October’s top lunar feature to observe
Harpalus
Type: Crater
Size: 40km
Longitude/Latitude: 43.5˚ W, 52.7˚ N
Age: Less than 1.1 billion years
Best time to see: Four days after first quarter (6 October) or three days after last quarter (20–21 October)
Minimum equipment: 50mm refractor
Harpalus is a 40km crater located to the north of the distinctive Sinus Iridum or Bay of Rainbows, a semi-circular bay of lava located on the northwest shore of Mare Imbrium. It sits within the section of dark lunar lava which defines the western extremity of the elongated Mare Frigoris, the Sea of Cold. Measuring 1,800km long and 200km wide, Mare Frigoris can be seen running along the top of Mare Imbrium, easily identified to the north of the distinctive 101km, dark oval crater Plato. The region to the northwest of Harpalus, adjacent to 41km Markov, is Sinus Roris, the Bay of Dew.
Harpalus is a relatively young feature on the Moon’s surface, estimated to be less than 1.1 billion years old. As such, its appearance is sharp and welldefined. A ray system surrounds the crater rim, with ejecta ramparts leading up to the outside edge of Harpalus. Inside the crater, a series of wall terraces lead down to a flat floor with three separated mountain peaks near the centre. All in all, the appearance is reminiscent of a miniature version of 93km Copernicus located south of the southern edge of the immense 1,250km Imbrium Basin. The bright and well-defined appearance of Harpalus’s rays categorise it as being of an age bracket known as the Copernican System, Copernicus itself is estimated as being less than 1.1 billion years old.
“A relatively young feature, its appearance is sharp and well-defined”
From Earth, Harpalus appears elongated into an ellipse, a consequence of foreshortening. If viewed from an overhead vantage point, the crater would appear more circular, but there are a number of rim sections that show irregularity, notably those to the east. If you fancy a challenge, possibly best suited for large aperture instruments or high-resolution imaging, there’s a tiny 1.7km craterlet on the floor of Harpalus, situated between the mountain complex and the southwest floor. Harpalus’s position within the western section of Mare Frigoris gives it some degree of prominence, the dark surrounding lava being nicely framed by the highland regions to the north and south. There’s also a lack of other prominent craters in this area, the largest within this section of lava being 10km Harpalus C located 100km to the north.
Between Harpalus and Sinus Iridum sits 23km Foucault, another sharp crater with a rectilinear mountain range touching its northern edge. Using Foucault as the starting point, draw a line from its centre through the centre of Harpalus and you’ll arrive at the large and very ancient form of 108km South. This crater couldn’t be further in appearance from Harpalus if it tried. Its ancient form is indistinct and its weathered floor pockmarked with many small craterlets. South is estimated to be older than 3.9 billion years, an age bracket known as pre-Nectarian. South abuts another ancient crater to the north, 144km Babbage. Like South, this has a very worn rim and a pock-marked interior dominated by 25km Babbage A and 14km Babbage C. To the northwest of Harpalus is an area with a trio of small craterlets, Harpalus E, G and H, with diameters of 7, 11 and 8km respectively.