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    Re: Agnostic gnomon and horary error
    From: Robin Stuart
    Date: 2024 Jun 3, 07:13 -0700

    Chuck V.

    Thank you for providing the scan of the photo. Unfortunately I don't think the method you propose will work in practice. If the photo had been taken from the north or south then you could still draw your blue tick but your yellow and white ticks would coincide. The method you suggest would be fine if the photo had been taken from the east or west and in the horizon plane of the armillary sphere (but then you could measure the angle of the gnomon directly). If you start at the east and  walk and around the armillary sphere the apparent angle of the gnomon will increase until it appears vertical from the north or south. The photo appears to have been taken just north of east.

    There is, I think, a more precise approach that could be possible. In the photo we see lots of ellipses. These are obviously oblique views well-defined circles, the horizon, the equator, E-Z-W etc. By measuring the ratio of the major and minor axes it is possible to determine angle that the photographer is viewing them from and get an estimate on of the photographer's position. From that the apparent angle of the gnomon could be calculated. This will take some careful measurements ad 3D vector analysis if anyone is game.

    There are some sources of error however. The photo is taken fairly close to the armillary sphere and perspective will play a role. Also a photograph with a good camera is a gnomonic projection (how fitting!) of the subject,

    Robin Stuart

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