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A Serendipitous Catch of Lunar Libration, Steve Lantz

A Serendipitous Catch of Lunar Libration

Acquisition type: Lucky imaging
A Serendipitous Catch of Lunar Libration, Steve Lantz

A Serendipitous Catch of Lunar Libration

Acquisition type: Lucky imaging

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Description

On July 2, 2021, I set out at 4:00 a.m. to image Jupiter. The seeing, however, was too terrible to do that. Since the waning crescent moon (44.5%) was available, I turned my telescope to that just for fun at 11h UT. Despite stacking 500 frames out of several thousand, the image was still not very good. So that was that until I had an idea a few days later. The July 2 image was of the southern lunar highlands. And I had previously taken an image of the same area on January 5, 2020 at 3h UT, but the moon then was just past first quarter (68 %). I wondered if these two images could display lunar libration (and also provide a contrast between illumination from the east vs. the west). The answer for libration was yes! I relied on a great web site, NASA's Scientific Visualization Studio (https://svs.gsfc.nasa.gov/4768), to help with the demonstration. First, though, data from the web site showed that only lunar libration in longitude would be large enough to be readily seen. So what is libration in longitude? The moon has an elliptical orbit and sometimes travels faster than average and at other times slower than average. When the moon is traveling faster, the spin isn't quite fast enough to keep the same side facing us, and we get to see a bit more of the leading limb side. This results in a positive sub-earth longitude (the lunar longitude where the earth would be seen straight overhead). The opposite happens when the moon is traveling slower than usual -- we see a bit more of the trailing side and the sub-earth longitude is negative. Data from the SVS site said that the sub-earth longitude on Jan 5, 2020 was -4.1 degrees and on July 2, 2021 it was 5.5 degrees. Would the images show this?

I mapped the two images to simulated lunar images on the SVS site to get them to scale and registered with respect to the moon. Then I added the moon's spin axis. I could then visually see the libration! But to make it more obvious. I prepared a graphic (the final image posted). I used Tycho and a prominent craterlet to the north of it as illustrations of the libration in longitude. If the moon traveled at the same speed all of the time, its synchronous rotation would keep the same side facing us all of the time. Every crater, mountain and see would remain at the same position relative to the moon's limbs. But as the green line segments show, on Jan 5 the moon was moving slower than average, meaning the spin ran the disk ahead a bit and we could see more of the trailing limb. Then on July 2, the moon was traveling faster than average and the spin fell behind, so we could see more of the leading limb!

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A Serendipitous Catch of Lunar Libration, Steve Lantz