Posidonius (Figure 10.22) is one of this author's favorite lunar craters. It has many similarities with Gassendi in that it fits into a corner where a lunar sea (Mare Serenitatis) borders more rugged highland terrain (north of the Taurus
Mountains). However, it lies diagonally opposite to Gassendi on the face of the Moon, as Posidonius lives in the northeast corner and Gassendi lives in the southwest corner. Posidonius is 95 kilometers in diameter and its system of rilles is just as superb as its rival on the other side of the lunar disc. Like Gassendi, the southeastern wall of Posidonius butts up against a significant crater: the 50-kilometer crater Chacornac. In addition, the floor of Posidonius contains at least one decent crater (Posidonius A) and numerous hills and peaks among the network of rilles. Under excellent seeing the rilles on Posidonius' floor are fascinating to behold. Posidonius is best placed a few days after full Moon when the evening terminator approaches the region and throws the rilles into sharp relief.
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