Besides a popular Pink Floyd album, what is “the dark side of the Moon”? Most people have not given much though to what it means or how it got its infamous name. For me the first image I get is of the moon I know and love turning over to the dark side of the force, much like Anakin from Star Wars when he becomes Darth Vader. In reality, however, there are no red lightsabers or Sith involved, just science…
The dark side of the Moon is a somewhat misleading name because it may lead a newbie astronomer to believe that the dark side of the Moon never actually has any sunlight touch it. It’s not a stretch to assume that the dark side would always be dark. What the dark side really is though, is the side of the moon that never reflects any light towards Earth. An observer from Earth can never see the dark side of the Moon, a more accurate name for the dark side of the Moon is “the far side of the Moon”. The far side of the Moon is the side of the Moon facing away from earth. The meaning of dark in the earlier name refers to the unseen or unknown nature of the side, rather than the absence of light on that side.
“But wait,” a newbie astronomer might ask, “since the Earth is constantly rotating, and the Moon is constantly orbiting the Earth, wouldn’t an observer from Earth eventually see all sides of the Moon?”
A person with this questions asks so rightly, but they are unaware that the moon is in synchronous rotation with the Earth. That means that the Moon spins on its axis at the same rate that is orbits the Earth. As the Moon orbits the Earth, it spins at just the right rate so that the same side of the Moon is always pointing towards Earth. It may seem like a phenomenal coincidence, but many moons that orbit close to their planet like ours are locked into synchronous orbits as well.