How much of the moon can you see? Most people would say fifty percent, but they’d be wrong. You can’t see more than fifty percent at a time, but look upward for a year, and you can see nearly sixty percent of the moon’s surface. We’ll tell you how.
Look up in the sky on a night of a full moon and you might think you’re seeing all the moon you can. But before the night is even up, you'll see more. Although someone on Earth can only see half the moon at a single time, and the sun can only illuminate half the moon at a time, put in enough time and the motion of the moon lets us see the universe's extended cut. The motions that let us see more are called the librations of the moon.
The first libration comes from the fact that the moon is tilted with respect to the sun’s light. If the poles of the moon were exactly where light and darkness met as the moon went around the sun, we’d lose out on a considerable amount of data. But because the moon is tilted so its north pole is slightly away from the sun during one part of its orbit, and slightly towards it two weeks later, we can see the north pole of the moon lit up in one part of the month, and the other in half a month. This is called the lunar libration.
There’s also the daily libration. The Earth is a big place with respect to the size of the moon, and when it spins it covers a lot of ground. Moonrise is seen from one side of the planet and moonset is seen from the other. Stand in the right place and you’re seeing the moon from two opposite edges in a single day. This lets you see little slivers of extra moon on each side.
Lastly, there is the orbit of the moon itself around the Earth. The moon turns at a steady rate, but its path around the Earth is not a steady circle; it’s an ellipse. Imagine being at the center of an oblong race track. If you face the long flat side of the track as the cars drive by, they’d take a long time to get from your right to your left. If you faced the sharp curve, the cars zip from your right to your left much faster. The moon does the same, but as it moves around its track, it also rotates. The rotation of the moon stays at a steady rate, but the movement of the moon along its orbit, when seen from Earth, seems to speed up and slow down. This mismatch makes the rotation seem to lag sometimes, and speed up at others, giving us extra glimpses at its east and west sides.
Add to this the fact the moon itself waggles around a little in space as it rotates, and we can see far more of the moon that we might think. About fifty-nine percent of the moon is presented to the careful astronomer who’s looking throughout the year.
Top Image: Bird In Paradise via Shutterstock