The Moon

LIBRATIONS

Most people know that the Moon always keeps the same face toward the Earth so that from Earth we can never see its backside. What many people don't realize is that from Earth we can see a bit around the "edges" of the Moon, so that from Earth we can in fact see more than half the surface of our satellite: about 59% of its surface. This effect is called Librations.

The Moon's librations have a variety of causes. One is that the Moon's rotation axis is tipped slightly so that sometimes its north pole is tipped a bit toward us while at other times its south pole is tipped toward us. This occurs for the same reason the Earth's northern hemisphere is tipped toward the Sun for half the year and its southern hemisphere is tipped toward the Sun for the other half of the year. These tipping librations allow us to see a little over the top and a little under the "bottom" of the Moon.

Another cause of librations is that the Moon moves along its orbit at a speed that changes, a consequence of Kepler's second law. However the Moon spins at a fixed rate. Thus, it can present exactly the same face toward Earth only on the average. This allows us to see a bit around its leading side for part of the month and a bit around its trailing side for another part of the month.

Finally, we get to peer a little around each edge of the Moon as a result of the Earth's rotation. It is exactly like having a box on a table in front of you with one side of the box facing you. If you shift your head slightly to the right you can see slightly around one side of the box. If you shift your head slightly to the left, you can see slightly around the other side of the box. The Earth's rotation shifts a given observer on Earth from right to left in a similar fashion.