Posidonius 31.8°N, 29.9°E

Posidonius is a spectacular FFC crater, with a diameter of 95 km, lying on the northeastern edge of Mare Serenitatis. The crater walls are about 2.3 km above the crater floor. Long after the impact, the floor was uplifted by post-impact volcanism, which created the Rimae Posidonius rille system.

One of the rilles runs directly north at the foot of the western crater wall. At the end, the rille seems to run out into a series of crater pits. Along the eastern inner wall of the crater, following its curvature, there lies a prominent ridge (possibly landslides from the inner wall). Near the centre of the crater's floor is the crater Posidonius A (c. 15 km). Posidonius exhibits many remnants of central peaks. Northeast of the crater Posidonius A, separated by a broad rille, lies a central mountain with two sharp peaks. When observing with larger telescopes, they may be used to estimate seeing conditions and the resolution of the optics. The two peaks have a separation of 1.9 km which, at the Moon's average distance, corresponds fairly exactly to 1 arcsecond.

Chacornac

29.8°N, 31.7°E

An almost completely destroyed crater, 51 km in diameter, directly adjoining Posidonius to the south. The crater rim has a slightly hexagonal shape. Rimae Chacornac, some small linear rilles on the crater's floor and south of the crater, are visible at high magnifications.