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". >

On 1/24/12 9:46 AM, Attila Kinali wrote: > On Tue, 24 Jan 2012 11:05:58 -0600 > "Lee Mushel"<herbert3@centurytel.net> wrote: > >> If you're looking for a really interesting topic to read about, the >> development of an accurate ship-board clock is really fascinating! And it >> wasn't done overnight! > > If you have a few references on books to read, you shouldn't keep them > for yourself ;-) > Dava Sobel's book is good. The Great Arc by John Keay also has useful information on this kind of thing. the Institute of Navigation (ION) has an $50 CDROM of almost 300 celestial navigation papers (http://www.ion.org/shopping/begin.cfm look down at the bottom) Lots about timekeeping, etc. in there. (got that one loaded on the iPad for long plane trips)

On 1/24/12 10:26 AM, Chris Albertson wrote: > have you ever tried to measure an angular distance using a hand held > instrument while standing on the deck of a moving boat in the open > ocean? try it and you will see why they wanted a clock. You > really can't measure an arc minute reliably we should expect about 15 > arc minute accuracy if you are standing on a moving ship. A few very > skilled people could do better. > > The moon moves what? about 10 degrees per day so in practical terms > you can get time to about 30 minutes. But other sources of error > would add to that. But still knowing even the hour is very good > that puts you in the correct time zone > you can do much better than that by looking for occultations (and they work with the new moon as well). If you know what day it is, you know about where the moon is, and you can look up in a table which stars get occulted when. Then you just watch through binoculars. I'd say you can get within ten seconds without much trouble, assuming you can find the star.