The Sky Guide Chal­lenge

The small­est fea­tures on the Moon.

Sky at Night Magazine - - CONTENTS -

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.

Iridum (left) and crater Grimaldi; if you can see both of these with the naked eye we salute you

The bas­ket­ball player (red) and Lady of the Moon (blue) make for sur­pris­ingly good nav­i­ga­tional aids

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