Good li­bra­tions

WHEN: 5-12 March and 23-24 March

Sky at Night Magazine - - THE SKY GUIDE -

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.

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