If you looked at the last full moon and thought that Luna was showing a slight “bulge” – as we say in polite company – it wasn’t your imagination. For the first time, NASA scientists have satellite images confirming that the Moon is with bulge and the Earth is to blame.
Actually, we’ve known for some time from ground observations that the gravitational dance which the Earth and Moon spin around in has caused both celestial bodies to develop planetary spare tires and shapes more oval than spherical. It’s not as noticeable on Earth because the planet retains so much water that the bulge manifests itself in tides.
The effect on the Moon is called the lunar body tide. The Moon is solid except for a molten core so the bulge is smaller – 20 inches (50 centimeters) – on both the facing side and the far side.
That measurement was unconfirmed until recently when scientists used data from two NASA lunar satellites working in tandem to compare the heights of the bulges in relation to the force of gravity in those spots. Multiple readings from both satellites were essential since the Lunar Reconnaissance Orbiter maps the height of features on the lunar surface while the Gravity Recovery and Interior Laboratory satellite maps the gravitation field.
Although the same side of the Moon always faces the Earth, the bulge is on both sides and actually moves around the lunar surface a few inches at a time, according to Erwan Mazarico, a NASA scientist at the Goddard Space Flight Center.
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.
At 20 inches in height, it also gives some clues on how big the shock absorbers on the next lunar rovers need to be.