A Davis Mark 3 “emergency sextant” is all you really need.
How to Find Where You Are from the Sun is the title of George Buehler’s slender but utterly worthwhile pamphlet on basic celestial navigation.
Buihler’s approach to celestial navigation is similar to his approach to boat building. In nutshell, he rejects the mystique and reverence, the twee, if you will, in favor of a this-is-some-thing-mere-mortals-can-and-have-done-and-you-can-do-it-to affect. From the introduction to Buehler’s Backyard Boat Building:
Building a boat is simple. Believe it or not, the hardest part is just to stop talking and actually start. From that small step, all you need do is fasten one piece to the nexy and eventually you’ll finish. You just have to keep plugging away at it. If you’ve ever remodeled a house, tended a large garden, written a Ph.D dissertation, rated a child or anthing lese that didn’t offer instant gratification, then you certainly can build a boat.
Celestial navigation, on the other hand, does offer (near) instant gratification. Here’s the basic principle:
If you are standing on the equator at (local) noon, on the equinox, the sun will be directly overhead.
From this one simple truth your latitude an be calculated by measuring the angle of the sun at noon and reducing that number through a series of calculations that account for the time of year, your elevation above sea level at the time of observation, thickness of the atmosphere and one or two other things I’ve probably forgotten to mention.
So, for example, if the sun is at 45 degrees, at (local) noon, on the equinox, and you’re observation is taken at sea level, you’re are at 45 degrees latitude.
How do you know it’s local noon? You’ll have a good guess from your watch, then you’ll take a series of sightings. The sun will rise, rise, rise, rise, hang, fall, fall, fall. The “hang” is local noon. You’ll take the angle observed when the sun hangs, and run that through your reductions.
Latitude is just that easy. If you’re not in a rush, you don’t even need a watch. You can just laze away and take sun-shots around midday so you don’t miss it.
But if you have a watch, you can find your longitude too. Here’s the principle:
If you’re watch is set to Zulu time, aka Greenwich Mean Time, and you observe local noon at 12AM GMT, that means you’re exactly half a day around globe. That’s half way around the world; 180 degrees longitude.
If you’re watch is set to Zulu time, aka Greenwich Mean Time, and you observe local noon at 6AM GMT, that means you’re exactly a quarter of a day around globe; 090W degrees longitude.
If you’re watch is set to Zulu time, aka Greenwich Mean Time, and you observe local noon at 6PM GMT, that means you’re exactly 3/4 of a day around globe; 090E degrees longitude.
(I haven’t had coffee yet, and may have gotten these backwards or inside out, but you get the principle, right?)
Now celestial navigation can get more complicated. You can take morning shots and evening shots, shoot the moon, the stars and the planets, and if you were depending exclusively on celestial navigation to know where your are, that stuff would be handy to know. We never saw the sun yesterday, not at noon, not at all; and today’s 50/50.
But basic noon sun-shots are simple enough that it’s just silly not to know how do them. Because (like the NY Lotto says) Hey, you never know.