East Side of the Moon

Because of various quirks in the Moon’s orbit around the Earth, its rotational speed and the angle at which its equator varies from its orbital plane, we actually get to see almost 60% of the Moon’s surface instead of just 50%.  At different times over a 30 year period, we may get to see the Moon apparently tilt north or south or east or west so that parts of the Moon which are on the “dark side” from our earthbound perspective suddenly become visible for a brief period.  This apparent lunar tipping is called “libration”.  If you check Sky & Telescope you can find out which part of the lunar limb is tilted in our direction (I believe Astronomy magazine has a similar feature).

I happened to be observing the night of January 2, 2015 when I noticed that the east limb of the Moon looked dramatically different from the way I was used to seeing it.  As luck would have it, the Moon’s libration tipped the east limb significantly in Earth’s direction, offering a view I think I had only seen a couple of times over the many years I’ve been observing.  The image below was taken with my Celestron NexStar 6” SCT at f/10 and my Celestron Neximage 5 Solar System Camera.  It is a stack of the best 10 out of 92 frames processed first in Registax and finished in Photoshop Elements.

You will note that there are two dark gray areas on the left edge of the Moon in this image.  The top one is Mare Marginis and the lower one is Mare Smythii.  Normally, if you see these maria at all, you’re seeing only a sliver of the western (right) portions.  On this night, I could see virtually all of both features, even the walls or hills on their far shores.  Of course, being on the edge of the lunar disk, both appear severely foreshortened.  I took several 10 and 15 second videos at a few different settings and managed to get one that was reasonably well focused and not overexposed.

Mare Marginis is quite different from its neighbors, Mare Smythii and Mare Crisium (the dark, flat plane to the right of Marginis and on the right edge of the photo).  Unlike the latter two maria which are circular ancient impact basins, Mare Marginis has a highly irregular shape.  At its largest extent, it stretches for 260 miles.  It was formed by thin layers of basaltic lava filling a low lying area covering about 38,400 square miles, slightly larger than Indiana.

Mare Smythii lies on the lunar equator and is south (below) Mare Marginis.  Mare Smythii was named for British admiral and astronomer William Henry Smyth (1788-1865). It is a circular impact basin formed by a large asteroid impact very early in the Moon’s history during the pre-Nectarian epoch between 4.5 and about 4 billion years ago.  This early impact basin was filled with basaltic lava during the Moon’s next two geologic periods, the first layer being deposited during the Imbrium period from about 3.8 to 3.2 billion years ago.  Buried beneath the basaltic layers and under the basin floor is a mass concentration identified by gravity sensors on a lunar orbiting space probe.  Mare Smythii is 232 miles in diameter and covers about 64,500 square miles…about the same area as Florida.  As big as Mare Smythii is, it’s considerably smaller than the much easier to see Mare Crisium, which is just over 350 miles in diameter and covers over 109,000 square miles, slightly more than Colorado with Delaware thrown in.

Jay Albert, April 11, 2015