Alphonsus

The middle crater in the Ptolemaeus-Alphonsus-Arzachel chain, famous for dark-halo craters.

Visual Description

Alphonsus appeared as a well-defined crater between the larger Ptolemaeus to the north and Arzachel to the south. The crater showed good wall preservation with visible terracing. The floor contained a small central peak and several dark patches. The crater demonstrated intermediate features between the ancient Ptolemaeus and younger Arzachel.

Notable Features

• Central peak on crater floor
• Dark-halo craters (volcanic vents) on floor
• Part of prominent three-crater chain
• Rille system on floor (telescopic)
• Evidence of volcanic activity

Notes

• Named after King Alfonso X of Castile
• Best observed 8-10 days after new moon
• Famous for dark-halo impact craters (pyroclastic deposits)
• Coordinates: 13.4°S, 2.8°W
• Ranger 9 impacted here in 1965