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    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?

Clive.

J.C.Sutherland
Wythburn
West Hanney
WANTAGE
Oxford, UK
OX12 0LH

   

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