bulge on Moon
Illustration of Earth as seen from the moon. The gravitational tug-of-war between Earth and the moon raises a small bulge on the moon. The position of this bulge shifts slightly over time. NASA's Goddard Space Flight Center

Scientists combined data from two NASA missions to study the moon’s lopsided shape and how it changes under Earth’s sway, something which has not been seen from orbit before.

Based on the data from NASA’s Lunar Reconnaissance Orbiter, or LRO, and the Gravity Recovery and Interior Laboratory, or GRAIL, scientists were able to examine the entire moon, and not just the side that can be observed from Earth, according to NASA.

“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,” Erwan Mazarico, a scientist with the Massachusetts Institute of Technology, who works at NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a statement.

According to scientists, the moon’s slightly irregular shape is one result of its gravitational tug-of-war with Earth as the mutual pulling of the two bodies is powerful enough to stretch them both. The activity has a strong effect on Earth's oceans, and is the driving force behind tides.

Although Earth’s distorting effect on the moon is difficult to detect, scientists have managed to detect a bulge about 20 inches high on the near side of the moon and a similar one on its far side. The position of the bulge shifts a few inches over time, according to scientists.

“Although the same side of the moon constantly faces Earth, because of the tilt and shape of the moon’s orbit, the side facing Earth appears to wobble,” NASA said, in the statement. “From the moon’s viewpoint, Earth doesn’t sit motionless but moves around within a small patch of sky. The bulge responds to Earth’s movements like a dance partner, following wherever the lead goes.”

Scientists used data provided by LRO’s Lunar Orbiter Laser Altimeter, which maps the height of features on the moon’s surface. They chose spots that the spacecraft has passed over more than once, compared the measurements taken at the same spot and calculated whether the height had risen or fallen from one satellite pass to the next -- a change indicated a shift in the location of the bulge.

According to scientists, the new results are consistent with earlier findings as the estimated size of the tide confirmed the previous measurement of the bulge.

“Having confirmation of the previous values – with significantly smaller errors than before – will make the lunar body tide a more useful piece of information for scientists,” NASA said.