We may have walked on it 45 years ago, but scientists have only now discovered the true shape of Earth's moon. And while it appears to be a perfect sphere, it's actually "like a lemon with an equatorial bulge," one researcher tells the New York Times. "Imagine a water balloon flattening out as you spin it." Using a laser altimeter, the researchers produced topographical maps of the moon's shape in what Gizmodo calls "unprecedented detail." But why the bulge?
Scientists theorize that the moon formed almost 5 billion years ago when a large body slammed into Earth and debris got blasted into space. And while they have suggested for decades that tidal forces helped shape the moon, it wasn't until recently that they were able to map out the bulge that froze into place as the moon cooled, reports Space.com. They determined that tidal heating likely stretched the molten moon's crust while it was still forming, and that the bulge formed later as the moon was cooling during a tidal surge that is now essentially frozen in place. (Earth may not be the only planet that helped form the moon.)