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Thanks for your interesting replies, they have helped a lot. From
what's been said, it
doesn't look a prime vertical sight can give a lat and long value for
a certain time, which
is a shame. However, can it give values with a certain error margin?

Thanks

Paul


On 1 Aug, 18:08, Gary LaPook  wrote:
> Gary writes:
>
> Remember, "the sun rises in the east" that you learned as a child. Of
> course we then learn that it doesn't rise exactly due east  but
> sometimes north and sometimes to the south depending on the season. At
> 44� of latitude the azimuth at sunrise varies from 56� to 124� which is
> never more than 34� from due east. At lower latitudes the azimuth
> changes even less, at the equator its maximum difference form due east
> is only 23.5�, the maximum declination of the sun.
>
> So advancing an LOP derived from a sunrise observation would provide a
> maximum angle of cut of about 30� or less which we know will at least
> double the possible error in the latitude determined. Figure in  the
> error due to variable refraction of a low altitude sight and you end up
> with what would only be a very approximate latitude.
>
> George Huxtable wrote:
> >Paul Lee asked-
>
> >| Hi everyone,
> >| 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?
>
> >Later, he added-
>
> >"The local sunrise had been determined in retrospect by an astronomical
> >program.
> >The prime vertical sight would be taken about this time."
>
> >===================
>
> >Reply from George.
>
> >No, it's not possible.
>
> >The comments about unpredictable low-level refraction, though true in
> >themselves, are missing the point. (True, I might add, except for Bill's
> >lower limit of 30 degrees, which would exclude half the sky, and most
> >observations. Lower limits of 5 or 10 degrees are more appropriate,
> >depending on the level of precision that's sought)
>
> >You are only making one measurement, presumably an altitude of the Sun, at a
> >moment when you have calculated (not observed), that it will be exactly to
> >your East. You are not observing the time of local sunrise, simply taking it
> >from some program, which will have asked for both latitude and longitude
> >before predicting that time. Your longitude could have been derived from the
> >prime vertical observation, but how did you provide the latitude, to feed
> >into the program? If you have had to tell it the latitude, you can't then
> >expect a prediction that it provides to tell you the latitude.
>
> >As a a general rule, if you have two quantities to find (such as latitude
> >and longitude) you have to make two independent observations to do so.
> >Calculating the time of sunrise, from a program, is not observing it. IF you
> >could allow precisely for the low-level refraction (which you can't, because
> >it's so variable), you could TIME the moment when (say) the Sun just peeped
> >over the horizon, and then due to refraction, its centre would be at an
> >altitude of about 50'  below the true horizon. If you timed that moment, it
> >would be just like measuring a sextant altitude at a particular moment, and
> >from it, and an almanac for the Sun, you could derive a position line at
> >that moment. Then you could cross that with a North-South position line from
> >the prime vertical observation, and where these lines meet is where you are,
> >in lat and long.
>
> >But beware; even that would not work well in the circumstances you describe,
> >when you say "The prime vertical sight would be taken about this time."
> >Unless the Sun's azimuth has had plenty of time to change, between sunrise
> >and your prime vertical observation, then the two resulting position lines
> >would not have a decent angle between them, and latitude of the crossing
> >would be found very imprecisely. That would exclude such a procedure at
> >dates anywhere near the equinox. Of course, prime vertical observations of
> >the Sun are themselves impossible throughout the Winter half of the year.
>
> >If both the azimuth and the altitude of the Sun (or any other body) could be
> >precisely measured at sea, at the same instant, they would comprise the two
> >independent observations that are called for, to determine latitude and
> >longitude, in one go. That's true at the moment of prime vertical, or any
> >other moment. So if you could observe (not predict) that the Sun really was
> >exactly on the prime meridian, to your East, and simultaneously measure its
> >altitude, you could get both latitude and longitude. That's possible for a
> >surveyor on land, with a firmly-planted theodolite, knowing his direction of
> >due North, but not for a mariner at sea, who has only his compass.
>
> >I am puzzled, though, why Paul is interested in prime vertical observations.
> >Is it for historical studies? When the Sumner method came in, improved by St
> >Hilaire in the mid 19th century, a position line could be determined from an
> >observation of any body at any time. Two such observations provide two
> >position lines. Where they cross is where you are. Simple as that. It's a
> >method which applies everywhere, at all times. Then, such special-cases as
> >prime-vertical became irrelevant, though noon sights lingered on, because of
> >their arithmetic simplicity.
>
> >Prime vertical observations are no more than a special case of a Sumner
> >line, providing a North-South position line (longitude), which has to be
> >crossed with a different observation (often, near noon) to provide a
> >position in lat and long.
>
> >George.
>
> >contact George Huxtable at geo...@huxtable.u-net.com
> >or at +44 1865 820222 (from UK, 01865 820222)
> >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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