Twin craters, couples, doublets, and triplets
Telescopic observers of the moon have noticed them since selenography's early days: twin craters. Some of these twins are very pronounced and don't require very powerful telescopes. A typical example: Cardanus + Krafft in the western part of Oceanus Procellarum. Most of the lunar surface's twin craters are too small to observe through common telescopes, such as the unnamed couple of bowl-shaped craterlets just west of the Fresnel Ridge (Hill 305) near Apollo 15's landing site.
There are triplets too, such as the well-organized trio on the floor of Endymion.
The list below is an overview of well-known and unknown twins, couples, doublets, and triplets (to observe through binoculars and telescopes) (small, common, and powerful telescopes).
Note that the word "twin", as used here, means only that a pair of nearby craters is similar in size and appearance. It does not necessarily imply that they were born at the same moment. But some - such as Ariadaeus and its A were. Simultaneous impacts can be identified by a slightly curved ridge (septum) occurring where the two rims touch. For craters that do not touch it is difficult to determine if they were formed together.
Overview of interesting couples and triplets
- Beer and Feuillée. A well-known pair. An unnamed catena runs east-southeast of Beer, which is an interesting test-object for common and powerful telescopes. Frequently photographed during the mission of Apollo 15 (orbital photography).
- Cardanus and Krafft. A well-known pair in the western part of Oceanus Procellarum. Both craters are connected to each other via Catena Krafft. The two craters are observable through common telescopes, the connecting catena through powerful telescopes.
- Carmichael and Hill, near Sinus Amoris. An interesting pair for small and common telescopes.
- Doppelmayer J, K, and L. A trio in the centre of Mare Humorum. Observable through common telescopes.
- Draper and Draper C, in Mare Imbrium near the Montes Carpatus. Observable through common telescopes.
- Triplet (row of three craterlets) on the floor of Endymion. Discovered by Major Molesworth (1894); see page 159 of T.W.Webb's Celestial Objects for Common Telescopes, Volume One; The Solar System. A curiosity, to observe through powerful telescopes!
- Eratosthenes A and B (south of Wallace). A powerful telescope is required.
- Fra Mauro H and HA* Slight overlap but somewhat old so can't tell if there was a septum; however H is considerably fresher than HA so probably not simultaneous formation.
- Fontenelle C + F, which look somewhat like the keyhole-shaped Fauth, although both craterlets (Fontenelle C and F) were depicted as two separate craters on Rukl's chart 3.
- Hercules J and K, which is a neat pair; observable through common telescopes.
- The unnamed pair of bowl-shaped craterlets west of Hill 305/ Fresnel Ridge. See: AS15-P-9804 (right half of the photographic strip). This is a VERY interesting pair to test your telescope's optics (if it's possible to split the pair into two craterlets, then your telescope is O.K.!). A powerful telescope is required.
- The unnamed pair of craterlets on the floor of Kies, which is also an interesting test-object!
- Galle B and BA, north of Aristoteles. Ideal for common and powerful telescopes.
- Democritus B and BA, southwest of Lamech's Danjon (which is the incomplete formation between Kane and Gartner).
- The unnamed pair of craterlets northwest of Pytheas (at 21°50' North/ 22°15' West). This is a very good test-object! Nearby this pair, there's an interesting cluster of craterlets (at 21°10' North/ 22° West). In this region (around Pytheas) there are lots of secondary craterlet-chains, related to the formation of the large crater Copernicus.
- The well-known couple of craterlets on the floor of Plato. This is probably the most observed one of the small couples. Sometimes it's as if this couple is "not there", and because of this strange "absence", lots of articles were written, to describe the "disappearance" of Plato's couple! (see also the LPOD of April the 14th, 2008: a spectacular orbital Kaguya photograph of Plato!).
- Gambart B and C. Observable through common telescopes. Gambart C was once called Moreux by F.C.Lamech.
- Helicon and Le Verrier. A well-known pair in Mare Imbrium, near Sinus Iridum. Observable through small telescopes and binoculars.
- Lichtenberg AA and anon, at 29° North/ 63°20' West. Common to powerful telescopes are required.
- The isosceles triangle (triplet) east-southeast of Markov. A curiosity, to observe through powerful telescopes!
- Messier and Messier A (formerly known as W.H. Pickering). The most well-known pair on the lunar surface. Not exactly a twin, because one of the two is elliptical. The Messiers were frequently photographed during the Apollo program (orbital photography).
- Secchi A and B (both captured on Lunar Orbiter 2's Frame 2024).
- Ritter and Sabine, at Mare Tranquillitatis's southwestern rim. An interesting pair for small and common telescopes.
- Ritter B and C. These are two bowl-shaped craters just north of Ritter. In between these two, there's a small craterlet which is a fine object to test the optics of your telescope! (source: the dedicated Belgian moon-connoisseur Antoine van der Jeugt).
- Smithson (formerly Taruntius N) and Taruntius O, in northeastern Mare Fecunditatis (Sinus Successus). Observable through common and large telescopes.
- Taruntius K and P, at Dorsum Cayeux. Observable through common telescopes.
- Sirsalis and Sirsalis A (Sirsalis A was once called Bertaud by H.P.Wilkins). Observable through common telescopes.
- Steinheil and Watt. A well-known pair near Mare Australe. Observable through common telescopes.
- Strabo's triplet (Strabo L, B, and N). Observable through common telescopes.
- Theon Junior and Theon Senior, southwest of Mare Tranquillitatis. An interesting pair of bowl-shaped craters, for small and common telescopes.
Simultaneous Impact Twins
- Ariadaeus and A. Simultaneous formation
- Simultaneous impact? doublet SE of de Gasparis. Yes. DeGasparis E and F.
- Triplet east of Encke (with Encke M in it). At least two craterlets of this triplet are simultaneous impacts. A common or powerful telescope is required to split the row into three craterlets!
- Doublet and triplet in Landau - simultaneous or secondaries?
- Plato K & KA
- North of Moro Dome on this Apollo 15 image
- South rim of Boyle three touching craters, two with straight wall between them.
A.J.M.Wanders, Op Ontdekking in het Maanland (Het Spectrum, 1949).