[time-nuts] establishing your position w/o gps

Chris Albertson albertson.chris at gmail.com
Tue Jan 24 12:48:28 EST 2012


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.boriani at 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.chris at gmail.com>wrote:
>
>> On Tue, Jan 24, 2012 at 3:16 AM, Attila Kinali <attila at kinali.ch> wrote:
>> > On Tue, 24 Jan 2012 11:04:08 +0000
>> > "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
>> >
>> >> In message <20120124115848.312d60bd4fccce4f3e71c136 at 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
>>
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-- 

Chris Albertson
Redondo Beach, California




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