Forget what your eye may tell you about a perfect full moon hanging in the night sky, researchers say; it's not a perfect circle, but in fact slightly flattened, and it's even got bulges on its sides.

It's squashed and even a bit elongated, with the bulges -- the thickest parts of moon's crust -- on the sides farthest from and nearest to the Earth, they say.

"Like a lemon with an equatorial bulge," described planetary scientist Ian Garrick-Bethell of the University of California, Santa Cruz.

"If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator," he explained. "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."

And yet the moon hardly spins, always keeping the same face to the Earth, so why does it have an equatorial bulge?

"There's no plate tectonics like on the earth," Garrick-Bethell notes. "Why is it so deformed?"

For 200 million years after the formation of the moon it possessed only a thin, weak crust while the interior remained molten, scientists say.

A gravitational process known as tidal heating stretched that crust while it slowly cooled atop the molten interior, giving it its present squashed appearance.

The bulge at the equator likely came later as the moon, still spinning around but slowing down, moved away from the Earth, freezing tidal surges in place as the moon's spin slowly ground to a halt.

Garrick-Bethell and his colleagues studied highly accurate topographical maps of the moon created using a laser altimeter, then ran computer calculations to recreate the processes that could have affected it shape.

The math proved out, they reported in the journal Nature.

"There is an expected ratio you get for each of those two tidal processes," Garrick-Bethell says. "We found the exact ratios you would expect for each process."

An off-center distribution of the moon's mass was likely created by subsequent episodes of volcanism on a large scale starting around 4 billion years ago and lasting almost 2 billion years.

Like the tidal events before, they changed the moon's appearance, the researchers say.

"The moon that faced us a long time ago has shifted, so we're no longer looking at the primordial face of the moon," Garrick-Bethell says. "Changes in the mass distribution shifted the orientation of the moon."