Why are all the planets in our solar system perfectly round? --- Austin, Long Island, N.Y.
It turns out that the eight planets in our solar system are almost spherical, as are many of the moons, but they aren't perfect spheres.
Take earth as an example. Because it is spinning, it tends to bulge at the equator. The bulge is not that big because the planet isn't spinning that fast. But it is measurable. It gives the planet a shape called an oblate spheroid. If you slice the planet through the equator it has a bigger diameter than if you slice it through the poles. The equator slice is about 26 miles bigger.