Thursday, January 30, 2014

Full Moon Soon

For voluntary lunatics like us, 2014 begins auspiciously. The month began with a new moon on the 1st and will end with another new moon tomorrow on the 31st of January too. I stress that word, new, because, well, it will also be a full moon tomorrow; here is why.

What with the snow and cold—and hearing about Atlanta (Atlanta!??!) being paralyzed by ice—I might have overlooked this fact, distracted by the immediately here and now. But by happenstance I was reading one of Stephen Jay Gould’s books of essays, Eight Little Piggies, in which he reminded me that until 1959 of the current era no humans could possibly imagine the backside of the moon. In that year the Soviet Luna 3 probe photographed the far side; in 1968 Apollo 8 orbited the moon; hence Frank Borman, James Lovell, and William Anders were the first three humans to see that side of the moon with their own eyes.

Now it so happens that when the Moon is New for us, it is Full Moon on the other side, on the one we sometimes refer to as the dark side of the moon. We would, of course, thus designated it; we are always seeing things from our peculiar perspective. I found a full image of that side on Wikipedia here—then got to wondering how come the moon persists in staring at us with one of its faces only.

The name of the explanation is synchronous rotation, also called tidal locking (a good visual example is here). The moon rotates around its own axis as it simultaneously rotates the earth. But its own rotation takes exactly the same amount of time as its circuit of the earth. Therefore one half of it is always oriented to our planet. The deeper explanation (hidden in that name, tidal locking, is that the earth’s gravitational force deforms the moon slightly, which aids and abets the moon’s rotational behavior.

What we see is what we get. But what we see depends on where we are.

1 comment:

  1. Nice picture. Thanks! It is odd that the rotation of the moon is so perfectly times with its orbit of the Earth that we never, not even over a thousand years, get to see even slivers of the dark side of the moon.

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