Scientists say they have finally discovered why the moon is shaped a bit like a lemon -- somewhat flattened with a bulge on each side. As detailed in a new paper published online in the journal Nature on July 30, 2014, it's all about tidal and rotational forces.
"Early tides heated the Moon's crust in different places, and those differences in heating in different areas gave the Moon most of its shape," lead researcher Ian Garrick-Bethell, an astrophysicist at the University of California, Santa Cruz, told the Agence France-Presse.
In other words, during the moon's infancy some 4.4 billion years ago -- when it was still super-hot as the result of an impact between Earth and another object -- the Earth's gravitational (tidal) forces molded its shape ever so slightly.
"Later on, those tides warped the outside of the moon while it was cooling, and it froze in that warped shape," Garrick-Bethell told the Agence France-Presse.
Scientists have known for some time that the moon is not perfectly round. But it's been difficult to determine exactly how the bulges formed, since deep craters on the lunar surface obscure the moon's original shape.
So a team from the Massachusetts Institute of Technology, which also contributed to the study, created a topographical model that sealed many of the moon's crevices in order to paint a more complete picture of what the moon looked like billions of years ago.
From that model, Garrick-Bethell and the team were able to determine that the bulges formed gradually in the first 200 million years of the moon's existence, due to the combination of forces.
"If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator," Garrick-Bethell said in a written statement. "On top of that you have tides due to the gravitational pull of the Earth, and that creates sort of a lemon shape with the long axis of the lemon pointing at the Earth."