"Dark Side of the Moon"

Today the space probes Ebb and Flo crashed into the mile-high edge of a crater on the moon after an extremely successful mission to map the subtle gravity changes all over the moon's surface. Yesterday, one well-meaning news reporter stated the lunar satellites were to crash on the "dark side of the moon", and man, did he get a bunch of people writing to set him straight!  

You see, supposedly there really is no dark side of the moon, because the sun shines on all parts of the moon over a one-month period. This makes sense, but I'm here to tell you there really IS a dark side of the moon after all, and here's why.

Standing on the moon looking up towards earth, you'd discover that the phase of the earth is exactly opposite the current phase of the moon. If the moon is full, the earth is barely lit (a "finger nail earth"), and when the moon is but a fingernail of light, the earth is full and very bright in the lunar sky.  This is all because of the way the earth-moon-sun geometry all works out, with the sun at a very long distance away from the relatively close-together earth and moon system.

This all means that from the perspective of someone standing on the near side of the moon, the sun is very bright when its up for half a month, and the earth is very bright in the sky for the other half of the month.  If you're on the near side of the moon, the earth always hangs up there pretty much in the same spot in the sky, always looming large, especially when the sun is down and the earth is all lit up. 

Now consider the view of the sky for a lunar explorer standing on the far side of the moon, where the earth is never visible. Half of the month the sun is very, very bright, as before, but for the other half of the month the sky is awesomely dark, with the stars, milky way, and a few planets brilliant and unblinking in the inky blackness.  

In summary, one side of the moon is always bathed in the glow of the sun or the earth-shine, and the other side of the moon spends half of each month in inky blackness.  The far side of the moon truly can be considered the "dark side of the moon".

* * *

Astronomical algorithms are a favorite subject on mine. This past month I republished a 2012 edition of Sun Position, my Kindle book that presents two very high accuracy algorithms for calculating exactly where in the sky the sun can be found.