Earth's gravitational pull is so powerful that it creates a small bulge on the surface of the moon. For the first time, scientists have observed this bump from orbit, using NASA satellites.
The gravitational tug-of-war between Earth and the moon is enough to stretch both celestial bodies, so they each end up having a slight oval shape, with the tapered ends facing each other.
On Earth, this gravitational tension shows up in the form of tides. The moon's pull has a strong effect on Earth's oceans because water has so much freedom of movement. [The Moon: 10 Surprising Lunar Facts]
The corresponding distorting effect on the moon, called the lunar body tide, is more difficult to see, because the moon is solid except for a molten core. But Earth's pull raises a small bulge about 20 inches (50 centimeters) from the surface on the near side of the moon and a matching bulge on the far side.
"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 who works at NASA's Goddard Space Flight Center, said in a statement.