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    Re: Timesight at sunset?
    From: Marcel Tschudin
    Date: 2013 Jan 10, 21:10 +0200

    Your exercise is of interest. Some of you may remember that my interest is refraction near the horizon. I was able to make during a few years observations of the setting sun. Some of these observations consisted also in timing the last sun ray.

    Greg, you analysed Doug's observation and wrote:
    The online calculator has your sunset observation within 6 NM. Not bad. Your watch could be slow some seconds though. Give it a time check double check.

    The reason is possibly not the watch. If I interpret a first analysis of my observations correct then the last ray seems indeed to happen a moment earlier compared to the time of the upper limb touching the horizon. A preliminary result indicates this difference to be about 2 to 3 moa (scattering +/- by about the same amount). At the latitude of my observations (41 N) this corresponds to a timing difference of about 15 s.

    Performing the sight reduction again but setting now  the instrument reading instead to 0 moa to the value from the preliminary analysis, 2.5 moa, then the intercept does indeed reduce from -5.7 nm to -2.6 nm.


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