Though it looks like a big, delicious, perfectly round cookie in the sky, the moon is not perfectly round – it’s lopsided and somewhat egg shaped, though only slightly. Though scientists have been aware of this, it’s not something easily observed from Earth, or even when orbiting Earth. That is, until now: By combining data from two missions, NASA has finally observed changes in the moon’s shape and how it responds to Earth’s gravitational pull.

“The deformation of the moon due to Earth’s pull is very challenging to measure, but learning more about it gives us clues about the interior of the moon,” said Erwan Mazarico, a scientist with the Massachusetts Institute of Technology in Cambridge, Mass., who works at NASA’s Goddard Space Flight Center in Greenbelt, Md.

When they talk about deformation, they’re speaking in small, barely-noticeable quantities. On both the side facing Earth and the one directly opposite, the surface bulges about 20 inches. That’s due to the gravitational “tug of war” between the Earth and moon. We feel the effect here as well, though because Earth is so much larger it manifests itself mostly through the tides.

Because the moon is solid, its distortion was much more difficult to detect. Mazarico and his team team drew on studies by NASA’s Lunar Reconnaissance Orbiter and NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, mission. Because orbiting spacecraft gathered the data, the scientists were able to take the entire moon into account, not just the side that can be observed from Earth.

What’s interesting is the way the Earth and moon respond to one another. The position of the moon’s bulge actually changes over time due to the angle of its orbit, even though the same side always faces Earth. In turn, from the moon’s perspective, Earth appears to move within a small patch of sky, pulling the moon’s bulge with it. NASA describes the movement of the two as something of a dance.

By using two measurements of the moon’s deformation, NASA hopes the information will become more useful to scientists in the future.

“This research shows the power of bringing together the capabilities of two missions. The extraction of the tide from the LOLA data would have been impossible without the gravity model of the moon provided by the GRAIL mission,” said David Smith, the principal investigator for LRO’s LOLA instrument and the deputy principal investigator for the GRAIL mission.