Popular Astronomy

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June 2017

Section Report 2017-6-28

This Section Report features an image of the Moon at last quarter, made by David Davies on the morning of 19 April. The image is a mosaic of two images captured with an APM 107mm Apochromat and a ZWO ASI 224MC colour camera.

Looking at David's superb image it is easy to identify the ray craters Copernicus, Kepler and Aristarchus, beacons in the Oceanus Procellarum. To the north Sinus Iridum (The Bay of Rainbows) is clearly defined and near to the western limb the dark craters Grimaldi and Riccioli can be seen. In the battered southern highlands Clavius, Maginus, Longomontanus and Tycho are near to the terminator and dramatically lit. At this phase Tycho's ray system does not dominate as it has earlier in the lunation but we see one ray streaking off across Mare Nubium and coming close to the crater Bullialidus.

The image also shows subtle colours that are all but impossible to discern visually, though some observers have reported seeing the plateau by brilliant Aristarchus, across which Schroter's Valley winds, as having a reddish tint. To me it always appears as mustard yellow or yellow ochre, but David's image brings out the ruddy hue reported by many observers.


Tycho appears again in Mike Hezzlewood's drawing, made earlier in the year. This relatively young crater was formed by an impact that would have been easily seen by observers on Earth had there been anyone to see it. Unfortunately it occurred when dinosaurs were the dominant life form on Earth. The 85 kilometre diameter Tycho is of course famous for its rays which, viewed when the phase is almost full, make it the most prominent crater on the Moon. The rays can be seen to spread out huge distances across the face of the Moon and can be traced over a distance of more than 1500 kilometres. A darker ring of about 150 kilometres diameter surrounds the crater. Tycho has terraced walls that reach to around 4800 metres in height and a trio of peaks. Mike's drawing was made when the waxing gibbous Moon was ten days old.

Dave Finnigan's image of the craters Geminus and Burckhardt was made on 1 May, a day before first quarter. These craters lie to the north of Mare Crisium and the crater Cleomedes, Part of which can be seen in the bottom left corner of the image. The 86 kilometre diameter Geminus is a complex crater, apart from the fact that it lacks rays it is similar to Tycho, with terraced walls that are clearly shown in Dave's image. To the east (right, in this image) of Geminus we see Bernouilli. Smaller (57 km) than Geminus and at first glance not such an imposing crater, Burckhardt is distinguished by the fact that it seems to have large ears! The 'ears' are clearly two older craters that were partly destroyed by the impact that made Burckhardt.

Added by:  Robin Scagell