A Moment of Science

And You Think

Some folks have a kind of middle-torso bulge known affectionately as “love handles.” Well, if you are such a person, take heart, the entire planet suffers from this same indignity.

In fact, any rotating body will have a tendency to bulge in the middle, as long as it’s made of a non-rigid substance. A spinning bowling ball stays round, but not a spinning water balloon. Though it seems pretty solid to us, the earth qualifies as a non-rigid body. Considered from a far-enough vantage point, earth’s molten interior, its oceans, and its shifting plates make it more like a water balloon than a bowling ball. Sure enough, the earth’s spin causes it to bulge.

The earth bulges around its equator by about twenty-seven extra miles, which is a three-tenths percent difference from its diameter measured pole-to-pole. The biggest planets in our solar system have even more dramatic love handles. Jupiter’s equator is seven percent larger and Saturn’s is eleven percent.

How does this happen? Think of the merry-go-round you find at playgrounds. The faster the dish spins, the harder it is for the kids on the outside to hold on. Kids in the middle have no trouble, because they are just turning around in one spot. Kids on the rim feel themselves flung outward.

Same with a planet. As it rotates, the poles are just turning in a circle, but the equatorial areas are being flung away. Over time an equatorial bulge develops. In some cases, such as Jupiter and Saturn, the bulge is so big it’s visible to the naked telescope.

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