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Lunar distance by photography
From: Paul Hirose
Date: 2024 Jun 22, 12:43 -0700

Speaking of photographic lunar distance observations, one method is
described in this 1893 paper:

"On photography as applied to obtain an instantaneous record of lunar
distances for determinations of longitude," U.S. Coast and Geodetic
Survey annual report for the fiscal year ending June 30, 1893 (part 2),
appendix no. 4, page 117 (126 in the PDF).

https://libguides.library.noaa.gov/coast-geodetic-survey/annual-reports

The method uses several short exposures of the Moon and a long exposure
of the star field, all on the same photographic plate. No special camera
is required. Since the Moon is photographed separately from the stars,
the camera settings can be optimized for each.

I don't fully understand the method, but it seems to me some
simplifications are possible nowadays. For instance, no plate measuring
machine is needed. A simple utility such as Windows Paint can get xy
pixel coordinates.

One complication not mentioned is the shape of the Moon's limb. I think
it's slightly elliptical in a photo (even if the lens has no distortion)
when the Moon is not on the optical axis. That occurs because the cone
of rays forms an ellipse on the image plane when the cone is not
perpendicular. In seems non-intuitive (wouldn't you notice in a photo
with the Moon off-center?) and I haven't investigated. But I think this
issue was discussed in another old photographic longitude article (also
by Runge?). I may have mentioned it here years ago.

There was a discussion of photographic lunars in December 2009, recorded
in the archives, but I haven't had time to look at it.

--
Paul Hirose
sofajpl.com