February 19, 2020

29.9.9: Can you Spot the Cave in Copernicus?

I’ve got a unique challenge for you, as you brush up on your lunar geography in anticipation for next weeks’ LCROSS impact. Next time you’re viewing the waxing gibbous Moon with your friends, amaze them (or make them think your totally crazy) by issuing the off-handed remark; “Did you know that there is a ‘cave’ in the crater Copernicus? The “cave” in question is, of course, an optical illusion. Its interesting to note, however, that in the pre-Apollo era, would-be Selenographers were faced with a lunar landscape that was much less straight forward. This first came to our attention while reading a February 2003 article in Sky & Telescope written by Steven O’Meara. The cave itself rests on the northern inner lip of the crater and is elusive unless caught at the precise sun-angle of 10.7 degrees above the local lunar horizon. This generally occurs around 10-12 days of age, and I encourage you to take a look early this week. We’ve never seen this phenomenon ourselves, so we’ll be probing new territory right along with you.

So, what is the cave? Apollo and unmanned orbiter imagery suggest that it is in fact a terraced slope embedded in the crater wall, or perhaps a collapsed crater itself…sketches show a swirling, whirlpool-like structure. Caves, arches, and other spurious structures have been claimed on the lunar surface as the light angle changes over the lunar month… keep an eye out for the “Cave of Copernicus” during this and other lunations… it’s (bad pun alert)..pure lunacy!

This weeks astro-word is…libration. During its path around the Earth, the Moon slowly “rocks” back and forth and side-to-side in a motion known as libration. This assures that the shadow angle of the forward, Earth-facing side of the Moon is never precisely the same from one lunation to the next. This also means that although the Moon is tidally locked with one face towards the Earth, we can occasionally “peer” over the edge, seeing a full 59% of the total lunar surface, albeit we see the limb at a very oblique angle. The reason for the curious motion is because the Moon is tipped about 6.6 degrees in relation to its orbit (causing north-south libration) which in turn is tilted 5 degrees in relation to the ecliptic, although the Earth’s own rotation also plays a tiny role. To confuse things even more, the Moon also speeds up and slows down along its elliptical orbit, causing side-to-side, or east-west libration. The term itself comes from the Latin libre, meaning to sway. This effect has been documented at least from Galileo’s time in the 1600′s, and can cause the familiar features of the Moon to change radically!

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