It is easy for the complete novice to mistake Langrenus (Figure 10.23) for Petavius (Figure 10.24) and vice versa. Like Petavius, Langrenus lies 60 degrees east of the lunar meridian, but it is 500 kilometers further north. Langrenus is a huge (132 kilometers) crater with fabulously terraced walls, seen at an oblique angle. However, unlike Petavius there is no prominent rille stretching across from center to edge.
Lying at the very tip of the Mare Fecunditatis (Sea of Fertility), close to the southeastern limb of the visible lunar disc is the other crater, Petavius. This is a magnificent formation despite the fact it is foreshortened into an ellipse by its proximity to the lunar limb. In fact, this foreshortening gives the crater a distinctive three-dimensional appearance at high powers, as the terracing on the inner eastern wall is more obviously seen; it is almost as if you were flying in lunar orbit toward the feature. Petavius is 177 kilometers in diameter, thus dwarfing even magnificent craters like Copernicus and Tycho, and it features a dramatic set of central mountain peaks. There are a number of clefts on the crater floor but the most obvious is the distinctive rille that radiates from the central mountains and heads southwest, making it all the way to the western crater wall. This rille reminds me of the arm of a centrifuge every time I see it!
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