How to Find Where You Are from the Sun: the basic principle.

How to Find Where You Are from the Sun: the basic principle.

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.




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13 thoughts on “How to Find Where You Are from the Sun: the basic principle.

  1. The ideal gift for an amateur astronomer (or the guy who has everything, including a seagoing vessel and a sextant) is the Nautical Almanac.

    I was consulting for a rather secretive executive placement firm. The CFO found me working late at night and advised me to double or triple my billing rate, though he wouldn’t do it for appearances’ sake and ethical reasons. Said clients wouldn’t take me seriously if I didn’t. Quite a guy. We became friends thereafter. Found out he was both an amateur astronomer and a sailor. So I gave him the next year’s Nautical Almanac as a Christmas present: it was then a simple matter of walking over to the book store in the Dirksen Federal Building in Chicago.

    In return, he gave me a hand-built 8″ Dobsonian mount telescope.

    I’ve always had a fascination for time and timepieces. There was once a magnificent collection in Rockford, Illinois, including a reproduction Harrison H-1 Clock, the first seagoing clock for the measurement of longitude. Captain Cook took a Harrison clock on his voyage.

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  2. David, did you ever read Dava Sobel’s Longitude, about the competition to find the best way to determine longitude? It was a most excellent read, and shows how recent was the discovery.

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  3. You are right that you never know. Few years back, admittedly before the proliferation of GPS (but well after beacon based and other electronic navigation had become ubiquitous) my navy frigate experienced a boiler explosion, leaving us without power or communications of any electronic sort. For two days we monitored our drift via solar and celestial navigation only. Didn’t exactly solve our larger problem, but it gave us sufficient warning should we need to drop anchor if we neared shoal water.

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    • Just in the last week or so we had a mishap with one of the local long-liners. She lost all power and had no navs or comms.

      Fired off her EPIRB and the Coast Guard couldn’t find her.

      Finally she came within cellphone range and was able to make contact. Nothing damaged, no one hurt or killed.

      I don’t think a sextant would have helped them, but it goes to show, shit does happen, and sooner or later it will happen to you.

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  4. Haha yeah. When we had our unpleasantness, it was only some very imaginative Electronic Technicians that managed to take the batteries from our laser ring gyro and rig them up to the MARS station (essentially, a ham radio) long enough to get hold of a civilian that lived near a Navy base, and have them deliver a hand written note to the security guards informing them of our, er, status. Then add the humiliation of having a Coast Guard cutter come and take our big bad warship under tow.

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  5. Did anyone hear about the woman who went to a camping shop and asked to buy a sextant?

    The man behind the counter said, “Madam, we will sell you a tent, and what you do in it is your business.”

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