square on the moon

Just in from a very short session, but I want to get it down while it's fresh in my mind.

While observing with the 12.5-inch, using 71x and the moon filter, from 4:00 to 4:30 UT, I noticed a distinct square on the moon, a little foreshortened by the curve of the moon, but it was there.  It was a relatively dark color, marble green.  It was as if there was a piece of square tape on the moon, left for a long time -- while the moon collected dust the part underneath the tape stayed clean -- then someone removed the tape leaving this clean green square on the moon.  It must have been a trick of the light (waxing gibbous, 86% illuminated).

The square started at Aristarchus and went to Schiaparelli; then turned at a right angle to Humason; then another right angle to Krieger, then back up to Aristarchus.  A corner of the square was bitten off by Aristarchus and Herodotus, which were still bright, but everywhere else within this boundary was a marble green, contrasted with the rest of the brighter surface.

I see on Rukl there are wrinkle ridges and mountains along the boundary line (Dorsa Burnet, some unnamed, and Rupes Toscanelli) -- somehow these define the boundary, either a change in elevation, or some blockage of light.  The square area is much more rugged than the surrounding area, suggesting it may night reflect as much light back and so appears darker.

It's a very surprising and unique sight.  I'm in early just to write this down but also because the moon went behind the meridian tree and the marine layer started coming in.  I hope it is clear tomorrow, I'll see whether the feature is still there.