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Dark Side of the Moon
Bad Astronomy: "That's as remote as the dark side of the Moon!"Good astronomy: "That's as remote as thefar side of the Moon!"
How it works:
I had the misfortune this morning to wake to the radio playing thesong "Dream Weaver" (I used to love that song when I was a kid, butas a friend of mine likes to say, "We are not responsible for songs we likedwhen we were 15 years old"). As the tired, hackneyed refrains went on, one inparticular caught my ear: " Fly me away to the bright side of the moon,and meet me on the other side."
Of course, thereis a bright side of the Moon, and youcango to it. But if you sit still, you can only be there for twoweeks. The bright side (and therefore the dark side too) is not a fixed place, but appears to move as the Moon revolves around the Earth.
Seen from the surface of the Earth, theMoon does not appear to rotate.This is because from an outside frame of reference, the Moon rotates once forevery time it goes around the Earth. So from our vantage point, the Moonis naturally divided into two halves: the hemisphere we always see, calledthenearside, and the hemisphere we do not see, thefarside.The farside has only been seen by probes or astronauts that have actually orbited(or at least passed by) the Moon.
This is very different from the dark side. As the Moon orbits theEarth, different parts of it get illuminated by the Sun. When the Moon isbetween the Earth and the Sun, we see it almost entirely in shadow. This iscalled the new moon. Half a lunar orbit later (two weeks or so)it is fully illuminated by the Sun, giving us a full moon. In betweenwe get a half moon, sometimes confusingly called a quarter moonbecause the Moon has completed 1/4 of an orbit. These are called the phases of the Moon (I have heard that some people think that the phases are caused by the shadow of the Earth on the Moon; but that is whole different can of Bad Astronomy).
So imagine you are standing on a spot directly in the middle of the nearside.You would see the Earth straight over your head. If it were new moon to someoneon Earth, the Sun would be behind the bulk of the Moon to you. For you, it would be local midnight! Two weeks later it would be fullmoon to someone on Earth, and it would be local noon to you; the Sun would behigh overhead. The half moon to someone on Earth means it would be either sunrise or sunset to you, depending on whether you are a week past new moonor a week past full moon, respectively. You can see that the dark side ofthe Moon is just simply thenight side of the Moon. It is no more a fixed feature than the night side of the Earth.
The Pink Floyd album may be one of the best selling albums of all time, but astronomically it's in eclipse.
Bad Addendum: At the end of the album "Dark Side of the Moon" by Pink Floyd, there is a voiceover thatactually says "There is no dark side of the moon. As a matter of fact,it's all dark. The only thing that makes it look alight is thesun." In a sense, that line is correct: the Moon is actuallyvery dark: it only reflects on average about 10% of the light that hits it (the specific reflectivity changes depending on the lunar surface; some parts are very dark, while others can be pretty bright).As a comparison, slate reflects about 10% of sunlight that hits it. The reason the Moon looks so bright isis that it is sitting in full sunlight, and so there is alotof light hitting it.
Special thanks to Bad Readers Josh Millard and Keith Collyerfor the Pink Floyd lyrics correction, and to Bart Schaefer for the Dream Weaver correction (he was 15 once too)!