Imaging lunar libration
Capture contrasting images that reveal the ‘extra’ areas at the Moon’s edges
he Moon is gravitationally locked to Earth, rotating once on its axis in the same period of time that it takes to orbit our planet. For us on the ground, this means we see the same familiar face of the Moon all the time. There is, however, a small variation in the Moon’s appearance over time, due to an effect called ‘libration’. This describes the cumulative effects introduced by the elliptical nature of the Moon’s orbit and its tilt relative to the orbit of Earth.
The Moon’s elliptical orbit means that its orbital speed around Earth varies over time; when it’s closest to our planet at perigee its speed is highest, and when it’s furthest from Earth at apogee its orbital speed is at its lowest. This variation allows us a peek around the eastern and western limbs of the Moon over time.
The Moon’s orbit is tilted to that of Earth by about 5˚ too, causing the Moon to appear to move above and below the ecliptic. As it does so, we effectively peek over the southern and northern edge of the Moon by a small amount. Together, these libration effects allow us to see 59 per cent of the Moon’s surface, which is 9 per cent more than if the Moon was rigidly locked to Earth.
T▲ Thanks to its elliptical orbit and tilt, we can actually image slightly more than half of the Moon’s surface
One good way to see libration is to look for the elliptical sea known as Mare Crisium, close to the Moon’s northeast limb. At extreme librations, this can look close to the Moon’s limb or quite far away.
Catch two Moons
By capturing images of two different moons you can see the differences in the positions of features caused by libration. October is a perfect opportunity to try this as there are two full Moons; the first rises at 19:15 BST (18:15 UT) on 1 October and it should be possible to get a decent shot from 21:00 BST (20:00 UT). The second is on the 31st, rising around 17:00 UT and favourable from 19:00 UT.
Once you’ve grabbed images of both, you can overlay them so that the features almost line up – they won’t precisely because of libration. The variation can be highlighted in a number of ways. For example, placing both images side by side, the minor differences in positions of features should be noticeable. A more dramatic method is to create an animated image which flicks between both versions.
The visibility of libration on a large scale (eg, in a full disc image) depends on the variance between latitude and longitude libration values over the comparison period. Another more ambitious project can be achieved by attempting to photograph every phase of the Moon over a month. The biggest hurdle to overcome here is the British weather; you would have to be very lucky to be able to catch one phase every day over a month, but it may be possible to grab shorter sequences that still show the effects of libration.
Recommended equipment: 1,000mm focal length telescope, DSLR camera or equivalent
Send your images to: gallery@skyatnightmagazine.com