Harry locates and sketches a Moon crater named after the great star cataloguer – Annie Jump Cannon
A sketch of the area at the edge or limb of the Moon containing the craters Plutarch, Seneca and Cannon. Sketch by Harry Roberts
The lunar limbs are difficult to study; the craters crowd together, their floor features hidden by shadows or by their own rims. Ridges can look like craters and hundreds of kilometres of lunar surface is compressed into a narrow strip – it’s hard to identify any given crater. One such region is that between Mare Crisium and the Moon’s east limb where the dominant feature is Mare Marginis, the sea at the “margins”.
The above factors made for a confusing subject – and I dashed away with the pencil hoping to capture a particular crater in the area that had long been on my list: Cannon (see sketch above), named for indefatigable Annie Jump Cannon (see below). It seemed at the time that Cannon was the half lit deep crater left of centre – but next day with atlas and freeware it was clear that Cannon was the shallower crater at the right side on the terminator – only just in my field. Still it was a record at last – and one that may not recur soon.
Parts of eastern Mare Crisium were roughly sketched to give some context for finding Cannon – with Cape Agarum labelled cA near large crater Condorcet. While mapping Agarum I noted what looked like an elongate “vent” with rilles at either end – an interesting feature worth a closer look.
Ring ridges are perhaps the dominant landform in this area – Crisium being a multi-ring impact basin – and remnants of outer rings confuse the view, two such are tagged rX and rY – they may be parts of the “Geminus Ring” (C. Wood, “The modern Moon”, Sky Publishing, note his caution about the rings). Raised terrain north (i.e. left) of Cannon is brightly lit, but a jumble of smaller impacts confuses the view. Here and there light streams through gaps between the row of big craters on the terminator, illuminating higher ground beyond. The two craters north of Cannon are Plutarch and Seneca, named for thinkers from Greece and Rome respectively, but Cannon – well, she’s different in every way.
Who was Annie Jump Cannon? She was perhaps the greatest of the female “computers” [at Harvard College Observatory] who created the Draper Catalogue of stellar spectral types in the 1920s – devising the final (and modern) classification system while personally classifying 225,300 of the original catalogue of 359,000 stars; a gigantic feat! She was no mere “robot”, and in fact redesigned the entire system, supplanting the earlier models, and refining the scheme as her insight grew. We may look at her work in more detail in a future post on stellar spectra, but it’s true to say that her system is the essential key to modern astrophysics and astronomy. We may well cry “Oh! Brilliant Analytical Feat – Great. Key Maker!” [This in an “in” joke. The names of the stellar spectral classes in the system began by Annie Jump Cannon are O, B, A, F, G, K and M with O referring to the hottest stars and M the coolest. The usual mnemonic to remember the order of these classes is Oh Be A Fine Girl/Guy, Kiss Me. Harry is suggesting a clever alternative – Nick]
Harry Roberts is a regular contributor to this blog and a member of the Sydney City Skywatchers.
