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Hi Greg,

The field of "transient lunar phenomena" or TLP studies has a checkered history that is literally centuries long. For the most part, the TLP crowd has counted themselves vindicated and moved on to other pursuits since NASA and other space agencies now actively study and observe meteor impacts and other transient phenomena (see PS). But the problem is that this does not prove that the historical observations of strange lighting phenomena were all real, and the original concerns over those observations remain. The biggest problem is changing lighting angles during the two-week lunar day. Aristarchus, for example, just as the Sun is rising at its location, catches the Sun's rays early, since it's tall, and stands out rather dramatically, since its intrinsic albedo is high. Then a couple of days later, it seems much more subdued. Many of these changes were recorded as TLPs by enthusiastic observers. This was a subject of considerable controversy in the 1960s before spacecraft were able to observe Aristarchus on a regular basis. There's a nice article here on TLPs: https://blog.moonzoo.org/tag/tlp/.

By the way, the Radon isotope detected is at exceedingly low trace levels. There are no visible outgassing events --no plumes or clouds of gas over Aristarchus. This Radon, the same invisible trace gas that is estimated to kill 20,000 people annually in the US alone, is an ordinary decay product from Uranium in rocks. Its presence at Aristarchus is often attributed to deep fractures in that plateau that allow the decay products to escape. Amazingly, not a single one of the historically recorded TLPs can be tied to any actual physical event. But don't get me wrong: Aristarchus and the nearby crater Herodotus and Schroter's Valley are amazing lunar features, well worth exploring at different Sun altitudes with backyard telescopes. The changing lighting angles create a different view every day. And it IS a bright crater with an albedo of something like 25%. It may be worth putting that albedo number in perspective. The Moon's average albedo is around 12% which is dark grey, comparable to an average paved road. In fact, you can get the correct photographic settings for a photo of the Moon at night by metering off an average bit of pavement in bright sunlight.

Now back to your image. Clearly Aristarchus does not stand out in the original, but it's a flaring beacon in the enhanced image. I strongly suspect that the filters you applied to the original had the net effect of brightening or increasing the local contrast in portions of your image along the Moon's terminator. I've prepared a pair of comparison images (before you posted the original) which shows the Moon at nearly the same phase alongside the Moon as you posted it originally. Notice that the terminator south of Aristarchus is oddly bright and distinct in your enhanced image. The real Moon always shows considerable darkening as you approach the terminator. Where did that go? Somehow, I would guess that the sequence of filters you applied selectively brightened the brighter points in the terminator region bringing those darker areas up in brightness closer to the mean brightness of the Moon. And since Aristarchus is already bright, it got turned into a veritable lunar bonfire! In fact, this is very similar to the problem that early visual observers faced. Aristarchus can look brighter than normal just by contrast against the dark areas at lunar sunrise. I'm convinced that this is a processing artifact.

As for digital lunars, the terminator is no concern. We don't use it. But the illuminated limb can also be subtly affected by image enhancement. Certain digital filters can make the limb grow or shrink as in traditional photographic "burning" and "dodging", in effect changing the lunar diameter. Of course, if you use the limb for your lunar distance observations, as you would with a traditional lunar, then this might throw off the measured distance by some tenths of a minute of arc. One test would be to verify the diameter of the Moon. You know the date and GMT. What would have been the true apparent lunar diameter (don't forget augmentation)? Another approach is to find the center of the Moon since this would not be affected by a little flaring of the Moon's limb (which would presumably happen symmetrically all the way around the limb). This is an annoying process without specialized software, but it's simple in principle. Just open the image in some drawing software and start drawing circles. By doing this repeatedly, recording the exact center you use each time (and deleting circles that aren't quite right after you draw them), you will eventually find a circle that perfectly matches the Moon's limb. You then know the location of the Moon's exact visual center. My online clearing tool doesn't (right now) permit clearing from the Moon's center, but you can adjust the star-to-center distance by adding on the Moon's known semi-diameter and and then clearing as a Far Limb sight. It's cumbersome, but there's an algorithm here that could easily be incorporated into some specialized software. It's better than trusting the photographed limb of the Moon in some cases.

-FER
PS: Some historical TLPs were probably small meteor impacts, and enough were seen by more than one observer to be taken seriously. NASA began a program to record small impacts some eight years ago when the Bush Administration was pushing hard for a manned return to the Moon. This program has continued on inertia, as most government programs do, and last week they announced the "biggest ever" impact on the lunar surface which would have been faintly visible as a fourth magnitude "star" for about one second. The small detail not too clearly stated in the NASA press release was that this was the "biggest ever" observed by this small team in the eight-year history of their program. The sensationalist "journalists" at space.com (and their associated sites and the partners they sell to) peddled this as "Huge Rock Crashes Into Moon, Sparks Giant Explosion". The rock in question is estimated to have been about one foot across. If I had a nickel for every abuse of the word "huge" on space.com, I would have a "huge" pile of money... well, at least an extra dollar a year in my pocket. :)

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