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The Face of the Moon

When you compare any picture or map of a full Moon with the Moon itself, you will see that the image does not match up with what is in the sky. Some maps are printed upside down for the convenience of users of telescopes that produce an inverted image. Even disregarding that, it is very rare that your view of the face of the Moon will exactly correspond with its picture.

At any one time we can see exactly 50% of the Moon’s surface. Because of various peculiarities of the Moon's orbit around the Earth, we are able to see over a period of time over half of the surface of the Moon. We would never be able to objects such as Tycho crater near the northern limb were it not for the slight variations in the Moon's orbit from a perfect circle called librations. The north-south libration is caused by the six and a half-degree tilt of the lunar axis. When the Moon is placed so that the northern end of the axis faces towards the Earth we are able to see the area around the Moon's North Pole. When the southern end of the axis faces towards us, we can see the South Pole of the Moon. The Moon does not revolve around the Earth in a perfect circle, but an ellipse. At various points of its orbit, the Moon rotates around its axis at a different rate from which it revolves around the Earth, so that we re able to peak around its limbs. As a result of the librations of the Moon, 41% of the Moon is in view all the time, another 41% is permanently hidden from those of us on Earth, and the remaining 18% can be seen over a period of time.

A sky almanac will give detailed information on the Moon's librations to enable you to know precisely what part of the Moon will appear on a particular night. Keep in mind that not only will the real Moon not exactly match a map of the Moon, but that in order to see a lunar feature around the Moon's limb, you may have to wait for an appropriate libration.


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