Elevenday Moon 121

diameter and 6600 feet deep, is termed by Wilkins and Moore "one of the most beautiful and important of all lunar walled formations." Tonight the terminator is far to the west, and the crater is illuminated fully, appearing as a bright ring. It presents an especially interesting spectacle around sunrise or sunset. At such times the relatively low walls appear high and steep, and the triple central mountain shows well as it does tonight. The north wall is broken by the Class 1 crater cassendi a, 20 by 24 miles across and 8500 feet deep, which also may be seen. Note that the young intruder is much deeper than its ancient host. On the south the walls of Gassendi virtually have disappeared over a stretch of some 30 miles, having been melted down by the lavas to a few mere hills. The crater actually is almost exactly circular in shape, but it is foreshortened by its limbward location into an ellipse, the exact shape of which charges noticeably from week to week owing to the effect of the libration. The floor offers a wealth of detail to telescopic observers. It abounds with rills and ridges.

Since Gassendi stands on the north shore of mare humorum, the cause of its ruined south wall is evident. Baldwin considers Mare Humorum, which is about the size of Arkansas, one of the oldest if not the oldest of the circular maria. He believes it was formed as a huge crater 287 miles in diameter excavated by a planetoid which struck the surface in a nearly perpendicular trajectory. The stupendous impact buckled the surface layers of the moon at a distance twice that of the crater edge and resulted in a circular anticline concentric with the mare, 594 miles in diameter, which he has been able to trace in part. Subsequently Gassendi and several other ancient craters along the mare shore were formed similarly by smaller colliding bodies. Then came the lava flows that gave Mare Humorum its present floor. Many years ago the dark floor was described as distinctly green in hue, but more recent (and probably much more accurate) measures have changed the description to reddish. Extensive systems of ridges and rills have been studied on the mare and in the surrounding region, most of them in arcs centered on the supposed point of impact. On the south shore of Mare Humorum may be seen the smaller but equally ancient Class 5 crater vitello, 25 by 29 miles across but only 4600 feet deep. One of the more prominent of the ridges mentioned above meanders northward from Vitello across the mare toward Gassendi. Perhaps with excellent conditions

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