January 11, 2015
Originally published January 11, 2004
The Straight Wall is the most conspicuous fault scarp on the Moon. Discovered by Huygens in the 1670s, the fault appears as an 120 km long nearly straight line of shadow for a few days after first quarter (lunar day 9). And then, as in this exquisite image by Thierry LeGault, near last quarter (lunar day 22) the Wall is a bright line reflecting the sunlight. This change from shadow to light shows that the east side of the Straight Wall stands higher than the west side. The absolute height is not well determined - from my shadow length measurements I calculated a value of about 450 m, but most other estimates make it lower. It would be a good project for amateurs to determine an accurate height. Schroeter discovered the lovely rille to the west which starts in a Cobra Head-like elongated depression that splits a small dome. But the least well known feature here is a delicate linear indentation that extends from a break near the northern end of the Straight Wall to the top right side of the image. This appears to be an old and shallow rille or fault that slightly displaces the Wall itself. Understanding of this feature would benefit from images taken under very low sun.
Yesterday's LPOD: Photo of the Century
Tomorrow's LPOD: Western Tranquillitatis
Author & Editor:
Charles A. Wood
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