How many times have you seen the moon? Seriously. How many times have you looked up and been like: "Oh yeah, the moon. Cool."?
If you are old enough to be reading this, the answer should be "a lot" (probably in the thousands).
But here is the real question: Have you really seen the moon?
I don't mean have you truly taken the time to stare at the Earth's only natural satellite, taking in its beauty and absorbing all its detail. That would be great, and all, but the real question I'm asking is this: How much of the moon do you get to see when you do see the moon?
Half. You get to see only half of the moon.
Now, I'm ignoring the issue of the moon's phase here because no matter what phase it's in, the moon always shows us the same face — meaning the same hemisphere. (A slight variation in the moon's motion called libration actually allows us to see a bit more than half of the moon's surface.) It's as if you had one of these high school globes of the Earth but it would only show you the part with China. You couldn't rotate it to see the Western Hemisphere.
That's the situation with the moon. Billions of years of gravitational interaction between the Earth and the moon have left our satellite "tidally locked." Formally, that means its "day" and its "year" are the same. The time it takes to spin on its axis (think of it as the moon's day) is the same length as the time it takes to orbit the Earth (think of it as the moon's year). So the moon is always showing us the same hemisphere. That's the one with the dark blotches (called "maria").
But there is another side of the moon — and you have never, ever seen it directly. It's kind of amazing to contemplate that something so familiar to us could also be so hidden.
But thanks to the glory of science, you can see the far side of the moon (it's wrong to call it the dark side). And for your Tuesday viewing awesomeness, I give you this beautiful video of the moon in all its two-hemispherical glory.
Notice how the far side of the moon looks really different from the near side? Why is that? Well, we will leave that question for another time.
But I'll leave you with this question now: What else lies in plain sight but remains hidden from us?
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4