Good librations

WHEN: 5-12 March and 23-24 March

The Moon rotates once on its axis in the same time it takes it to complete an orbit of Earth. The net result of this is that we always get to see the same face of the Moon – well, almost always. Variations in the Moon’s orbital speed caused by its elliptical orbit combine with variations from a 5º orbital tilt, to give us an extra peek around the edge on occasion.

This cumulative effect is called lunar libration. Catching sight of lunar features in what are known as the libration zones – the regions of the Moon right on the limb that move in and out of visibility for earthbound observers – can be tricky. The complication arises because a favourable libration must coincide with a favourable lunar phase when the Moon is in a good position in order to be useful. There’s no benefit in a feature being libration favoured if the phase places it in the dark of a lunar night.

On the evening of 9 March, libration favours the Moon’s eastern edge. The Moon’s 92% phase at this time means this region will be brightly lit and devoid of relief shadows. This works out well for viewing for nearby seas, which include the Mare Humboldtianum, close to 125km-wide crater Endymion. Another libration sea is the Mare Marginis, (close to the limb by the Mare Crisium) and the Mare Smythii (south of the Mare Marginis). Also look out for dark-floored, 138kmwide crater Neper, which sits between these two seas. Libration is favourable for these features from 5-12 March.

A similar good libration for the western limb occurs from 23-27 March but this time the circumstances aren’t so great. The Moon will be a waning crescent and very low just before sunrise on these dates. If you can get a look, part of the tantalising Mare Orientale will be on view.