Monday, July 6, 2009

Astronavigation / Celestial Navigation, Part 2

Last night, a (briefly) clear sky gave me a chance to try a key element of practical celestial navigation. Following the example of Marvin Creamer and countless forgotten sailors of the pre-sextant era, I attempted to measure the altitude of some celestial bodies using no instruments whatsoever (partly because, aside from my telescope, I haven't made or bought any yet).

Why no instruments? Obviously, this decreases the accuracy of the observations that you can make; not so obviously, it is still possible to be accurate enough to make measurements that are accurate enough to be useful. Marvin Creamer made fairly accurate landfalls all around the world using this method, generally maintaining his latitude to within half a degree of the intended value , and any experienced navigation venturing deep into desert or out of sight of land before the coming of "modern" instruments like the sextant almost certainly relied on such techniques. So, it is possible, but is it useful?

Well, sometimes; today, people like myself who like to use the "road" less travelled have the benefit of tools like GPS (I recommend the excellent, cheap, tough Garmin Etrex); however, the very nature of independent travel in remote places means that equipment may well be lost, broken, or simply out-of-charge. By learning how to navigate without any instruments which aren't permanent (I hope!) parts of my body, I'll be able to travel with a permanent built-in backup for the GPS and the compass (you'ld be surprised how many sailors run into GPS trouble). Apart from hypothetical equipment failures, navigation is a subject that I enjoy for its own sake; there is something deeply magical about looking up into the night sky, knowing the stars by ancient names from foreign tongues, and, simply by looking, to find yourself.

Back to practicalities. At this latitude, the July sky isn't really dark at 23:00, so there weren't very many bodies to measure - simple identification was tricky, because only the very brightest bodies were visible. Even worse, not being at sea, I didn't have a clear horizon either. Nevertheless, I extended an arm and spread thumb and forefinger as far apart as they would go; for the average person, the span between thumb and forefinger will cover about 15 degrees of sky. I levelled my arm at the invisible horizon, and began to measure.

Waving my arm around the summer sky, I measured the altitude of a rising moon (itself covering only 0.5 degrees of sky, a useful checking-fact) at about 10 degrees and the bright orange of Arcturus (Alpha Bootes) at about 38 degrees. I sighted Dubhe (part of the Plough Constellation) too, although I forget the number; then, checked the observed altitudes against what Mobile StarChart app on my phone said they should be. I got the moon nearly dead-on - its real altitude being about 10.5 degrees - and Arcturus turned out to be just over 40 degrees above the horizon. The error for Dubhe was higher, about 3 degrees.

For a first attempt, with no clear horizon, these measurements strike me as acceptable; if I had been using them to find my latitude, I would've had a pretty fair chance of finding my home country. Marvin Creamer did a lot better, though, so the next time I've got a clear horizon after dark, I'll give the procedure another try. In the meantime, I'm now wondering what simple instrument I might be able to construct to get those errors a little smaller. Ideally, these will be simple instruments, such as a lost sailor on a small boat might plausibly fabricate. Also, wouldn't it be nice to measure latitude and longitude?

Coming soon...

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