The Sky Guide Challenge
The smallest features on the Moon.
Our June challenge is deceptively simple and requires no equipment. We simply want you to look at the Moon and find the smallest feature you can identify.
The Moon’s Earth-facing side shows many bright and dark regions. The bright regions are the reflective lunar highlands, while the dark areas are lavafilled impacts. The dark seas, or maria, form patterns often likened to familiar things, such as rabbits and insects; there’s also a Man in the Moon, a basketball player and the Lady of the Moon. Although these have no physical relevance, they are useful for navigation. We’ll use the basketball player and Lady of the Moon – outlined red and blue here – because they cover most of the lunar surface and are easy to recognise.
The basketball player faces away from us. His head is formed from Mare Imbrium, a large circular sea measuring 1,250km across. His neck and shoulders are formed mostly by the Oceanus Procellarum, stretching 2,500km north to south. The Mare Frigoris represents his shooting arm, a thin strip of dark lava 1,800km in length but only 200km wide, and it’s this narrowness that makes it hard to pick out. The basketball itself is depicted by the 620x570km Mare Crisium, near the eastern limb. This is relatively easy to see.
The Lady in the Moon looks at the basketball player side on. Her hair is represented by the Mare Serenitatis and the Mare Tranquillitatis, both easy targets with diameters of 650km and 700km respectively. The Mare Fecunditatis sits to the southeast of Tranquillitatis and its 600x500km shape is also relatively straightforward. The lower-rear portion of the Lady’s hair is marked by the Mare Nectaris and is far harder to make out, the sea being only 350km in diameter and having a lighter surface appearance than many of the others.
The Lady’s eye is marked by the Mare Vaporum, darker than Nectaris but a tricky 230km across. The mouth is depicted by the Sinus Medii, the darkest portion of which stretches for 120km north-south and 386km east-west. The north-south dimension equates to an apparent size of 1 arcminute, which is around the limit of the human eye’s resolving power. They are best seen around full Moon.
Harder still is the Bay of Rainbows, or Sinus Iridum, forming the basketball player’s ear. Its opening is 246km (2 arcminutes) wide but the bay itself only extends from the shore of the Mare Imbrium by 140km (40 arcseconds). The best time to look for it is when the terminator is near.
If you have managed all of these, try for the ultimate challenge and look for 150kmwide crater Grimaldi, the floor of which is conspicuously dark against its bright surroundings. Its foreshortened shape changes size with libration; when favourably placed it can reach 75x45 arcseconds.