The Moon is a steady, unchanging companion. We always see the same “face” — the same lunar hemisphere always faces our way. But it turns out that we actually see more than just the face — we see a little behind the ears, too, thanks to a combination of effects known collectively as libration.
One of those effects is caused by the Moon’s tilt on its axis, which is a bit different from the tilt of Earth’s axis. For half of the four-week cycle of lunar phases, the north pole tilts slightly toward Earth. For the other two weeks, the south pole points our way. So over the course of a month, it looks like the Moon is nodding up and down a little, allowing us to see slightly beyond the poles.
Another effect is caused by the Moon’s orbit, which is slightly lopsided. When the Moon is closest to Earth, it moves a bit faster in its orbit than average, and when it’s farthest, it moves a bit slower than average. From Earth, that looks like a sideways shake of the head. Each shake allows us to see a little around the Moon’s eastern and western limbs, bringing slivers of the far side into view.
Combined, these effects allow us to see 59 percent of the lunar surface — leaving just two-fifths perpetually hidden from sight.
And the Moon is in fine view tonight. Aldebaran, the bright orange star that marks the eye of Taurus, is close below it at nightfall, and even closer to the Moon as they set in the wee hours of tomorrow morning.
Script by Damond Benningfield, Copyright 2013
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