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Comment by lambdasquirrel | Hacker News Reader

I don't know if I entirely buy the line on "34-35 degrees off the horizon." Unless we knew the focal length of the lens it was taken with, and which camera it was taken from, you don't actually know that. A wide-angle lens is going to have a much larger field of view than, say, a 100mm macro. And different camera systems have different angle of view for the same focal length.

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You don't actually need the focal length, it doesn't help accuracy that much but can help you line up the sky to the photo a bit quicker. Anyway, if we did, all that info was in the metadata anyway:

Camera: Nikon D5 F-stop: f/2.8 Exposure time: 5 sec. Focal length: 28mm Max aperture: 3

You know what the angle between the stars in the constellation are, you don't need to know the field of view of the camera.

I think what you're suggesting is that because we know the angle between a pair of stars, we can use that to infer the angle to the horizon by measuring the distance between each in pixels.

I don't think it's a safe assumption that there is a linear relationship between the angles. Like, two pairs of objects in the image, in which the angles subtended by each pair relative to the viewer are the same in real life, may not be separated by the same number of pixels in image space depending on the lens used.

Furthermore, this could be thrown off by even a small hill or mountain obscuring the astronomical horizon.

If you were doing this professionally you would, because lenses introduce distortions of various kinds - typically barrel or pincushion depending on the focal length and/or the lens.

So any view of the sky is an approximation, and will introduce subtle errors that make it harder to pin down an exact location.

They probably won't be huge areas, but this would be a harder problem if the time and longitude had to be exact with no existing guesses.

both the focal length and the field of view are irrelevant, you know the angles between the stars so you can infer everything.

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