Popular Astronomy

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A saintly trio (SPA News Circular 273)

On the evening of 15 August after sunset the crescent Moon hangs above the western horizon. Take a look at the Moon through steadily-held binoculars or a telescope and you’ll see that the sunrise terminator has rolled back to reveal a prominent ‘chain’ of three craters west of Mare Nectaris (the Sea of Nectar), about one-third of the way up from the bottom of the Moon. These magnificent craters are Theophilus, Cyrillus and Catharina, each of which were named after Christian saints of old.

Above: Steve Bush’s observational sketch of Theophilus, Cyrillus and Catharina, made on 19 April using a 90 mm MCT, 200×

Theophilus (100 km across) is an imposing structure whose prominent circular rim rises 1,200 metres above the level of Mare Nectaris to its east. Inside, broad walls descend in a series of terraces some 4,400 metres to its floor, which contains three hefty central peaks rising to 1,400 metres high. Theophilus’ ramparts display extensive ridges, grooves and secondary impact craters that spread east and north across the marial plains to distances exceeding 100 km.

Theophilus overlies the northeastern wall of Cyrillus (98 km), a similar sized crater that shares many of the same topographic features, only it’s considerably more ancient and eroded than its neighbour, with lower and less orderly ramparts. Its inner southwestern wall hosts Cyrillus A (15 km) and three rounded mountains on its floor rise to heights of 1,000 metres. A mass of ridges breaks from Cyrillus’ southern wall and runs south, across the northeastern ramparts of Catharina (100 km); under low illumination, Cyrillus appears connected to Catharina by a broad valley. Catharina has an eroded rim that is dented by a number of craters. Much of Catharina’s northern floor is occupied by the flooded Catharina P (49 km), parts of whose southern edge are breached and level with Catharina’s floor. Can you see anything of these craters when all the shadows have receded?

Recent observational drawings have been received from Dave Balcombe, Steve Bush, Peter Grego, Dale Holt and Graham Sparrow; lunar images have been received from Anthony Ayiomamitis, Mike Brown, Dave Finnigan and David Scanlan.

Added by:  Graham Sparrow