Like planet Earth, the moon isn't perfect. But while Earth's slightly elliptical shape has been explained due to its rotation and gravitational pull of the moon, these factors didn't explain the moon's lemon-like form.

Astronauts have spent a long time trying to figure the moon's appearance, and now researchers from the University of California, Santa Cruz (UCSC) believe they have a solution.

The researchers released a paper that says tidal forces early in the moon's formational history accounts for its off appearance.

The moon rotates too slow today, and is too far from Earth to explain its flattened shape, according to the study. But in its earlier days, while still mostly liquid, the moon spun faster and was closer.

The effects of these two factors were frozen into place as the moon matured and solidified, according to the researchers.

When the moon first cooled, only its crust hardened, a molten core remained, an ocean of liquid rock, according to the study.

The pull of Earth's gravity influenced the moon's insides.

"If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator," said Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at UC Santa Cruz, according to a press release issued by the university. "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."

"So there's a variety of interesting things that could happen, at that time when the Moon was really hot, that could change its shape," Garrick-Bethall added.

The study was published this week in the journal Nature.

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