Can you spot the Moon’s eastern libration features this month?
The Moon is tidally locked to Earth, rotating once on its axis in the same time it takes to complete each orbit. From Earth this means we get to see the same familiar features revealed and hidden as the Moon passes through its phase cycle. Looking at the Moon at higher magnification reveals that there is slight variation in this view. This month’s challenge is to take advantage of this effect to spot some tricky edge features.
The variation is mostly caused by the Moon’s elliptical and tilted orbit. The ellipticity causes a variation in the Moon’s orbital speed which allows us to see slightly further round the eastern or western side of the Moon’s globe. The tilt causes the Moon to bob up and down relative to the plane of Earth’s orbit (the ecliptic plane). The tilt isn’t much at around 5°, but it does allow us to see further round the Moon’s southern and northern edges.
The variations are collectively known as lunar libration and over time 59 per cent of the Moon’s globe can be seen. Actually, seeing 59 per cent of the Moon’s globe is easier said than done as in order to see a specific edge feature, the libration has to be favourable as does the lunar phase. In addition, the Moon has to be above your horizon and, of course, the weather has to play ball.
This month there is a good opportunity to spot eastern libration features; lunar features which can only be seen under favourable libration. The very thin, 2%-lit waxing crescent Moon on the evening of 3 March shows the three eastern seas, Mare Humboldtianum, Mare Marginis and Mare Smythii. The magnificent 207km crater Humboldt is also well positioned on this date. This Moon sets 70 minutes after the Sun, so isn’t an easy target.
On the evenings of the 4th and 5th, the Moon appears at a greater phase and stays up progressively longer after sunset. Libration remains favourable for the features mentioned, but starts to become less optimal after the 6th.
One aspect which does work in our favour during March is the position of the waxing Moon in the early evening sky. The ecliptic makes a steep angle with the western horizon around sunset at this time of year. Despite its orbital tilt of 5° from the ecliptic plane, the Moon tends to stay close to the ecliptic in the sky, and consequently, it’s approach vector towards the western horizon in March also appears steep. This gives it a good altitude post sunset which is ideal for us when we want to observe or image it.