I blame Pink Floyd. But perhaps I shouldn’t, since I’m sure the album title “The Dark Side of the Moon” reflected the very popular misunderstanding that THERE IS NO PERMANENT DARK SIDE of the moon.
I wouldn’t have even thought to write about this except that Time magazine – a journal which I hold a fair amount of respect for in its reporting breadth and accuracy – had an article today commemorating the Apollo 8 astronauts and they make the very same mistake.
The moon orbits Earth. Fairly simple. Earth orbits the sun. Again, fairly straight-forward (unless you’re a Bible literalist, a flat earther, etc.).
While Earth orbits the sun, it also rotates once on its axis per day. The side facing the sun is the day side and the side facing away is the night side. But, since it rotates, all parts of Earth experience both day and night once per day (let’s ignore those poor fools folks in the Arctic Circle).
Now, just as the earth rotates on its axis as it orbits the sun, the moon also rotates on its axis while it orbits Earth.
Let’s ignore the mechanics of the vocabulary and simply say that “tidally locked” means that an object always shows the same side to the object that it is locked to. That’s why the moon always shows the same side towards Earth (ignoring nutation and libration). If it weren’t rotating on its axis, then we would always see different sides of the moon, just as the sun would always see the same side of Earth if Earth weren’t rotating.
So now we have a rotating, tidally locked object around Earth. And because it rotates, it has BOTH night and day! Yes … there is a “dark side” to the moon just as there is a dark side to Earth, but that side is always changing because the moon rotates.
When we see a new moon (which we can’t see unless its blocking the sun), we are seeing the same side we always do. Except that it’s dark. It’s completely dark because it is now night on that side. When we see a first quarter moon (half full, sets around midnight), the side we see is still the same side we always do, but half of it is lit and half of it is dark – night and day.
When we see a full moon, again, we see the same side we always do, but now the half that’s dark is completely facing away from us, and the entire part that is in day is what is facing us.
Time Magazine’s Error
“Orbital mechanics also demanded that the maneuver occur on the dark side of the moon, entirely out of radio contact with Earth. At 68 hours and 58 minutes into their journey, the crew buckled in and vanished around the moon’s far side.”
The lunar phase during December 21 and December 26, 1968, was between 9-56% full. In other words, the “far side” was not the “dark side,” despite the author of this article, Jeffrey Kluger, using them synonymously.
You may wonder why I choose to point this out, apparently quibbling over “small” details. It’s because the title of this blog is “Exposing PseudoAstronomy,” and the purpose is to, well, expose pseudoastronomy. Not understanding that the lunar “dark side” is not a static location on the moon is a very popular misunderstanding. The fact that it’s propagated through into a highly respected news outlet, admittedly, was something I found very annoying. I’d been wanting to address this issue for awhile, and this gave me an opportunity.
Why is it important? Because if you think there’s an actual side of the moon that’s always dark, you don’t understand how the moon orbits Earth. Why there are phases. Why we always see the same side. Why a solar eclipse can ONLY happen when there’s a new moon and why a lunar eclipse can ONLY happen when there’s a full moon. And as someone who has taught college astronomy labs and tried to work with students to dispel these myths, I’m hoping that this post may save at least one future student from believing this common mistake.