GPS... can find you within about 3' of your real location.
Sextant... if you're good at taking sights (I'm not, yet) you can likely get within a few miles of your precise location.
I've spent the last five years saving money, getting rid of crap at my house, now I'm rebuilding a basement (sigh) and I've spent time learning to sail, practicing, and understanding the physics of sailing.
Somewhere in there I've picked up some diesel mechanics (not much yet, but simple trouble shooting), plumbing (enough to rebuild bathrooms!), knots, knot work, net making, knife making (trust me it has to do with sailing), "Rules of the Road", safety and many, many other skills associated with skippering a boat around a pond, lake or across an ocean.
What I have NOT "grasped" yet is celestial navigation. Oh, I understand the basics. I can accurately determine my Latitude at night using the North Star (polaris) and I have the mechanics more or less firmly planted in my mind on calculating my longitude from time (Noon sun sight for instance) and doing most of the math.
But, really, I have yet to take a sextant (I have two plastic sextants) and make a real sight and do the calculations.
You need to have a horizon to see. I can see mountains to the south, west and north of me. Houses to the east. No horizon. So need to build an "artificial horizon" (pretty easy, water in a pie tin should do it, or cooking oil).
I have bought somewhere around $200 worth of books, all taking a different direction on explaining celestial navigation.
But in a nutshell (a small one) I learned the most important thing I've ever learned:
Navy guys using the sextant on a ship and being competitive, practicing and trying to out-do one another RARELY got closer than 10 miles - which they considered "Good Enough". Occasionally someone could get within a mile (Fantastic).
If I am crossing an ocean why do I need to be more accurate than 10 miles again? I don't!
All I need to know is roughly where I am - as long as I'm not headed directly for a shoal water, reef, island chain I'll crash into.
When you are close enough to hit an island or see reefs, you USE YOUR EYES!
Seriously, I have spent all this time preparing for this and reading everything I can read, absorbing as much as I could and by normal, common sense, I already knew all this. I couldn't see a need to use a sextant in a lake. No need at all. I can use a compass to take bearings off objects on the shore and plot my location based on that on a map. No problem.
What is it about using a sextant though that has bugged me so much the past couple of years?
Easy. Math is not my BEST subject, I've always hated it to some degree and almost every, single text on the subject explains the details IN DEPTH of HOW celestial navigation works.
Ok - to make you all get this like I finally did.... do you NEED to know how an internal combustion engine works to drive a car? Hell no.
If you can get a simple step by step instruction on how to take the sight, correct for errors (Dip, Index, refraction, etc) then fill in a set of numbers from a book (or use a calculator that has the tables in it) and plug in numbers - then you can figure out where you are, within reason.
Why bother with all of this?
Simple.
GPS units are electronic, as are computers, calculators, chart plotters, and numerous other electronic devices requiring batteries or electricity of some sort to work.
If you take a lightning strike, the chances of your stuff working is pretty low after that. EMP will take even your hand held units off line.
So.. knowing Celestial navigation using a sextant, watch and some tables is good to know.
Even if it is "hard to do".
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