The Dark Side of the Moon

Source: Wikipedia

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.

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4 thoughts on “The Dark Side of the Moon

  1. Nice! This is definitely a misconception that I had growing up. By just observing the moon from Earth, I never would have known that the other side of the moon isn’t actually always dark. The synchronous rotation of the moon is a tricky concept to grasp too. I enjoyed the post and your explanation, although a visual of the rotation or a link to one would’ve been nice. No worries though Google is a click away.

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  2. I really like the analogy you made to Star Wars. It helps clarify that “dark” does not necessarily mean bad or evil, but rather unknown or unseen. However, I am a little confused by the synchronized orbit. If you were on another part of the earth would you see a different side of the moon? Or does the location not matter at all?

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    1. Consider it like this, if the Moon had a face, it would ALWAYS stare right at the Earth no matter how much it rotates or orbits the Earth; however, as the Moon orbits the Earth different portions of it’s “face” get illuminated since the Sun always illuminates one half of the Moon (even if we can’t see it!). So, as the Moon orbits we can only see the one particular side of the Moon and the amount of illumination that that side gets depends on the time of month/phase of the Moon.

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  3. From your TA: Great post! It took me until college to understand this. Another way to say this is that the moon is tidally locked, i.e. it’s orbital period matches its rotational period.

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