NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Advice concerning sextants
From: Paul Hirose
Date: 2011 Jan 06, 15:11 -0800
From: Paul Hirose
Date: 2011 Jan 06, 15:11 -0800
Patrick Goold wrote: > My boat > being on the hard, I had to take the sight from my front garden and that > required using an artificial horizon. Many problems with this. Wind > disturbs the reflecting surface even with the glass covers in place. A suggestion I've floated here before -- use the sextant as you would at sea, estimating the horizon as best you can. Of course accuracy is low. But for practice this method has many advantages. • It's so much quicker and easier to shoot without the bother of a fiddly, jiggly artificial horizon. This is especially nice when the weather is not! • The knack of rocking the sextant ("swinging the arc") to make it perpendicular to a sea horizon can be practiced. The corresponding movement with an artificial horizon is not the same. • Stars and planets are not a problem. You can plan a round of evening stars and shoot them much as you would at sea. Stars are difficult with an artificial horizon. • The necessary compromise between star visibility and horizon quality immediately becomes obvious. Too early in the evening and you can't see the stars. Too late and the horizon is indistinct. The books tell you this, but nothing beats seeing it for yourself. • You'll also realize why many navigators precompute approximate azimuths and altitudes for star shots, and preset the sextant. They can find their stars while there's still plenty of light for a sharp horizon. • The relatively low accuracy allows many shortcuts. Time isn't very critical. Instead of shooting a limb, you can simply estimate the center of the Sun or Moon. Angles may be read to the nearest 5' or so. On the other hand, there's no reason you can't read the vernier to .1' and apply all the corrections for the sake of practice. • If desired, the scope may be removed. It doesn't get you any more accuracy. In fact, with no magnification and both eyes open you can better guess where the horizon lies. If you've never tried shooting the Sun or Moon that way, I think you'll be surprised at how easy and comfortable it is. (But I find the scope helps with stars.) I'm not saying you should abandon the artificial horizon. It's far more accurate. However, estimating a natural horizon is a better approximation to using a sextant at sea. --