How Libration Reveals some of the Moon’s Far Side

Many people think that the whole of the Moon’s far side is entirely hidden from view. This isn’t quite true. There is a region around the boundary between the near and far sides which is sometimes visible due to a phenomenon called libration. Libration causes the Moon to appear to wobble from side to side and up and down when viewed from Earth. As a result, 18% of the Moon’s far side can sometimes be seen; so only 82% of the far side, or 41% of the Moon as a whole is completely hidden from view.

There are three different causes of lunar libration. The first is due to the tilt of the Moon’s rotation axis, the second is due to the ellipticity of the Moon’s orbit and the third, which has nothing to do with the Moon’s rotation or orbit is much smaller, and is due to the size of the Earth.

Libration type 1 – caused by the Moon’s axial tilt

The Moon’s rotation axis is tilted by 6.68 degrees with respect to the plane of its orbit around the Earth. If, at a particular time, the Moon’s North Pole is at its greatest tilt towards the Earth then half an orbit later its South Pole will be at its greatest tilt.

The diagram below shows a two dimensional slice through the Earth-Moon system. The Moon’s near side is shaded light grey and the Moon’s far side is shaded dark grey. If you take a location on the  far side near its South Pole, marked with an ‘X’ in the diagram then, at time A, no one on Earth can see location X as there is no direct line of sight.  Location Y on the far side but near the Moon’s North Pole is visible.

For clarity, the sizes of the Earth and the Moon in relation to the distance between them has  been exaggerated.

Half an orbit later (marked as time B), the Moon has also completed half a rotation on its axis. Location Y cannot be seen from Earth, but Location X can.

Libration type 2 – caused by the Moon’s varying speed in its orbit        

The Moon takes the same time(27.32 days) to complete a single orbit around the Earth as it does to rotate once on its axis. Its rotation speed is constant. Because the Moon moves in an elliptical orbit, its orbital speed varies, being faster when it is closer to the Earth and slower when it is further away.  This causes the Moon to swing from side to side when viewed from Earth.

The next diagram is from a view point looking “downwards” above the plane of the Moon’s orbit around the Earth. Although there is strictly speaking no up and down in space 😊

1. If we start at time A when the Moon is at its closest to the Earth, regions x and y which are on the far side of the Moon, just beyond the near side/far side boundary can’t be seen directly from Earth.
2. Between A and B, the Moon is closer to the Earth than average and moves more rapidly in its orbit.  The distance between its position at time A and its position at time B is also less than one quarter of an orbit. So, the Moon covers this distance in less than 6.83 days. In this time the Moon has made less than a quarter of a turn on its axis. Region y is now visible from Earth.
3. Between B and C, the Moon is further from the Earth than its average value and so is moving more slowly in its orbit.  The Moon covers the distance between B and C in more than 6.83 days and makes more than a quarter of a turn on its axis .
4. At C, the Moon’s rotation has caught up and it has done half an orbit and half a rotation on its axis since A. Neither regions x nor y are visible from Earth.
5. Between C and D, the Moon is further from the Earth than its average value and  moving more slowly in its orbit.  It covers the distance between C and D in more than 6.83 days.  The Moon does more than a quarter of a turn on its axis. As a result, at C region x becomes visible from Earth.
6. Between D and A, the Moon is closer to the Earth than its average value and is moving more rapidly in its orbit. It covers the distance between D and A in less than 6.83 days.  It returns to position A having performed exactly one orbit and rotated once on its axis. Once again neither regions x nor y are visible from Earth

Libration type 3 – caused by the Earth’s rotation

This carries an observer first to one side and then to the other side of an imaginary line joining the Earth’s and the Moon’s centres. This allows them to look first around one side of the Moon and then around the other as the Earth rotates.

• At moonrise, marked with an A, a viewer can see the green region of the Moon, but not the orange region.
• At moonset, marked with a B, a viewer can see the orange region of the Moon, but not the green region. 

Of the three types of libration, this has the smallest effect. Interestingly, it is latitude dependant. At the equator it can bring up to 0.5%  of the Moon’s far side into view. in Manchester, England (latitude 53.5^oN) it is 0.25% and at the Earth’s poles it is zero.

And Finally…

I hope you’ve enjoyed this post. On the subject of the Moon, if you’ve not done so already you may wish to take a look at my e-book about the Moon which I’ve just revised and updated for more details, click the link below

Moon e-book