December’s top lunar feature to observe

Pallas

Type: Crater

Size: 50km

Longitude/Latitude: 1.7o W, 5.5o N

Age: Approximately 3.9 billion years

Best time to see: First quarter (11-12 December) or six days after full Moon (25-26 December)

Minimum equipment: 50mm refractor

Pallas is an ancient lunar crater appearing close to the centre of the Moon’s disc as seen from Earth. The Moon’s equivalent to Earth’s prime meridian intersects the lunar equator in the dark 350km lava bay known as Sinus Medii (‘central bay’). Fifty-kilometre Pallas is located in the rough highland region immediately north of Sinus Medii.

Pallas is ancient and battered. It sits between two prominent craters which couldn’t be more apart in their appearance. To the north is bowl-shaped 18km Bode, a relatively young feature with a sharp, slightly non-circular crater rim. To the southeast is 58km Murchison, which is even more ancient than Pallas. Murchison’s rim appears moderately broken south of Pallas and open to Sinus Medii in the southsoutheast section. Pallas and Murchison are open to one another thanks to a small 5km gap in Pallas’s eastern rim.

The rest of Pallas’s rim is quite identifiable despite its battered appearance. An 11km interloper, Pallas A, interrupts the northwest rim section, easy to identify as it sits directly south of Bode. Pallas itself has a well-defined central mountain complex; a feature not present in either Bode or Murchison.

To the south of Pallas lies a hotch-potch of broken crater rims. Here lie the remains of 26km Pallas E and 18km Pallas F, partially rimmed features both with smooth floors. The small 3km craterlet Pallas W sits close to the inner eastern rim of Pallas F, a good test for a 200mm instrument.

Pallas B is a 4km crater located 47km to the southwest of Pallas’s central mountains. It’s a useful crater in that it marks the rough position of what could be a rather unique feature on the lunar surface, a tiny 1.5km crater known as Stuart’s craterlet. This tiny feature is surrounded by a small ring of bright ejecta, something which you would normally associate with a young lunar feature. It is located near the southwest edge of the elevated region directly to the east of Pallas B.

The story of Stuart’s craterlet is interesting and begins on 15 November 1953, when the physician and amateur astronomer Dr Leon H Stuart took a photograph of the Moon that appeared to show a bright event 16km to the southeast of Pallas. This was dismissed by professional astronomers, who suggested it was likely to be a local effect, perhaps a meteoroid vaporising in Earth’s atmosphere.

It wasn’t until many years later that Dr Bonnie Buratti saw Dr Stuart’s image and began an investigation. The upshot of this research was the identification of a 1.5km crater within images taken by the Clementine spacecraft. This tiny feature appeared to match what would have been expected from the impact suggested by Dr Stuart’s photograph and some now believe that Stuart had imaged an asteroid hitting the Moon’s surface. Stuart’s craterlet is pretty tiny, but not beyond the range of modern highresolution imaging setups.

The 23km crater Ukert sits 112km to the northeast of Pallas. The ridges and elevated ranges around it are illuminated around the time of a first quarter Moon to produce the clair-obscur effect known as the ‘Lunar V’.