April’s top lunar feature to observe

Ptolemaeus

Type: Walled plain

Size: Length 154km

Longitude/Latitude: 1.8˚ W, 9.2˚ S

Age: Older than 3.9 billion years

Best time to see: First quarter (9 April) or six days after full Moon (23 April)

Minimum equipment: 10x binoculars

Ptolemaeus is an immense crater that sits just below the centre of the Earth-facing side of the Moon. It is battle-worn, with a rounded rim that has been heavily eroded by the impacts that followed after its formation. It’s an interesting exercise to circumnavigate Ptolemaeus’s rim, counting the number of small craterlets that lie within it. To its credit though, despite its age and battered appearance, it is whole.

Ptolemaeus has a diameter of 154km. Inside its eroded walls, the crater’s floor has been smoothly surfaced in lava – well almost. Although it looks smooth under direct sunlight, when the Sun appears low in Ptolemaeus’s sky, bumps and depressions are revealed in the crater floor which hint at what lies beneath. The ghostly forms of smaller submerged craters can be seen, the vaguest hint of a raised rim betraying their existence. It’s really worth spending time here, trying to detect the barely visible.

Bumps and depressions in Ptolemaeus’s floor hint at what lies beneath

Not all craterlets inside Ptolemaeus are as subtle. 9km Ammonius is the most obvious with several smaller craterlets around the 4km size, such as Ptolemaeus L, Ptolemaeus S and Ptolemaeus D.

A whole peppering of smaller craterlets can be revealed using high-resolution imaging. Ammonius is circular and easily seen using a 100mm instrument. It’s also a superb example of a bowl-shaped crater, its curving walls leading down to a small, flat floor, just 2km in diameter lying 1.9km below the crater’s rim. Ptolemaeus B sits just north of Ammonius. It has a diameter of 11km but is not prominent under direct illumination. This ghost crater is best seen when the Sun is low in its sky.

The circular boundary surrounding Ptolemaeus’s plain is rudely inverted along its southern section. Here, fractionally younger 118km Alphonsus has stamped itself onto the lunar surface, its northern edge overlaying the southern edge of Ptolemaeus in the process. Like its larger neighbour, Alphonsus is a walled plain, but it does at least have a small central mountain complex. A region of bumps runs almost north-south along the centre of Alphonsus, another feature best seen under oblique illumination. In addition, look out for numerous dark patches on Alphonsus’s floor. These have been formed by pyroclastic fire fountains, regions where dark material has spewed forth from tiny crater pits. The fire fountain craters located near Alphonsus’s northeast rim have been given the names Ravi (1.6km), Monira (1.1km), Jose (1.2km) and Soraya (1.9km).

If you’re in the region, look for the 32km, irregularly shaped Ptolemaeus E, located near Ptolemaeus’s southwest rim. Next to it lies 16km Davy G, marking the northeast end of a crater chain known as Catena Davy. Stretching for 57km towards 35km Davy, it is formed by tiny craterlets around the 1km diameter mark. A number of these have familiar names such as Harold, Delia, Priscilla, Osman and Susan.

At certain times close to first-quarter, specifically colongitude 2.9˚, the Sun casts a peak shadow over Ptolemaeus’s floor, which reaches over to Ammonius. For a short period and best seen using smaller instruments, the shadow resembles the form of the head and neck of the Loch Ness Monster, a clair-obscur effect known as Nessie.