Deirdre & Peter -
Sorry to take so long to get back to you about this.
I have been searching in vain for some other observation made under lighting conditions similar to those Deirdre would have experienced on August 20th, but so far have failed. I am sure there are many; it is just hard to find them because the millions(?) of lunar photos and observations posted on the internet are not organized in any systematic way. I don't, myself, even have access to a systematic list of the carefully-timed plates available in the standard printed Lunar Atlases. Of the internet-posted observations, many, sadly, fail to note the circumstances as clearly as Deirdre and Mike Portuesi do on their sketches.
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The difficulty of identifying lunar observations -- and our very human ability to interpret them as representing something other than what they may actually be -- has been something of an obsession for me since reading (a couple of years ago) the 1979 article by the famed American selenographer Ewen Whitaker (author of
Mapping and Naming the Moon, Cambridge University Press, 1999) in which he claimed to be able to date the first vague lunar sketches of Galileo by identifying the features depicted in them. Whitaker probably has/had more experience in the identification of lunar features than any other human being. Yet, just as Deirdre's sketch can be "read" with almost equal ease as a depiction of Letronne or Mersenius, I am truly skeptical that anyone can unambiguously identify most lunar sketches without knowing the circumstances (date/time) under which they were made. It bothers me that Whitaker can say, with the greatest confidence, that a particular dot along the terminator in a Galileo drawing "must" be thus-and-such peak or crater (yielding a definite date and time for the drawing), when to the layman it could represent almost anything.
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Returning to identification of Deirdre's drawing and my search for a matching photo, we have the advantage of knowing the date, and therefore the exact lighting circumstances, at the time of her observation. But there seem to be relatively few souls hardy enough to get up at three in the morning to stand in the chrysanthemums and admire the slender waning crescent of a 25 -26 day old Moon. Or perhaps those that do are just too sleepy to record their observations as carefully as Deirdre. Mike Portuesi's sketch of Mersenius, like the Apollo photo of Letronne, and most others I have found, is taken with the opposite lighting of a waxing gibbous Moon, easily observable in the early evening.
Although I could not find a suitable observation of Letronne, I
did find several that would seem to rule out Mersenius as a possibility. The most accessible of these is a photo taken by Belgian amateur Jan Simons on October 10, 2004 at 4:33 UT:
www.astronomie.be/Jan.Simons/myastroshot.../200410100433ut.html
I should perhaps have mentioned earlier that in connection with my obsession I developed a PC program, similar to VMA, that permits one to visualize/document the lighting conditions at any date/UT, and (unlike VMA) easily predict past/future dates/times at which the same lighting will recur. In collaboration with Danish moon enthusiast Henrik Bondo I have been developing this into a freeware application which, should you be interested, you can download at Henrik's site:
www.HenriksUCLA.dk
. We call it LTVT (Lunar Terminator Visualization Tool). It may not have the pizzazz of a home-grown Irish product like Gary Nugent's
LunarPhase, but it does serve a purpose, and it is free.
Jan's photo shows Mersenius on the terminator with the entire rim, including the east wall still in bright sunlight. LTVT, like VMA, places the theoretical position of the lunar terminator at a lunar longitude at the time of Jan’s photo a little past midway between Mersenius and Letronne. The altitude of the Sun as seen from the center of Mersenius would be +2.5 degrees, while from Letronne it was -4.3 degrees. Clicking on the center of Mersenius and asking LTVT for other dates on which the Sun would be at the same angle we find that on Aug. 20, 2006 Jan's lighting angle over Mersenius would have been observable at 10:16 UT.
This is nearly five hours after Deirdre finished her sketch, and well into the morning in Wicklow. Yet even then, Mersenius would (as shown in Jan's photo) still have appeared as a full circular crater with a bright central peak and the east wall still in full sun, not lost in the lunar shadows as it appears in Deirdre’s drawing.
So I think it is fair to say that despite the strong resemblance in the features, the drawing cannot represent Mersenius as Deirdre could have seen it from Wicklow on Aug. 20 at 03:30 – 05:35 UT.
This does not in any prove that it is Letronne, but it does seem to rule out Mersenius.
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But all is not lost! According to LTVT the lighting that Deirdre
did see will repeat, but about an hour later, on October 18th. So if Deirdre, or anyone else in Europe, has a chance to look, at say 5:00 UT on Oct. 18, they will see what Deirdre saw at 4:00 UT on Aug. 20 and so on. Someone looking that morning should be able to easily recognize the features that caught Deirdre's eye.
Should it be cloudy, there is also a chance to see the Aug 20 2006/4:00 UT lighting on Dec. 16 at 9:13 UT, though the Sun will be a bit up by then (3.8 degrees). Additional opportunities, from Ireland, to see the terminator near Letronne in its Aug 20 2006/4:00 UT position will be May 13 2007 3:51 UT, June 11 2007 15:07 UT (sun up) and July 11 2007 02:10 UT (LTVT can give you the list beyond that).
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From my location in California, I don't have a good chance to see Deirdre's lighting until Feb. 13 next year. If you haven't solved the mystery by then I'll try to remember to take a look!
-- Jim
