> > Here's a toss-in: A question for Nav 101 levels:
> > How can you tell the latitude you're at if you were to
> > be taken out of your office or home right now, no chance
> > to get anything, and all you've gotten on you is what
> > you can use. You're taken out into the woods somewhere
> > on Earth. What's your latitude?
>
> Here's my guess, but i suspect I'm about to learn something:
> Woods is a more difficult problem vs water, since you probably don't have a
> good horizon. You could probably rig a poor-man's sextant using a plumb line
> (shoestrings?) to get a decent vertical and fake a perpendicular line to
> that as a horizon line. Then at night take a sight on Polaris, use a
> wristwatch as a makeshift protractor (6 degrees/min) and get your lattitude.
> Should be good within 5 degrees or so if you're careful. But 5 degrees is
> about 300NM so its not a huge help.
In the woods, you can create an artifical horizon much simpler
with a small pool of water or oil. With the sextant, bring the
reflection of the sun in the pool to match up with the sun so
that you see one sun. This does have the limitation that the
sun can't be more than 45 degrees up for most of us. Your line
trick is a good one, especially if you use a fluoruscent colored
line so it's easy to see. Just remember that there is less eye
height to subtract, and none when using the artifical horizon.
Another, but more cumbersome, way is to put a horizon on the
sextant itself, one of those little monocles that you sight thru
and has a bubble displayed to one side. $5 or less at any good
survey shop or hardware shop (sometimes).
Anyway, here's the easy way: Jam a stick vertically in the
ground and mark where the shadow is. Every so often mark it
as the day passes and you'll get a curve drawn in the ground.
>From the point furthest from the stick (created at local noon)
measure the angle from the ground to the tip of the stick.
That's your latitude, pretty close. It's easier if you happen
to have some string in your pocket, but.... You can use the
wristwatch trick here to measure.
> > A question for Nav 201 levels:
> > All you have is a clock/watch. You don't know where you
> > are. How many hours away from Greenwich are you?
> > You don't know what your watch is set to. You can
> > use any pre-1800 tools available to navigators, but no
> > maps or charts. There's no-one around to ask questions
> > of, either.
>
> If you happen to know the lat/long of magnetic north, and have a compass,
> you >might< be able to narrow it down out via variance between the magnetic
> reading and a polaris sighting in the northern hemisphere. But I don't think
> it will get you close enough, and areas with the same variance can span
> multiple time zones.
Yep, a problem. Plus, the areas aren't consistent and wander
all over the place, thus you could be in one particular area
of variance, and know which one, but where in there were you?
You could be one or more longitudes away from where you thought
in some instances.
> Since greenwich is an arbitrary location, there's no way to do this without
> celestial tables. This would work: Assume the watch keeps decent time, and
> we're in the northern hemisphere. But the actual time it is set for is
> unknown...but an hour on the watch is a true hour. At night, get a polaris
> direction. Use this to draw the NORTH line on a sundial, and also to
> determine lattitude. At noon the next day, the shadow will point due north
> at local noon (or close enough). Set the watch. You now know the time close
> enough for this experiment. Navigators pre-1800 still had celestial tables,
> no? Presume we have an accurate sextant, or have built a workable makeshift
> one. At a specific local time, assume you are at the same longitude as
> greenwich. Using the celestial tables backwards, check to see if the stars
> (moon wouldn't work well for this example cuz its position changes way too
> fast) are where they should be at this time in greenwich. If they are,
> you're in the same time zone. If they are not, find an entry which puts you
> an 15 degrees (1 hour) ahead of greenwich, and check for fit with the stars
> again. lather rinse repeat, or do it backwards if you have to. Eventually
> you will find a close enough fit. It should get you within an hour anyways.
> There's probably a better way than that. I'd love to hear them. I don't have
> any practical experience with celestial navigation - I only know the theory
> behind it. This would be a time consuming method because the tables
> presumably have their output in longitude, so you'd have to find a cell in
> the table with the assumed longitude and work back to find the time and the
> correct stars.
>
> Am I close?
Actually, dead on in this case! They did have tables then,
and sextants. I pointed this method out to the guy that
asked, and said it was the only way I could figure out. He
said that was the way to do it. I'm sure there is another
way to do it, and one I came up with was that if you could
get the GP of any object off a table, you could then use the
DR of Greenwich and work from that, correcting the DR to the
actual position, which would provide your longitude, and the
answer. I'm a little fuzzy on the exact math here and my
books are at home. I'm hoping to use this winter to write my
own little program to do all the math work for me, and I'm
sure that by the time I finish the program I'll have a better
memory of the math involved.