Dark side of the moon

In http://www.straightdope.com/classics/a2_112.html, the Master discussed why we always see the same side of the moon, due to synchronous rotations.

My question is: Is the same side of the moon I see in the United States the same side of the moon I’d see if I were in Africa or Australia? If the dark side of the moon never faces the earth, then how the hell does the moon rotate?

The short answer to the first question is “yes.” THe same side of the moon always faces the Earth, no matter where on Earth you are.

As to the second question, to quote Pink Floyd, “There is no Dark Side of the Moon, really.” The moon does rotate, at the same rate at which it revolves around the Earth. The same side therefore always faces Earth, but the same side does not always face the sun. The sun rises and sets about every two weeks on the Moon’s surface.


But … I don’t get it. If the moon rotates on its axis, and in turn revolves around the earth, wouldn’t the quote-unquote “dark side” of it be facing the earth at some point?

Or, am I completely mis-understanding you and that side does face the earth during the phase of the moon when it’s new (and thus unviewable from earth)?

The rotation is in sync with its orbit. Put a quarter and a penny on the table, and move the penny in a circle around it. Say you want the edge nearest the date to always be closest to the quarter. You’ll quickly realize that the penny needs to rotate to accomplish this, and that it needs to rotate once for each orbit. That’s how it works.

You’ll also see that the penny-edge farthest from the date never gets near the quarter. That’s the farside of the moon.

Got it, thanks, Saltire.

(Really, folks, I am a college graduate.)

Just to add a little item to clarify (or possibly make things even more complicated): The moon is not rigidly “locked” into orbit with the earth. Although the same face is toward the earth, it does “rock” side to side and up and down a little, a process called “libration”. As a result, you can actually see somewhat more than 50% of the lunar surface from the earth (over time, of course, not all at once).

Saying “There is no Dark Side of the Moon” has always bothered me, I have to admit. Of course there’s a Dark Side of the moon. It’s just not permanently dark. The side facing away rom the sun is in shadow.

There’s a difference between the Dark Side of the Moon, and the dark side of the moon. The Dark Side of the Moon is usually taken to be the Far Side of the Moon. That’s where Gary Larson got his sense of humor.

Who cares about semantics, it was a great album, after all. :slight_smile:

And the reason for the libration is that the rotation rate is constant, but the orbital speed is not (it’s faster when it’s closer to the Earth), so they can’t match up exactly all the time, only on average. IIRC, libration allows ~60% of the surface to be visible at some time, rather than the 50% you’d get were the orbit circular.

If you’re gonna quote Pink Floyd, at least go for the whole nine yards. To wit: “There is no dark side of the moon, really. As a matter of fact, it’s all dark.” I have generally interpreted this bit to be telling us “Hey, dumbass, the moon doesn’t give off light, it’s reflecting the sun.” Of course, being British and all, Mr. Floyd would never have used a phrase like “hey dumbass.”

p.s. I know there’s not really somebody named Pink.

A local radio station played DSOTM over President Clinton’s “Lewinsky Apology” speech. It ended during “On the Run.”

Oh, and for the dawn of the 2000’s, I almost timed DSOTM perfectly. As the ball was dropping, “…and the sun is eclisped by the moooooon…”