrap drive.

The following homely metaphor is sometimes used to explain why there are two tides: the earth and moon, which are really dual planets, are like two figure skaters spinning around one another while holding hands. Centrifugal force naturally tends to pull them apart, but their clasped hands (i.e., their mutual gravitational attraction) keep them together. Similarly, centrifugal force tends to fling the ocean outward on the side of the earth away from the moon. On the near side, the water is tugged moonward by lunar gravity.

There's just one problem with this explanation. It's wrong. Cecil has consulted with the physics division of the Straight Dope Science Advisory Board and is satisfied that centrifugal force (OK, inertia, if you want to get technical) has nothing to do with why there are two tides.

The real reason is this. The pull of gravity drops off rapidly with distance. Lunar gravity tugs on the side of the earth facing it a lot, on the earth itself a medium amount, and on the opposite side of the earth relatively little. In short, the far-side tide is a result of the moon attracting the earth, leaving the ocean behind. Which, looking back at your letter, I guess you already knew and didn't find convincing. If so, Kathleen, come on. Would I lie to you?

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