The Moon's "terminator" is simply the line between the light and dark areas of the moon, i.e. the line between the sunlit portion of the moon and the shadowed portion. To the naked eye this looks like a smooth line, but if you look closer (with some binoculars, or zoomed in with a camera) you can see it's actually ripply, with the various craters, ridges, canyons and mountains both creating shadow, and poking up into the sunlight.
Those "poking up into sunlight" features are a potential source of "UFO" or "Alien Structure" photos from the moon. The above example is some that cropped up recently. A video of the moon appears to show some kind of object in the dark area of the moon. Because it's "behind the line" there's an illusion that it's actually flying above the surface of the moon, perhaps even an orbiting spaceship!
However all it is is the top of a ridge at the edge of a crater, in this case the crater Fracastorius. A great resource for tracking these things down is NASA's Moon Phase and Libration visualization 2017, aka "Dial-A-Moon". (There's also a 2016 version, for older images). You can simply enter in the month and day of the photo, and the closest hour (converting to UTC/GMT time) and it will give you an image of the moon with accurate shadows on the terminator.
The great thing about the moon is that it's essentially the same from wherever you are looking at it on Earth, so you don't even need to know the exact time of the video, you can simply adjust the day and hour until the crescent matches.
When you find the matching image of the moon, you can click on the image to download a high resolution image with the craters along the terminator labeled.
You also get more information about the moon with a useful graphic showing where it is relative to the earth (not to scale).
Using this you should be able to track down pretty much any Moon terminator UFO. It can also be very useful in identifying things that are in shadow on other parts of the moon, as well as "structures" that are near the terminator (and hence casting unusually long shadows).
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