A good libration for the Moon’s northern limb

WHEN: 20-26 December Hermite

THE MOON APPEARS to rock and roll on its axis as it trundles around the sky each month. This effect, called libration, allows us to see an extra nine per cent of the lunar surface as it progresses through its orbit, compared to what we’d normally see if the Moon were truly locked to show us just one face.

The rocking and rolling occurs in such a way that it brings features at certain positions around the Moon’s limb into good view from time to time. These ‘libration zone’ features are interesting to hunt down, but can be a challenge to identify because they tend to appear foreshortened. What may be a well-defined area of large craters can soon become a jumble of thin ellipses when seen edge on.

The appearance of libration zone features are further complicated by the Moon’s phase, which has to be favourable in order to see anything meaningful. Libration and phase work together to bring the Moon’s northern region into a favourable view from

Worn crater Hermite is one of several on view during this period

of favourable libration – but be warned, it will appear quite elliptical

20-26 December. So if you’re lucky enough to get something ‘astronomical’ for Christmas, this is a great time to try out your new kit. Turn to page 60 for details on how to create a lunar mosaic of the Moon’s northern limb.