# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Latitude of prime vertical sight**

**From:**George Huxtable

**Date:**2007 Aug 1, 09:11 +0100

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 george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---