During the last post to this blog I commented on the "Mare Crisium Sunset Ray" a couple of times. I thought I'd try to post the photo of the Mare Crisium area taken last weekend along with some photos and sketches from past years to try to explain this fleeting event on the Moon a little more thoroughly.
Mare Crisium is one of the smaller "seas" on the Moon; large flat plains of volcanic rock that are left over from giant asteroid collisions that took place billions of years ago when the Moon was first forming. It sits on the eastern edge of the Moon, and I've always found it showy because of it's dark color and because it has some pretty lofty mountains that ring it, making it a real beauty through the eyepiece when it's experiencing sunrise or sunset. This photo of the Full Moon shows the location of the Mare, using a red square:
From two to three days after Full Moon, sunset occurs on Mare Crisium. The terminator (the line that divides day and night) creeps across the plain, craters that lie within Mare Crisium grow long shadows, and the mountains that surround it also cast shadows over the dark material of the plain. I was observing and sketching this area on the morning of February 24th, 2008 when I saw that there was a long streamer of sunlight on it's southern edge, caused by the sun shining between a gap in the mountains. I was able to see this again when I observed it on August 19th that year. Unfortunately, I haven't seen it since then. A "day" on the Moon lasts 29.5 days on Earth, and this phenomenon only takes place over a matter of hours during each Lunar Day. I haven't been able to catch the Moon at exactly the right time to see it, but hope to soon. If I do, and especially if I manage to photograph the Sunset Ray, I'll try to post it on this blog.
To try to show what the Mare Crisium Sunset Ray looks like, and get an idea of when it might be seen, I thought it would be worthwhile to show some photos and sketches that show how this area looks as sunset happens. This is a photo I took on November 1, 2012. It shows the terminator about halfway through Mare Crisium. It's still to "early in the evening" on the Moon for the Sunset Ray to show up. Craters within Mare Crisium are showing a lot of relief because of the low sun angle and their floors are in shadow. The terminator here is at about 62.0 degrees East.
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This is the same photo from November 1, 2012 with some of the craters within and outside of Mare Crisium labeled. Mare Crisium itself is about 570 km (350 miles) in diameter. Two of the most obvious craters within it are Pierce (diameter = 11 miles) and Picard (diameter = 14 miles). Both of these craters were starting to get deep shadows across their floors. Also starting to show a lot of floor shadow was the small but sharply defined crater Greaves (diameter = 9 miles). Along the edge of Mare Crisium are two interesting flooded craters, Yerkes and Lick. These are probably craters that existed before the impact that created Mare Crisium, and when dark lava flooded that big crater that created the plain, it also flooded the floors of Yerkes and Lick. All we see are the crater rims that managed to stay intact. This photo also shows some craters around Mare Crisium, mainly for reference; Macrobius, the bright ray crater Proclus, and the flooded crater Tebbutt. The Mare Crisium Sunset Ray seems to be associated with the mountains just north of (above) Tebbutt.
This is a sketch I made at the telescope while observing on the morning of February 24, 2008. This was the night I first noticed the Sunset Ray. It shows some of the craters that were in the photo before. The longitude of the terminator at the time was 56.9 degrees East. Since the terminator (or sunset line) moves a little over half a degree of longitude per hour, this shows how the area looked the equivalent of ten hours after the photo above it was taken. The floors of Pierce and Picard are still in shadow. Greaves also has a floor in shadow, but the shadows cast by the mountains along the western rim of Mare Crisium have nearly covered up that whole crater. The flooded crater Yerkes is still visible, but the other flooded crater Lick is also almost covered up by the mountain shadows that are starting to cover the Mare. The mountain shadows were long enough to meet the sunset line on the south (lower) part of Mare Crisium, except for the Sunset Ray which stretched across it just above the crater Tebbutt.
This is a sketch made at the telescope of the same area on the morning of August 19, 2008; the second time that I was able to spot the Mare Crisium Sunset Ray. At the time the longitude of the terminator was at 55.8 degrees East, so it shows the area the equivalent of two hours after the first sketch, and the equivalent of about 12 hours after the photograph. The craters Pierce and Picard by this time were so deep in shadow that they looked like black ovals on the plain. So did Greaves, and it was at the edge of the shadow being cast by the mountains to the west and about ready to be covered up. Yerkes was still visible and the low sun angle made its low rim stand out more. Lick was no longer visible since it was totally covered by mountain shadow. Tebbutt was also starting to get covered by shadows. The Sunset Ray was thinner now and shorter, but still visible just north (above) Tebbutt.
This is a photograph I took last weekend on the morning of Saturday, March 30, 2013. The longitude of the terminator here was 50.6 degrees East, so it's the equivalent of how Mare Crisium looked 22 hours after the first photograph, or how it would look 12 hours after the first sketch and 10 hours after the second sketch. Mare Crisium itself has now experienced sunset and only the mountains on the west rim are showing. (I thought the bright spot within the Mare was part of the rim of Yerkes crater when I first published photos of the Moon from Saturday in my previous post, but now I'm thinking that it's an isolated high point just inside the rim of Mare Crisium.)
Here's the same photograph, with the craters Macrobius (diameter = 40 miles) and Proclus (diameter = 17 miles) labeled. These craters are mainly for reference so this photo can be compared to the first one in this entry.
By compiling these photos and sketches, the Mare Crisium Sunset Ray seems to last, at most, 18 hours or so during each Lunar Cycle, but I have a hunch it can be observed for only about 12 hours at most every 29.5 days. I hope I can catch it again sometime soon, and also image it. It really is stunning to see! The fact that such fleeting details can only be seen for such a short time each month is one of the mind-blowing things about observing the Moon to me!
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