A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2018 Jun 27, 19:23 -0700
When you shoot the Noon Sun with small zenith distance, for example if you were near 23° N latitude this week, you run into certain issues. Swinging the arc requires greater skill, and you absolutely have to do it right (keeping the Sun centered in the field of view as you spin on your heels), and the maximum altitude is only stable for a minute or even less time. A NavList member asked me a question about a high altitude sight in the tropics two weeks ago, and that got me thinking again about an alternative, which I call a "meridian-anchored" Sun sight.
We "swing the arc" with the sextant in order to ensure that the sextant is vertical and to ensure that we are, therefore, measuring the minimum angle between the Sun and the point on the horizon directly below it in azimuth terms. But suppose we just throw that out and allow ourselves the luxury of being able estimate azimuths. Using the vessel's compass, or, if near land, using terrestrial features and markers in the distance, we can determine a point on the horizon that is close to true north or true south. Then we can measure the angle between the Sun and that fixed point on the horizon. Imagine taking sights every thirty seconds for some period of time around local noon. We still get a noon curve with a maximum (or minimum!) around local noon, but it lasts much longer than the usual peak of the Sun's altitude at noon. In a case where the zenith distance is small, and you might have only one minute to get the proper altitude right, you might have fifteen or twenty minutes to get the meridian-anchored altitude. And if you happen to be facing north when the Sun's altitude peaks to the south, it's no problem since you can just subtract the resulting maximum altitude from 180° to get the correct noon altitude (or since we want zenith distance, you can just jump to that: if it's over 90, subtract 90; under 90, subtract from 90).
Having spoken to quite a few celestial navigators who have tried sights at small zenith distance, I have had the impression that this is something that people do anyway, as a fallback, and assume it's invalid theoretically but close enough in practice. What I'm suggesting here is that it is excellent in practice and also theoretically valid. If you can reliably aim your sextant at a point on the horizon that's due north or due south plus or minus a few degrees, within ten minutes of noon, and measure the angle between the Sun and that point, the angle that you get will stably match the noon altitude (or 180-that) for a convenient, extended period of time around local noon.
That's a "meridian-anchored" Noon Sun sight.