Lunar Libration

By Betty Robinson,

The Moon only presents one half, or 50%, of its surface to us, what we call the near side of the Moon. (We never see any more than a small amount of the far side of the Moon from Earth because of the tidal lock between Earth and the Moon, but that’s for another blog post.)

However, we can actually see about 59% of the Moon’s surface over time, due to a phenomenon called libration. Libration is the real or apparent “wobbling” of an object, in the left-right direction and/or in the up-down direction. As a result of lunar libration, we get to see more of the Moon’s surface. Libration is noticeable to any observer, as shown in the two images.

The lunar libration predicted for January 2021 is based on data collected by NASA’s Lunar Reconnaissance Orbiter (LRO) satellite, which has been orbiting the Moon since 2009. We can see both the left-right libration and the up-down libration in these two images. On Jan. 1, 2021, Mare Crisium looks to be a bit higher and not fully visible as compared to the image for Jan. 25, 2021. Credit: NASA Scientific Visualization Studio and LRO

In the two images, notice how the size of the Moon seems slightly smaller on Jan. 25 than on Jan. 1. This isn’t a manipulation of the images. The difference is because the Moon’s path around Earth is not a perfect circle; it’s elliptical. Think of an oval, or a slightly flattened circle, with Earth positioned closer to one end than the other. So at one point the Moon is slightly closer to Earth, and at the opposite point it is slightly farther from Earth. The Moon’s elliptical path is important to the concept of left-right libration.

On Jan. 1, the Moon is 386,460 kilometres from Earth. On Jan. 25, the Moon is 397,371 kilometres from Earth. The difference is about 11,000 kilometres. So, because the Moon is farther away from us on Jan. 25 than on Jan. 1, it appears to be smaller in the sky to us, just as any object far away from us appears to be small. The size of the Moon itself does not change.

Back to the photos. So, what is happening here? Why can we see different positions of the Moon’s features? Why don’t we always just see the same features in the same spots?

Left-right Libration

The Moon rotates on its axis, just like Earth does. They both rotate west to east. So, if you could look down on Earth and the Moon from high above the north pole, you would see them rotating in a counterclockwise direction.

One Earth rotation takes 24 hours, our definition of a day. One Moon rotation takes about 27 days. It also takes the Moon just over 27 days to go around Earth once. As a result, we only see one side of the Moon. It seems like the Moon isn’t rotating at all, but it is.

Here is where the elliptical path comes in: in the late 1500s, a famous astronomer called Johannes Kepler figured out that objects such as Earth, the Moon, and the planets do not orbit in circles, but ellipses. Kepler then figured out that, because of the elliptical orbits, the objects move at different speeds, depending on where they are relative to the Sun, or Earth in this case. For example, when the Moon is at its farthest point from Earth, its speed is just under 1 kilometre per second. When it is at its closest point to Earth, its speed is just over 1 kilometre per second. (1 kilometre per second = 3,600 kilometres per hour.)

After the Moon’s closest approach to Earth, the Moon is still moving faster in its orbit. But the Moon’s rotation speed doesn’t change and it can’t keep up with the faster orbital speed. As a result, on the right side of the Moon we see a bit of the Moon’s far side before the Moon completes its full rotation.

After the Moon’s farthest distance from Earth, the Moon is still moving a bit slower in its orbit. So the Moon’s rotation is a little faster than its orbital speed. As a result, on the left side of the Moon, we see a bit of the Moon’s far side.

Up-down Libration

If you think of a line coming out of the centre of the Sun and going to the edges of the solar system, many planets would appear to sit on or near that line. The planets and other objects are said to be co-planar. Some don’t, though. Mercury, for instance. Mercury’s orbit crosses the imaginary line at an angle of about 7 degrees. And Pluto’s orbit is at an angle of about 17 degrees to the line. The Moon’s orbit is not co-planar with Earth’s orbit. The Moon’s orbit is inclined about 5 degrees to Earth’s orbit.

The concept of up-down libration is related to this 5 degree tilt. Also of note is the tilt of the Moon itself. Earth is tilted about 23.4 degrees, which causes our seasons. The Moon is tilted only about 1.5 degrees, so no seasons on the Moon! Anyways, back to libration. It’s so easy to move from topic to topic in astronomy because everything is so interesting!

As the Moon orbits Earth, sometimes it’s above Earth’s orbit, and sometimes it’s below Earth’s orbit. As a result, the Moon’s north pole is sometimes tipped toward Earth, and sometimes it’s tipped away from Earth. So at times we can see a little more of the south side as the Moon goes above our orbit, and a little more of the north side as the Moon goes below our orbit.

There are other factors that affect how much of the Moon’s surface we can see, such as our latitude on Earth and the Moon’s precession. But the main two factors are the left-right and up-down libration.

Watch the Moon with binoculars over a month, and try to see different positions of its features as shown in the NASA images.

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