Celestial Navigation makes my head hurt

Posted by Kromey at 7:10pm Jan 2 '14
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One of the things I've long wanted to do is learn the art and science of navigating by the stars (and the sun and the moon, but that's a touch less poetic). I long ago mastered using maps and compass to pinpoint one's location by way of using landmarks [although those skills are rather rusty and need a good refresher], but what if you don't have a map (yet somehow still need a fix on your location) or are somewhere where landmarks aren't viable for navigation? If it were me -- especially since my Dead Reckoning skills would leave me dead! -- I'd be screwed.

Anyway, my parents got me a book to learn it -- Celestial Navigation for the Clueless -- as well as a nautical almanac. Now I just need a sextant, but in the meantime I'm starting to work through the book to get a handle on the concepts.

And the concepts hurt my brain.

First brain-bending concept: Polaris, the North Star, isn't truly north. Its GP (Geographical Position) actually describes a circle with a radius approximately 47 miles from the true North Pole. Still, it's good enough to take a sighting on it to get your latitude to within ~55 miles (mean error, meaning sometimes more!), which is good enough for rough navigation -- and indeed the early mariners were quite successful with only this! [Okay, this isn't truly brain-bendy, just world-concept-turning-overy, as I'd long believed that Polaris is north.]

Here's the one that truly breaks my brain, though:

To figure out longitude, you can take a sun sighting at local solar noon (when the sun is at its peak) and use the current GMT -- for each (time) minute away from GMT noon, you're 15 (longitude) minutes away from the Prime Meridian, i.e. the time difference between your local noon and GMT noon (in minutes) times 15 is your longitudinal position on the Earth.

Well, almost. We know the Earth's orbit isn't circular, and as a result moves faster and slower relative to the sun at different times of the year, resulting in solar noon at the Prime Meridian actually varying several minutes plus or minus (what we call "true noon", i.e. 12:00:00 GMT, is actually the Solar Mean Time, i.e. if you were to average all the times of solar noon at the Prime Meridian in a year's period you'd get 12:00:00 GMT). Not too brain-bendy yet, just have to apply a correction to your sighting/to your calculated longitude based on the time of the year.

Here's the brain-bendy part, though: The difference between solar noon and "real noon" at the Prime Meridian is different than the difference between solar noon and "real midnight" at the International Date Line, i.e. longitude 180⁰! This means that the sun seems to move faster and slower in the sky during the course of a single day, speeding up (or slowing down) in the afternoon before slowing down (or speeding up) after midnight!!

My brain is hurty now! :-(

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