Moons gravitational pull

Myth 1. The moon has a permanent dark side. Most grammar school students know that the moon presents only one face or side to the Earth. This is (roughly) true and gives rise to the idea that there is a permanently dark side of the moon, a thought immortalized in Pink Flyod’s music and elsewhere.

In fact, the side of the moon that is perpetually turned away from Earth is no more dark than the side we see. It is fully illuminated by the sun just as often (lunar daytime), and is in shade just as often (lunar night), as is the familiar Man in the Moon face we see.

The Earth-facing side of the moon gives rise to another misconception that many people share, namely that we see only 50% of the moon from Earth. In fact, only about 41 percent of the moon’s far side (a much more accurate and preferable term than dark side) is perpetually hidden from earthly observers. A diligent observer on Earth can, over time, observe about 59% of the moon’s surface. This is because a phenomenon called libration causes the moon’s viewing angle, relative to Earth, to change slightly over its orbit.

Lunar libration is due to the fact that the moon’s orbit around Earth isn’t a perfect circle. Instead, it’s a slightly elongated circle called an ellipse. Imagine a race car on an elliptical track. At each elliptical end of the racecourse, the car is flung out slightly due to the change in angle. It is a bit like rounding a corner. The result for the moon is that it occasionally exposes slightly more of its surface on the eastern or western extreme (depending on the location in the orbit). That’s why, as viewed from Earth, about 59% of the moon’s surface is exposed over the course of the moon’s (roughly) monthly orbit around the Earth.