The orbit of the Moon
The Moon revolves around the Earth in the same sense that the Earth revolves around the Sun. Looking down on the North Pole, the Moon appears to make an anticlockwise orbit around the Earth.
The Moon's orbit is an ellipse with the Earth at one of the foci, and it obeys Kepler's Laws in the same way that planets do. Perigee is the point in its orbit when it is closest to the Earth and at this time the Moon will appear largest in the sky. At apogee the Moon is at its furthest from the Earth and appears smallest in the sky.
Each day the Moon rises in the east and sets in the west as a result of the Earth's rotation. It moves about 13° eastwards against the background of stars as a consequence of its revolution around the Earth.
The 27.3 days it takes the Moon to complete one orbit around the Earth is about the same as the time taken for it to complete one rotation. This synchronous rotation means that it always shows the same face to the Earth.
However, libration means that it is possible to see around 59% of the surface. It has three components:
i) Diurnal libration
As the Earth rotates the view of the Moon changes. An observer on the surface of the Earth sees slightly around the eastern limb of the Moon at moonrise. At moonset the same observer sees slightly more around the western limb of the Moon.
This arises from the changed position of the observer because of the rotation of the Earth. A parallax effect means that the Moon looks slightly different at moonrise and moonset.
ii) Longitudinal libration
The Moon moves in an elliptical orbit around the Earth. As a result it does not move at a constant speed (this is Kepler's second law)
However the Moon rotates at a constant speed. When it is moving faster in its orbit, this allows observers on Earth to see around the trailing edge of the Moon. When it is moving slowly it is possible to see around the Moon's leading edge.
iii) Libration in latitude
The Moon's axis is tilted by 6.7° to the plane of its orbit around the Earth. This means that sometimes the Moon's North Pole is tilted towards the Earth and sometimes tilted away.
As a result it is sometimes possible to see beyond the lunar North Pole and the South Pole is hidden. Conversely it is sometimes possible to see beyond the South Pole and the North Pole is hidden.
The combination of the three kinds of libration allows observers on Earth to see 59% of the lunar surface. However 41% remained hidden from view until the space age.
Questions to think about
- Why do we not see an eclipse of the Moon or an eclipse of the Sun every month?
- The Earth has a diameter of 12,756 km, whereas the Moon is only 3476 km across. How many Moons would fit inside the Earth?
- Why did we have no knowledge of some of the Moon's surface prior to 1959? How is it that we can see 59% of the surface from the Earth?