Ball Game

By Bob Berman|Wednesday, October 01, 1997
Ever wonder why the universe seems to be made mostly of spheres? Why the planets and stars aren’t cubes, blobs, planks, or the shape of diamonds? This month celestial objects of varying profiles parade through the night sky, offering us a perfect chance to examine the roundness issue.

The reason all our brighter luminaries--the moon, sun, planets, and stars--are spherical is a combination of geometry and gravity. A sphere is the geometric form with the smallest surface area--no spot on its surface is higher than any other. A massive object has sufficient gravity to self-contract, so its atoms pull in toward the center, forming a perfect ball.

Meteoroids and tiny moons have minuscule masses, so their gravity is too weak to pull them into globes. They remain irregular shapes. But visible, distant celestial objects are all big--or we wouldn’t see them-- and hence have the gravitational oomph to be spherical. In reality, though, not everything big and bright is big and round. The fun is finding the large odd shapes.

A large body can avoid roundness only if it spins fast enough to bulge at its middle, or if a second massive object lurks nearby to distort it. October’s sky is an orrery of objects displaying one or the other effect.

Consider two worlds with nearly the same size and density: Earth and Venus. Venus hardly spins at all, needing a lazy 243 days to complete a single rotation. Therefore it is perfectly round. (See it low in the southwest after sunset.) Our planet whirls along at 1,040 miles an hour (and, of course, rotates in just one day), forcing Earth’s tropical diameter to bulge out 26 miles.

You can observe this phenomenon by looking at the full moon on October 15. Because the moon rotates slowly, at 27.32 days, its deviation from an absolutely perfect sphere is imperceptible--its 2,160-mile diameter has a bulge of just four miles, less than one part in 500.

That same night, the full moon floats near its antithesis: the most out-of-round planet in the known universe. Point any telescope to the star next to the moon and rings won’t be the only curious thing you’ll notice. A pastel yellow Saturn will be flat. The sixth planet’s fluffy composition, just seven-tenths as dense as water, coupled with its huge size and wild ten-hour rotation, yields a squashed planet, with the same oval appearance as the setting sun. A tunnel drilled through Saturn’s equator would be 25 percent longer than its polar diameter.

A few nights earlier, on October 10-11, you might let the moon guide you to brilliant nearby Jupiter. Any telescope will show you a conspicuously flat fifth planet. With an equatorial rotation speed of 28,000 miles an hour--faster than astronauts have ever traveled--its waistline bulges like a couch potato’s.

Then turn to Vega, the brightest star high in the west, closely accompanied by five medium-dim stars. The southeasternmost of these is Beta Lyrae, whose light changes over the course of 13 days, so that sometimes it looks brighter, and sometimes dimmer, than its nearest companion, Gamma. Beta’s single point of light is actually two enormous stars that orbit close enough for their atmospheres to intermingle. These massive suns pirouette so dangerously near each other--22 million miles from center to center--that they are both grossly distorted into the shape of a football. No wonder the amount of light they send us varies as they rotate.

Strange, distinctive objects of the night--that win the heavyweight title for being amazing in spite of losing their round.
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