A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Aug 22, 13:06 -0700
If weather permits, tomorrow morning the Moon will be at Last Quarter (of the month) which means the Sun-Moon lunar distance will be close to 90°, and the phase of the Moon will be exactly half-full (N.B.: Last Quarter means Half Moon!). They used to call this being "in distance". Tomorrow and for the next couple of days conditions will be optimal for shooting lunars and getting good results from them. Historically this was the most popular time for shooting them.
Even if you've never shot a lunar before, tomorrow may be a good chance, weather-permitting. Preset your sextant to 90°, swing in the "horizon" shades and aim at the Sun (aha! the original purpose of those "horizon" shades), and then rock the sextant a bit until the Moon appears in the index mirror view (*). Adjust until the bright, leading edge of the Moon just kisses the limb of the Sun. Take your time to eliminate any overlap or gap. Lunars do not demand constant attention on the micrometer. Hold the sextant as steady as possible and get it right. When you like it, record the GMT/UT to the nearest five or ten seconds. Then clear and check your observation using my web app: http://www.reednavigation.com/lunars/lunars_v4.html. It's easy and extremely accurate.
For best results, shoot four to six lunar distance arcs in a row and average the times and then the angles. With a good metal sextant and a five-power or better scope, you can expect accuracy of half a minute of or better easily, and with just a little practice you'll get a tenth of minute of arc accuracy from an averaged set. If you use a plastic sextant instead, you may only get 2' accuracy or 1' with practice and frequent zero-ing of index error.
* Alternatively, especially if the Moon is lower in the sky, aim at the Moon through the horizon glass (no shades) and swing in the usual Sun shades in front of the index mirror. Aim at the Moon and rotate the instrument until the line through the horns of the Moon (nearly equivalent to the Moon's N/S axis) lies horizontal in the field of view (perpendicular to the frame of the sextant). At this point, the Sun should pop right into view on the darkened index mirror side of the field of view. Then continue...