A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Oct 25, 19:44 -0700
In your original post, you wrote:
"Seems there no strong enough horizon filter to prevent dazzling from inadvertant Sun view. Should I add one or am I using incorrectly?"
I am worried that you may be a victim of some of the entropy that has crept into celestial navigation in recent decades. You say that you don't have strong enough horizon filters. But you can use a sextant just fine that has no horizon filters at all ...unless you follow a procedure regrettably advocated by certain navigation instructors where you start out by aiming the sextant right at the Sun and then "drag" it down to the horizon by a steady rotation of the index arm opposite the rotation of the instrument. This technique is totally un-necessary, but it makes some navigators feel skillful, and they insist on peddling this pointlessly tricky methodology to beginners. If you're doing that, stop now.
Use your sextant by pre-setting to an approximate altitude of the Sun. You can get this by manual calculation, by software app, by experience, or by measuring with your hand using some simple tool (like a kamal) held at arm's length. For a quick calculation, this week at high noon, the altitude of the Sun is about 77.5°-latitude (in the northern hemisphere). For a "quickie kamal", a common index card, held long axis vertical, subtends about 11° when held at (adult average) arm's length. So if you count off three and a half index cards between the horizon and the Sun, then the approximate altitude is around 38°. Even sextants with telescopes have fields of view of several degrees width, so if you preset to 38° and the actual altitude is, let's say, 41° 36', you will still find the Sun in your field of view with barely any searching. So you use the sextant, not by pointing the horizon view at the Sun, but by pointing the horizon view where it belongs --at the horizon. With the approximate altitude preset, a little scanning left and right will quickly pick up the Sun and then you adjust for the correct altitude. Also, if you want to get it done even faster, you can line up the azimuth to the Sun by looking at the sextant's shadow on your shirt. The sextant's flat frame will cast a narrow "line-like" shadow when you have the right azimuth. Then just bring it up to your eye, and usually you'll have the Sun right in the field of view. All that "dragging" and counter-rotating is for showoffs... it's not practical navigation.