On 1/24/12 9:48 AM, Chris Albertson wrote:
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.
That's the Davis Mark 3 (which is basically a copy of a lifeboat
sextant). $50 from starpath.com (which also has the more expensive mark
15, and others)
Or get a copy of "Emergency Navigation" by David Burch, and you can make
your own instruments from materials close at hand.
All the sight reduction tables and such are available online for free
now. (although a paper copy is nice, and fairly cheap, being a
government publication. )
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.
It's not that hard to learn yourself, once you get the right conceptual
model. The trick is knowing how to use the sight reduction tables. (I
figure that if your GPS has died, so has your calculator, so you'd
better be able to do it with pencil and paper).
I haven't gone to the extreme of calculating the trig functions by hand
(wasn't that what Napier's wife did.. calculate log tables by hand
during long sea voyages)
I think you could probably do some interesting
compass/straightedge/protractor kinds of geometric constructions to do
sight reduction as well.
Doing a fix on land, in one place, is pretty easy. (Much easier than
standing on the deck of a boat that is moving). The only trick is
having an "artificial horizon" that doesn't move.. A pan of liquid works
nicely (molasses, thick motor oil, or corn syrup are your friends.
Water is bad.. ripples in the least wind)
Star sights are a bit trickier, just because the stars are dimmer and
harder to find. And seeing the horizon at night is also tough.
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
And hey, you can learn while you're on the way, like Jack London did, on
his way to Hawaii. Read "the cruise of the snark" (Project Gutenberg)
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".
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