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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).
    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

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