The moon, by all terrestrial appearances, is a big, perfectly round ball in the sky. Except, it’s not – it bulges around its equator and flattens at its poles, making it shaped more like an heirloom tomato than a round ball of cream cheese. Scientists have known this since 1898, but have speculated as to why. Now, thanks to UC Santa Cruz researchers, the verdict is in: Tidal and rotational forces in the moon’s infancy are to blame for its goofy shape.

“In 2010, we found one area that fits the tidal heating effect, but that study left open the rest of the moon and didn’t include the tidal-rotational deformation. In this paper we tried to bring all those considerations together,” said lead author Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at UC Santa Cruz.

Four billion years ago, the moon began to cool and solidify after the collision that caused it to break off in the first place. Prior to that, in a more malleable state, Earth’s gravitational pull combined with the moon’s rotation to cause the warping. “If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator,” Garrick-Bethell explained.

That doesn’t tell the whole story, though, so the UCSC team dug deeper. In the new paper, they coauthors incorporated other tidal effects into their analysis. They also took into account the large impact basins that have shaped the moon’s topography, and they considered the moon’s gravity field together with its topography.

Because of those impact craters, it was impossible to analyze the moon using spherical calculations. Compensating for that, they found that early-stage tidal heating and varying thickness in the crust was responsible for most of the moon’s major topographical features.

“In 2010, we found one area that fits the tidal heating effect, but that study left open the rest of the moon and didn’t include the tidal-rotational deformation. In this paper we tried to bring all those considerations together,” Garrick-Bethell said.

The rest of the features appeared to match up with the earlier “fossil bulge” theory, but this study is the first to establish multiple theories in combination.

What’s interesting is that the moon’s gravitational field doesn’t align with its shape, with the principal axis separated by about 34 degrees from the gravitational axis. Craters (and the mass that left with them) likely account for some of the shift, while internal changes are responsible for the rest.

The only mystery? The topographical differences between the near and far sides – tidal forces can’t explain those.