Hi If you spend some time on the auction sites you can find some fairly good (though not brand name) sextants on the cheap.
Bob -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris Albertson Sent: Tuesday, January 24, 2012 12:48 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] establishing your position w/o gps If you want to try your hand at position determination in the pre radio nav days you can buy a "studen sextent" It's a low cost plastic instrument sells for about $60. Better ones start at $200 with $500 to $800 for a good one. But it required much pratice and training to outgrow the plastic instrument. I took the class. I think most anyone who wants to sail on the ocean had better take the class just in case their GPS fails. I know some one who had both his primary and backup GPSes fail and he was still a week from Hawaii. They had to revert to the old techniques from the 1700's Much of pre-GPS position determination is not about finding your latitude and longitude. That is a modern notion. What they did and what sailors still do is find a "line of position". That means "I am some place on this line but I don't know where on the line" There are many ways to do this and they would work every method and find several lines. If they could see land they could shoot a compass bearing and draw a reciprocal bearing and know they were on that line. They would know the ship's heading and could estimate drift and know course over ground was parallel to that. They could always find a latitude line. Then if they did this right some of these lines would roughly intersect and they would know the position without need to know longitude. There were other methods to find lines that required an estimate of your speed and without clocks they resorted to chants and songs (jo ho, jo ho,...) As long as you sing the old pirate song at the same tempo every time you have a decent clock. Then you measure distance by tossing a big chunk of lumber overboard with a measured rope tied to it. The captains hated doing math by hand so they calibrated the rope by tieing knots at intervals so the natural unit was one arc minute at the equator and called it a "knot". My buddy who was headed to hawaii put both GPSes in the oven in the galley and after three days was able to get one of them to work a few minutes a couple times a day. That was enough. But he said he was within maybe 15 miles of where he thought he was Basically your estimated course line intersected with a line of latitude gives you longitude. On Tue, Jan 24, 2012 at 8:50 AM, Azelio Boriani <azelio.bori...@screen.it> wrote: > Yes, the first real push was the Longitude Act (1714) and the Harrison's > clocks. > > On Tue, Jan 24, 2012 at 5:36 PM, Chris Albertson > <albertson.ch...@gmail.com>wrote: > >> On Tue, Jan 24, 2012 at 3:16 AM, Attila Kinali <att...@kinali.ch> wrote: >> > On Tue, 24 Jan 2012 11:04:08 +0000 >> > "Poul-Henning Kamp" <p...@phk.freebsd.dk> wrote: >> > >> >> In message <20120124115848.312d60bd4fccce4f3e71c...@kinali.ch>, Attila >> Kinali w >> >> rites: >> >> >> >> >All this talk about telling the time using stars or the sun made me >> wonder >> >> >how did people tell what position their telescopes had back in the days >> >> >before GPS? >> >> Sailingships and trade was what pushed this. At the time of Columbus >> he was able to know his latitude within a few 10s of miles but even >> after returning to Europe he did no know how far around the world he >> had sailed. Was it 1/3rd or 2/3rds? They had no way to know. The >> problem was that on one had a clock that should keep time well enough. >> They used hour glasses on board ship for short duration time keeping >> but those were of no use on a longer ocean crossing. >> >> Later they discovered the idea of common view of the moons of Jupiter >> and they could measure the time from local noon some even on Jupitor >> while a person back home did the same thing. Later when he got back >> home they compare notes and then know the difference in longitude. >> Good ocean going clocks were still centuries away. But in the >> 1500's they could only know the location after the fact when they >> returned >> >> -- >> >> Chris Albertson >> Redondo Beach, California >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.