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    Re: Latitude of prime vertical sight
    From: Bill B
    Date: 2007 Jul 31, 22:52 -0400

    
    
    > Paul.Lee wrote:
    >> "I'm new to navigation, and am doing some work on the usage of prime
    >> vertical sights. I know it gives longitude, but if you knew the time
    >> of sunrise, is it possible to obtain a value for lattitude too? "
    
    Peter responded:
    
    > How would you derive latitude from a time for sunrise, Paul? I suspect that it
    > could give a rough indication, but what advantage would this method propose?
    >
    > Don't forget that due to refraction, measured sights of bodies close to the
    > horizon can be unreliable.
    
    Paul
    
    First, be aware I am a practitioner and not a theoretical guru.
    
    Peter hit the nail on the head in his second paragraph.  With latitude,
    longitude and declination as knowns one could theoretically predict sunset.
    For me it is a matter of successive approximations as declination changes
    with time.
    
    Flip it around, and given elevation of the sun at sunrise or sunset with no
    atmosphere (derived from the upper limb 0-10 degree table in the nautical
    almanac), time of observed sunset, and one other variable (latitude or
    longitude) one could determine the other variable.
    
    To expound on what Peter said, sights using a body below a certain elevation
    (I have read 30 degrees above the horizon as a good minimum rule of thumb)
    are suspect due to refraction anomalies.
    
    I also recall reading (can't quote the source--perhaps the almanac) that the
    observed time of sunset can be 4 minutes in time either way from predicted
    due to refraction. 4 minutes time *could* equal a 1 degree or 60 nautical
    mile error.
    
    Perhaps better to use Peter's posts on longitude from the sun at 90d or
    270d.  Latitude is no problem as "A good sailor always knows his latitude."
    
    
    Having constructed a straw man, let lose the hounds of theory. ;_)
    
    Bill
    
    
    
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