Monday, June 8, 2009

Astronavigation (Celestial Navigation) for Beginners

Knowing one's way around the night sky is a useful thing, if, like me, you have a telescope and want to know where to point it, or if, like me, you have ambitions to learn astronavigation. Until last week, sunset came early enough that I could get a few minutes of practice on every clear evening, standing in my garden and counting out the stars. This time of year, the orangey-red light Arcturus is usually the first that I see; the distinctive blue blaze of Vega is to the east, and when the sun's glow has faded a little more, Pollux, Castor, and Capella (actually 4 stars, an exotic double-binary) show up nicely.

The stars that I am really watching for, though, are Polaris (the north star) and Etamin; obviously, Polaris is very useful, in that it gives a navigator a course to steer anywhere in the northern hemisphere above maybe 10 degrees of latitude (ish) - but why my interest in Etamin (gamma Draconis)? Well, it so happens that my home port on the eastern seaboard of the north Atlantic is just a smidgen north of Etamin's declination (celestial latitude), which is 51 degrees, 29 minutes, 20 seconds. Now, Polaris has the useful feature of always (where always = "several hundred lifetimes") being 51 degrees and X minutes above my local horizon; Etamin, by contrast, whirls around the sky, never dipping below the horizon, but once per day passing through the zenith - what you might call "Etamin-noon".

In practical terms, this means that were I some day to be lost in the blue vastness of the North Atlantic, no GPS, compass, sextant or chrometer to guide me home, I could use Etamin to find the latitude of home, sailing north if Etamin passed north-of-zenith, and sailing south if it passed south-of-zenith. Once at the right latitude, I would need only to keep an easterly course, and a sharp look-out for pointy rocks. Of course, measuring the fixed, non-whirling altitude of Polaris is more convenient - it can be done whenever Polaris is visible - but that would require an instrument, ideally a sextant. Marvin Creamer, an American amateur sailor and retired professor of Geography, once sailed around the world on Globestar using techniques like this and no instruments whatsoever, making surprisingly accurate landfalls.

Unfortunately, during part of the year, Etamin-noon would fall during daylight hours - but even then, other bright stars at similar latitudes could give useful hints. Which bright stars pass directly over your home port / next port? Just follow the linked query at Wolfram Alpha to see a table listing the hundred brightest stars by declination, and you'll soon be on your way. A useful tool to help you practice is the (totally free) Mobile StarChart, a java applet you can install on your mobile phone - it only has about thirty star names, but is open source, so you could add more.

Living a long way from the sea? Astronavigation can also be pretty useful in the desert, and was much practiced by people like Popski. Must learn how to use a sun-compass one of these days.

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