Forty-five years after astronauts landed on the moon, scientists say they have finally discovered its true shape: slightly flattened, with a bulge on one side.

"Like a lemon with an equatorial bulge," said Ian Garrick-Bethell, a planetary scientist at UC Santa Cruz, and an author of the study, being published in the journal Nature. "If you can imagine a water balloon flattening out as you spin it."


For example, the moon barely spins, yet it appears to have the sort of equatorial bulge caused by rotation. And why would a giant ball of cooled liquid be anything but spherical?

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

To overcome the crater problem, he and his colleagues used highly accurate maps of the moon's topography, made with a laser altimeter, then ran calculations to see what the surface could have looked like before the craters formed.


The clues, Garrick-Bethell said, are all in the math.

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