Posidonius is a big crater of the Moon on Mare Serenitatis. There is a lot of interesting details to observe inside: peaks, ridges, craterlets, … In the following image taken yesterday night, 4 km wide craters can be spot as small white points in the smooth surface of Mare Serenitatis.
Posidonius measures 95 km in diameter. The second biggest crater (a bit ghostly) in the picture is Chacornac, just below Posidonius. Inside it is visible a small craterlet called Chacornac A (it measures 5 km in diameter).
The third biggest crater of the picture is Daniell (31 km wide) located in the upper middle side. Its shape is not circular, but oval. This is the cause of a strange effect in perspective when comparing it with the craters nearby.