Celestial Navigation and the Sextant

Forrest Brownell (forrest@slic.com)
Fri, 08 Jan 1999 19:30:16 -0500


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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RE Scott Foshee's question concerning celestial navigation:

I've often used a sextant on long canoe trips in the Canadian
subarctic. I find a comparatively-cheap American-made plastic
instrument, the Davis Mark 15, entirely satisfactory, though you'll
be disappointed if you hope to locate your position with pinpoint
accuracy.

The Davis, for example, requires frequent adjustments to eliminate
index error (a problem shared by most plastic sextants), and, with
extended use, develops significant "lost motion" in the tangent screw.
Under ideal conditions -- a clear horizon (or a well-shielded
artificial horizon ashore) and a well-practiced navigator -- you'll be
doing well to get a position line within two miles of your "true" or
actual position. This is almost always good enough for practical
blue-water or open-country navigation, but it can't compete with GPS!

The computations used in celestial navigation are quite
straightforward, and so -- somewhat surprisingly -- is the theory,
though if you really want to come to grips with the "maths," you'll
need to know something about spherical trigonometry. If you'd like a
quick overview -- more appetizer than entree -- I'd suggest you start
with the short chapter on celestial navigation in Captain (RN) John O.
Coote's _Yacht Navigation -- My Way_. Should you then find that
Captain Coote's robust prose whets your appetite, go on to Susan
Howell's _Practical Celestial Navigation_ or (if you want to learn the
maths) Merle Turner's _Celestial for the Cruising Navigator_.

On the other hand, if it all starts seeming too much like work -- and
it may, somewhere between side error and the law of sines -- and you
decide to stay an armchair navigator, I'd still recommend that you get
hold of Derek Howse's book _Greenwich Time and the Discovery of the
Longitude_. Dava Sobel's _Longitude_, good read though it may be, is
both incomplete and misleading in a number of points . Howse, the
former Head of Navigational Astronomy at the National Maritime Museum
in Greenwich (England), is more thorough than Sobel, more reliable,
and -- to my mind at least -- much more entertaining.

Good luck!

Forrest Brownell
South Colton NY