Moonwatch
August’s top lunar feature to observe
As Delaunay stretches towards the southwest, it creates a strawberry or heart-shaped outline
Delaunay
Type: Crater
Size: 46km
Longitude/Latitude: 2.6˚E, 22.3˚S Age: Older than 3.9 billion years
Best time to see: First quarter
(15 August) or six days after full Moon (29 & 30 August)
Minimum equipment: 50mm telescope
Delaunay is a misshapen 46km crater located near the Moon’s prime meridian. It’s located south of the Moon’s equator in a complex highland region to the east of Mare Humorum. There are two guides to its location which will be visible in the same part of the lunar phase cycle as Delaunay itself. To the north lies the large 136km walled-plain of Albategnius, while to the south is the smaller but sharper, 70km crater Werner. Delaunay lies roughly one-quarter of the way north of Werner along an imaginary line joining the centres of Albategnius and Werner.
Delaunay is unusual in that on a first visit it doesn’t really look like a regular crater. Its sides are irregular as a consequence of its great age and it nestles in an uncomfortable manner between 36km Faye to the northeast and 68km La Caille to the southwest. Indeed, it looks like there has been a border conflict to the northeast between Delaunay and Faye, the co-joined rim sections of both craters appear to have been pushed into a more linear shape than the usual arcs you would expect.
As Delaunay stretches towards the southwest, it elongates in a way that creates a strawberry or heart-shaped outline. There is a very distinctive ridge that runs down the centre of the crater. This is best defined to the north, thinning out into a narrow divider as it heads southwest. The ridge helps to create confusion here, as it looks on first glance as if Delaunay is actually two smaller craters with rims squashed together at the ridge.
The inner surface of Delaunay is complex and multi-faceted. The wider, northern portion of the ridge opens out into a small plateau shaped like an equatorial triangle with sides approximately 10km in length.
The region that could loosely be claimed to be the eastern rim appears as a similar triangular plateau of slightly larger dimensions. A number of rounded, elevated features can be seen towards the south of the crater, their raised surfaces eventually merging with the thinning central ridge.
On Delaunay’s western rim is the 6km satellite crater, Delaunay A, a reasonable test for a 100mm telescope. The other small craters that appear scattered around Delaunay are officially designated satellites of Faye and La Caille. Located 180km to the northwest of Delaunay’s centre is the impressive form of 98km Arzachel, the southernmost crater in the distinctive north–south run of three major craters, along with 154km Ptolemaeus to the north and 118km Alphonsus in the middle. The region to the east of this trio and north of Delaunay is peppered with parallel grooves. These are likely to have been formed at the time of the impact which created the massive 1,250km Mare Imbrium basin located further to the northwest. A good example of one of these grooves can be seen leaving the southeast rim of 31km Parrot C, which is located to the northwest of Delaunay.
It’s an interesting exercise to see how far you can follow this groove as it heads north towards the eastern rim of Alphonsus.