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HOW COME? How the moon tugs on tides

How does the moon affect tides? asks reader Simon VargheseActually, the sun, the moon and all the planets of our solar system tug on the waters (and land) of Earth. But only the moon and the sun have significant effects on ocean tides. Their gravitational pull, combined with the spinning of our planet, are what makes Earth's seas rhythmically rise and fall.

Gravity goes hand-in-hand with mass, and the sun is 27 million times more massive than our moon. Still, it's the moon that exerts the biggest influence on our planet's sloshing waters. Gravity weakens over distance, and with each doubling of the distance between one body and another, the gravitational tug decreases to just 25 percent of its former strength. At only 240,000 miles away (versus 93 million miles to the sun), the moon exerts twice the tidal-generating force.

But tidal forces aren't exactly the same as gravitational pull. The tidal force that a given "particle" on Earth would experience is the difference between the moon's gravitational force and the force that would be felt if that particle were located at the Earth's center of mass.

The sun is so far away that its much greater gravitational pull doesn't change very much from one side of the our planet to the other. But the moon is very near, and the Earth is 8,000 miles wide.

Result: The waters on the side of our planet that happens to be facing the moon feel a stronger attraction than the waters on the far side of our planet.

Put it all together, and as the moon orbits and the Earth turns, the pull of lunar gravity makes ocean waters under the moon bulge up several feet. (When bulge meets land, it's thrust up even higher, as much as 40 feet.) Presto: high tide. Away from the tidal bulge, lower-than-average waters mean low tide.

But high tide on one side of the planet also means high tide on the opposite side. Why? The Earth's rotation creates a centrifugal effect, in the same way we feel "thrown outward" in a car rounding a sharp curve. So as the planet spins, ocean waters move outward on the side facing away from the moon.

When the sun, Earth and moon are aligned, as they are when the moon is new or full, the tides are especially high and especially low. These are called "spring" tides.

The moon's gravity isn't strong enough to simply lift the waters vertically. Instead, as it circles the Earth, the "horizontal component" of the moon's gravitational force becomes most important.

As the passing moon tugs on the ocean's waters from just ahead of them, they flow forward -- and bunch up.

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