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    Re: Longitude by Sunset
    From: Marcel Tschudin
    Date: 2012 May 7, 15:09 +0300
    Gary, you wrote:

    Santa Cruz island is 25 NM offshore and 1,600 feet tall so substends about 30 minutes.

    In my observations the last recognisable brightness differences in the night sky (as viewed with my naked eyes) was when the moon center was about 70 moa above the apparent horizon. The dimming started obviously well before. The island should therefore be no problem.

    Greg, I wish you good luck for your observation. I am very much interested to hear (or rather read) what you observed.

    The visibility threshold is better observed with the setting moon where the observer can adapt to the location where the faint brightness differences are still visible. These faint brightness differences would hardly be found/recognised during the rising moon.

    I dug now my observations out of the archive:

    Location:
    Lat=40.9016432, Lon=29.1595805, Heye ca. 20m

    Time is in EEST=GMT+3h and height above horizon relates to moon center

    Last recognisable brightness difference:
    Date / Time / Temp. C / Press. / rHum. / H abv. app. Hor in moa
    22.06.05 / 05:05:00 / 18.5 / 1015 / 68% / 16.4
    17.07.05 / 01:12:30 / 22   / 1014 / 78% / 69.0
    18.07.05 / 01:49:30 / 23   / 1012 / 73% / 69.1
    12.08.05 / 23:11:20 / 24   / 1014 / 65% / 71.5

    I can not remember but I guess that the timing on 22nd June was an approximate one. I likely was so much excited to actually see what I found years before from some crude estimations that I probably forgot to check the time and estimated it some minutes later when writing down the details.

    The atmospheric data correspond to those measured at the airport across the Marmara Sea in about 30 km distance (Lat=41.0, Lon= 28.8). It is assumed that the data do not differ much since both, observer and the airport are close to the same sea.

    Marcel

       
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