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    From: JC Sutherland
    Date: 1999 Apr 24, 5:49 AM

        This is my first link with this group although I have been
    fascinated by the topics discussed for some time.
        It is time I introduce myself.   I am a yachtsman sailing in UK
    waters and the Channel. I have been teaching Navigation to my fellow
    yachtsmen for about  20 years. My interest in Astro-nav goes back to may
    RAF days which is now a long time ago.
    For the most part my interest has been in instrumental techniques for
    maximum precision in navigation.but I have a recent interest which you
    may like to help me with.
        "How can I predict the precise TIME and AZIMUTH of SUNRISE on any
    day and any place."
        For example: As far as I know the "Green flash" has only been
    observed at Sunset. Theoretically it should alsobe present at Sunrise
    but has anyone been looking at the right time and in the right direction
    to see it?
    The Altitude, Azimuth etc of the Sun for a range of values around 0 deg
    altitude can be calculated using the Cosine/Sin formula. Then values of
    Declination and LHA can be iterated to get the precise UT of sunrise.
    However this result would only be correct if the earth was spherical and
    had no atmosphere. The difficult bit is finding the right amount of
    Refraction to be applied.
        Even assuming "normal Pressures and Temperatures" there are several
    equations for calculating refraction but they do not work very well for
    zero or negative altitudes. Also I don't know of any that take any
    account of Latitude and I would suppose that refraction in the Tropics
    would be greater than in higher latitudes.
        The refraction applied to the Dip of the horizon will be based on
    the short distance between the Observer and the horizon. On the other
    hand the refraction applied to the Suns rays must take into account the
    greater amount of atmospher they pass through "even though the line of
    sight is exactly the same!"
        I am assuming that the moment of Sunrise is defined as the instance
    that the first flash of light is seen and incidently this would also
    define the precise moment of the observers Millenium on Jan 1 2000.
        It is nice to talk to you, what do you all think?
    West Hanney
    Oxford, UK
    OX12 0LH

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