Moonwatch
May’s top lunar feature to observe
Langrenus is a classic crater with a rim rising to a height of around 3km
Langrenus
Type: Crater
Size: 133km Longitude/Latitude: 61˚ E, 8.9˚ S Age: Between 1.1–3.2 billion years Best time to see: Three days after new Moon (14 May) or two days after full Moon (28 April and 27 May) Minimum equipment: 10x binoculars
Langrenus is a prominent crater located near to the Moon’s eastern limb. As such it gets affected by lunar libration, a rocking and rolling effect caused by the Moon’s orbit. Being so close to the eastern limb, from Earth, Langrenus appears foreshortened into an ellipse. Although it stays visible throughout the libration cycle, the degree of ellipticity waxes and wanes according to where the crater appears relative to the limb.
Langrenus marks the northern point of a trio of similar sized features, all located at a similar apparent distance from the limb. The middle crater is 177km Petavius, located some 500km further south. Continue in this direction for a further 320km and you’ll arrive at 126km Furnerius, the least distinct of the three. The location of all three craters makes them ideal for viewing when the Moon is in its early waxing crescent phase, a part of the lunar cycle which is particularly well placed for the Northern Hemisphere during the spring.
Langrenus is quite easy to find as it sits on the eastern shore of 600km x 500km Mare Fecunditatis, the Sea of Fertility. This is the large, dark area which is located due south of the distinctive dark oval of 620km x 570km Mare Crisium.
In terms of appearance, Langrenus is a classic crater. It has a well-defined rim rising to a height of around 3km. The crater itself is around 4.5km deep. A series of complex terraces lead down from the rim edge towards a flat floor. Few features can be gained from this floor with smaller instruments, although under oblique lighting it should be possible to determine that the northern half is more rugged than that to the south. For larger telescopes or high-resolution imaging setups, a good challenge comes from trying to resolve three tiny craterlets to the southwest of the central mountain complex. These features measure 2km, 1.4km and 1.0km, arranged in descending size on a line running southeast to northwest.
The dark lava of Mare Fecunditatis acts as a canvas for the impact ejecta from Langrenus. Being so close to the eastern limb, this is harder to see than, say, that of younger 93km Copernicus. In the case of Copernicus, being closer to the centre of the Moon’s disc, we get to see it head on and, coupled with the fact that its features are newer than those of Langrenus, it appears quite magnificent. It’s interesting to wonder how Langrenus would appear if it had a more optimised location as seen from Earth.
Head south of Langrenus and you’ll pass through a bumpy region containing Langrenus Q (12km),
L (12km), P (42km),
X (25km), G (23km) and E (30km). After passing through this region, you’ll arrive at 41km Lohse before reaching the ancient walled plain of 147km Vendelinus.
Travel 155km northwest from the centre of Langrenus and you’ll arrive at a distinctive trio of small, well-defined craters arranged as a rightangled triangle. These are 43km Bilharz to the west, 35km Naonobu to the north and 30km Atwood marking the right-angle.