• Don SpainEmail author
Part of the Astronomer's Pocket Field Guide book series (ASTROPOC)


Alphonsus (71 miles) is directly south of Ptolemaeus and its northern wall abuts on Ptolemaeus’ southern wall. Its walls are 1½ miles above the large flat floor. There is a small, but prominent central peak that is a little over a ½ mile high. The floor has some dark regions; the largest is on the western floor. The eastern floor has an extensive system of rills. This formation is a must show for yourself and your guests at first quarter.

Arzachel (59 miles) is the third of three great craters in a north-south alignment. Ptolemaeus and Alphonsus are its big brothers, but is no slouch itself. It is the deepest of the three with terraced walls that tower over 2 miles about its flat floor. There is a rill system on the eastern floor and an elongated central mountain mass that is over ½ mile high. The floor is rough with hills and small craters. The crater due east of the central mountain is 6 miles across and about a mile deep.

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© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  1. 1.Louisville Astronomical SocietyLouisvilleUSA

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