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> 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? "

One approach is to treat sunrise as an ordinary sight, except that it is taken 
without a sextant.  When the upper limb of the sun just becomes visible, then 
the altitude of the center of the sun is approximately minus 0 degrees 50 
minutes.  Record the time. Correct for height of eye (dip) and you have Ho. 
Then reduce the sight as usual to get a line of position. The -50 minutes 
accounts (approximately) for semidiameter, refraction, and parallax.

As far as accuracy is concerned, I'd put this in the category of "lifeboat 
navigation", i.e., the sort of thing one does when there is no better 
alternative.

If you advanced this line of position to the time of your prime vertical 
sight, then the two lines of position would give you both latitude and 
longitude (as would virtually any two lines of position).

Prime vertical observations were treated as special before the days of Sumner 
and St. Hilaire because they give you longitude. With line of position 
navigation, there is no longer any need to regard sights taken at the time of 
the prime vertical crossing as special, because you can get both latitude and 
longitude from any two lines of position (provided of course that they cross 
at an appropriate angle).

Best regards,
Chuck Taylor
  48 N
122 W






       
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