May’s top lunar feature to observe
Montes Apenninus
Type: Mountain range
Size: 950km long, 100km wide
Longitude/Latitude: 0˚, 19.9˚ N
Age: 3.2–3.9 billion years
Best time to see: First quarter (8–9 May) or six days after full Moon (22–23 May)
Minimum equipment: 10x binoculars
Montes Apenninus is the most impressive of the lunar mountain ranges visible from Earth. It borders 1,250km diameter Mare Imbrium, arcing from the eastern side of the Mare around to the south-southeast. It contains many lofty peaks, which cast striking shadows across Imbrium’s relatively flat lava during the late waxing crescent and early waxing gibbous phases. With a low Sun angle, the jagged peak shadows give a highly exaggerated impression of the mountains within the Apenninus range.
The northern edge of the range begins with Promontorium Fresnel, a rugged outcrop on the southern edge of a lava corridor between 650km Mare Serenitatis to the east and Mare Imbrium. The Apennine mountains appear steep to the western side of the range facing Mare Imbrium; the eastern flanks fall away more gently as they extend radially from the centre of the Imbrium Basin.
The northern part of the range contains Mons Hadley, a mountain that rises to a height of 4,500m. To its south is the Mons Hadley Delta, another tall peak rising to 3,500m, which was seen and photographed as a backdrop during the Apollo 15 mission. A lava inlet to the west and southwest of Mons Hadley Delta is home to the most famous sinuous rille (winding groove) on the Moon, Rima Hadley. Snaking northeast and southwest of 6km Hadley C, the rille requires at least 200mm of aperture to see and is a joy to image using high resolution imaging equipment. The rille’s southern end is marked by four small craters; 5km Carlos, 8km Taizo, 12km x 2km Bela and 7km Jomo.
Progressing south takes us over more high-elevation, sheer cliff faces bordering Mare Imbrium. The largest crater to interrupt the range’s southeast flank is 22km Conon. It sits near 4,200m-high Mons Bradley.
Those with 300mm or larger instruments, or high-resolution imaging setups may like to try for Rima Conon, which sits 82km to the south of Conon in Sinus Fidei, the Bay of Trust. It is 45km in length, but only 2km wide.
There are more sheer cliffs at Mare Imbrium’s edge as the range continues to arc towards the southwest. Two standout peaks appear here in the form of Mons Huygens and Mons Ampere. Mons Huygens is the tallest mountain on the Moon with a height of 5,500m. As the Apennines arc toward a more westerly direction along the south-southeast edge of Mare Imbrium, the massive triangular form of 3,500m-high Mons Wolff can be seen. After Mons Wolff, the width of the range narrows considerably, the southern Apennine slopes being replaced by the lava of Sinus Aestuum, the Seething Bay, a large, flat 230km-diameter expanse of lava to the south.
Montes Apenninus ends near 60km Eratosthenes, a beautiful example of a circular terraced crater with a central mountain complex. Eratosthenes is younger than Montes Apenninus and its ejecta ramparts, outside the main rim, can clearly be seen overlapping the last peaks of the Apenninus mountains to the north.