I managed to score a Davis Mark 25 sextant off of ebay for under 100 dollars. it's taken a fairly large amount of practice but I'm able to get my position to a bit north of a mile.
It's a fun skill to acquire, and has lead me off in several other fun hobby tangents. -Eric On Tue, Jan 24, 2012 at 10:13 AM, Bob Camp <[email protected]> wrote: > 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: [email protected] [mailto:[email protected]] 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 > <[email protected]> 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 >> <[email protected]>wrote: >> >>> On Tue, Jan 24, 2012 at 3:16 AM, Attila Kinali <[email protected]> wrote: >>> > On Tue, 24 Jan 2012 11:04:08 +0000 >>> > "Poul-Henning Kamp" <[email protected]> wrote: >>> > >>> >> In message <[email protected]>, 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 -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> 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 -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- --Eric _________________________________________ Eric Garner _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
