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A perfect circle

The most round objects ever found may just be a few million miles away.

The sky is dominated by two seemingly perfect circles: the Sun and Moon. When safely behind the right amount of partially obscuring fog or low clouds, the Sun’s shape is obvious. Its equator’s tiny bulge throws it out of round by a mere 6 miles (9.66 kilometers) in its 864,337-mile (1,391,015km) width. If the Sun were modeled as a basketball, the difference between its equatorial and polar diameters would be half the width of a human hair. Circular perfection!

But perhaps this perfection is not unexpected. The sphere is the universe’s most common shape. Kids may wonder why stars never resemble cubes or diamonds. Answer: Every spot on a sphere lies the same distance from the center. A sphere also has the minimum surface area of any geometric shape. You’d use much more paint to cover a cube than to cover a ball of equal volume. So when a newborn star contracts from its own self-gravity, it pulls itself into the littlest form — always a sphere.

If the object spins quickly and its midsection bulges, it gets thrown out of the ball club. Through backyard telescopes, fast-spinning Jupiter and Saturn are clearly ovals.

But the Sun is different. Its slow rotation preserves its roundness. Our lethargic Moon, which has a similar spin of just under four weeks, differs from a perfect sphere by just 1.24 miles (2km). To the eye, the Full Moon is indistinguishable from a perfect disk. Ancient cultures, which revered the circle as nature’s supreme shape, never saw flawless spheres here on Earth. Only in the heavens was such perfection beheld.

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