Tuesday morning's observing / imaging session of the Moon was a memorable one for a couple of reasons. Ever since viewing and sketching the "sunset ray" on Mare Crisium twice in 2008, I'd never seen this feature since, so this was the first time I'd seen it through the telescope in over seven years! And this was in spite of several attempts. It was also the first time I'd ever photographed it. I seemed to have caught it right at the beginning, so it also gave me a much better idea of when the effect on Mare Crisium starts. Here's a rundown of what I know so far from photographs and sketches:
Lunar Colongitude = 121.49° March 8, 2015 3:20 UT
The photo above was taken early last March, at a time when the mountains along the southern rim of Mare Crisium first started to cast shadows on the plain. It was obvious even when I took the photo that it was still some time before the "sunset ray" was really noticeable.
Lunar Colongitude = 122.63° September 1, 2015 7:41 UT
My first set of photos from Tuesday morning really made the situation clear. It shows this area of Mare Crisium at the equiv 2 hours 15 minutes after the first photo. Those mountain shadows were much more stretched out, but it's also clear in the photo (and I also saw this through the eyepiece) that the mountain shadow to the north hasn't quite reached the terminator yet, so even at this point there wasn't a cut-off lit-up area in between those shadows.
Lunar Colongitude = 123.06° September 1, 2015 8:34 UT
The photo above shows this area just 53 minutes after the previous one, and by this time the northern mountain shadow had reached the terminator; making a true "sunset ray" in between. So now I know that the complete cut-off sunlit area starts around the time that the lunar colongitude lies at about 123°. This was the other real personal discovery on Tuesday morning before dawn!
Lunar Colongitude = 124.2° Sketched August 19, 2008 7:30 UT
Now that I have pretty much pegged down the start time of this monthly lunar phenomena, my main mission in the near future is to find the "end time" and hopefully photograph it. The sketch above was from the second time that I've ever seen the "light ray" in 2008, and I recorded the colongitude then as 124.2°, which is the equivalent of about 2 hours 15 minutes after the last photograph. At the time, the "light ray" seemed narrow and even more detached from the rim of Mare Crisium. (I'd love to get a photograph of it looking like this some day!)
Lunar Colongitude = 129.37° March 30, 2013 5:38 UT
Finally, this photograph shows the lit-up rim of Mare Crisium with the Mare itself all in shadow. It was taken at the equivalent of about 12 hours 30 minutes after the photograph taken when the colongitude was 123.06°. It's obvious at this point that the "sunset ray" is long gone. This photo gives me an extreme upper limit to how long it lasts, but until I make more observations of the Moon between when the terminator is between colongitude 124° and 129°, I'll never know for sure how long it lasts. I suspect it could be as short as four hours, though it could possibly be three times longer than that!
(Postscript - I posted the photo from 8:34 UT Tuesday September 1 on the Facebook page Telescope Addicts - Astronomy and Astrophotography Community on Saturday, September 5 and asked if any other group members had ever seen or photographed what I've been calling the "Mare Crisium Sunset Ray." One reply that I received was from Rick Scott in Arizona. He seemed to think that this was the famous "false arch" along Mare Crisium; where the shadows seemed to suggest a natural arch lies on the rim. This was later proven false. I've read about this in the past but I was never sure where, exactly, this feature was located. He may well be right!)
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