2.8 - Understand the causes of lunar libration and its effect on the visibility of the lunar disc

At any moment we can only see 50% of the Moon's surface whether illuminated or not. It is not possible to see any more at any one time.

We also know the Moon has a synchronous rotation and the same face is permanently turned towards us. Yet astronomers state that they can view 59% of the Moon's surface. This is possible by a process called libration.

During the lunar cycle we can view a different part of the surface not visible during other phases. At times a crater or mountain that might appear on the edge of the Moon's disk (we call this the limb) might another time appear away from the limb. Sometimes we see a little more around the north, east, south, and west of the lunar surface.

The image on the right shows the same area of the Moon at different times of the month. you can see how the Mare Crisium moves and other features are visible because of libration.

This explains how we can see 59% of the Earths surface. We cannot see the remaining 41% from Earth, only spacecraft sent into lunar orbit can do this.

There are several reasons why this takes place.

  1. The elliptical orbit of the Moon around the Earth.
  2. The plane of the Moon's orbit.
  3. Our own observing position.
  4. Like the Earth, the Moon also has a precession effect so that it wobbles slightly.

For the purposes of this section, west is left and east is right. We see three methods of libration of the Moon from Earth.

Libration in longitude

The Moon's elliptical orbit and rotation allow us to observe more of the Moon's surface on its western or eastern limbs due to the Moon's elliptical orbit.

Libration in latitude

The equator of the Moon is titled from its orbital plane. This allows us to see little over the northern or southern polar regions.

Diurnal libration

The rotation of the Earth allows us to see more of its surface when the Moon is near the Horizon as we are seeing the Moon from a higher angle. More of the Eastern limb can be seen at Moonrise, more of the western limb at Moonset.



Notice how the image of the left Moon shows Mare Crisium appearing near the edge of the limb. Due to libration we can see Maria Marginus and Smythii on another occasion in the right moon.