Vol 49, No 4, April 2012 - BAA Lunar Section

More documents

Recommendations

Info

The Plato ‘hook’ Phil Morgan On the evening of 2012 March 01, 18:25 UT, Col.13.1°, I was observing the Moon, but with nothing of particular interest on offer, I turned my attention to Plato and was struck by the sight of a particularly fine appearance of the famous Plato ‘hook’ effect. This was unexpected because I assumed that it would not be visible until the shadows had receded a bit more. Although the seeing was unsteady at first, when the mirror had cooled down it was quite good. Throughout the observing session the curve of ‘hook’ was always to the south. Just to check that this was indeed so I orientated myself so that the image of the Plato west facing shadows were facing down to my feet—that is to say that outer the western rampart was facing downwards instead of across my field of vision. Even with this orientation the curve of the ‘hook’ was always towards the right (southern) rampart, and those smaller shadows on the extreme northern half of the crater’s floor were always curving to the left (north). Other experienced lunar observers have also noted this hooked effect at a much greater shadow length than that recorded originally by Wilkins and Moore at Meudon in April 1952. At right (Figure 1) is an observation made by Eric Strach on the 1967 March 19, colongitude 13.2°, showing a curve of the longest shadow to the south. Below this at lower right (Figure 2) is an observation made by Roy Bridge on 1993 January 01 at 19:20 to 20:45 UT, colongitude 12.2 to 12.9°. 184 mm spec., 261×. This observation depicts a long shadow spire on the southern floor area curving slightly to the south. There will always be some differences in the predicted colongitude and the actual recorded shadow length. This is because the selenographical colongitude is measured from the mean centre of the lunar disc. Plato, being at latitude 52 degrees north, will always tend to be subjected to variations in its predicted appearance, due to the Moon’s axial tilt of 1.5 degrees. The resulting swing in the Sun’s selenographical latitude could, in the worst-case result in a variation of up to plus or minus 4 hours’ difference in the observed shadow length for a given colongitude. Even with this orientation the ‘hook’ was always facing the southern rampart. Apologies for the poor reproduction of Figure 2. No higher resolution version could be found. Ed. Figure 2. Figure 1. 8 BAA Lunar Section Circular Vol. 49 No. 4 April 2012
Join us at our Yahoo! Group: Lunar Topographic Studies Many more lunar observational drawings, including observations featured in previous Lunar Section Circulars, can be found at: http://uk.groups.yahoo.com/group/baalunarsection-topography/ BAA Lunar Section Circular Vol. 49 No. 4 April 2012 9
Page 1 and 2: Director: Bill Leatherbarrow The Br
Page 3 and 4: Topographical notes compiled by Pet
Page 5 and 6: Sunrise on Plato Nigel Longshaw In
Page 7: Encke, 2012 March 03 Peter Grego No
Page 11 and 12: Images Since March’s LSC, lunar i
Page 13 and 14: (detail below) BAA Lunar Section Ci
Page 15 and 16: Peter Oden took this detailed image
Page 17 and 18: BAA Lunar Section Circular Vol. 49
Page 19 and 20: Figure 1. A modified sketch of Aris
Page 21 and 22: 2012-Apr-06 UT 00:21-04:04 Ill=99%
Page 23 and 24: Graze occultations ZC 1198 on 2012
Page 25: Lunar data for April 2012, lunation

Vol 49, No 4, April 2012 - BAA Lunar Section

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?