The rotational forces on spinning heavenly spheres creates bulges at their equators. A faster spin exerts a stronger force and bigger bulges. However, the Earth’s moon only rotates about once a month. So, for centuries, astronomers wondered: With so little spin, why the lemon shape?

Billions of years ago, the Moon was made of rock sheets floating on a sea of magma, said Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at UC Santa Cruz. The Moon’s distorted shape resulted from the combined effects of early tidal forces on a hot Moon, plus rotational forces as the surface later cooled unevenly, according to his paper published in Nature.

Garrick-Bethell drew insight from similar ideas about the shape of Europa, the cool and watery moon of Jupiter. A detailed topographical analysis of the Earth’s moon, based on data from laser altimeters and studies of gravity-field perturbations, supported these conclusions. Also involved in the study was UC Santa Cruz professor of Earth and planetary sciences Francis Nimmo.

This work expanded on earlier studies, published in Science, which used tidal forces to explain unusual features on an area of the Moon called the farside highlands.