A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Mar 31, 15:19 -0700
It depends on what you're trying to accomplish when you shoot lunars. They can be used in several different ways. I'll stick to the two that are most familiar and relevant.
If you were shooting lunars historically --in the period from about 1763 to about 1850 at sea-- they were being used to determine absolute time, and several geometric aspects impacted that function. First, the Moon's motion had to be more or less in line with the position of the star or or other body. This implied that the traditional lunars stars were preferentially along the ecliptic. Aldebaran, Pollux, Regulus, Spica, and Antares all worked and were listed in lunars tables for over a century. Unfortunately, some parts of the ecliptic have no bright stars at all. So a few stars were included that were well away from the ecliptic, like Altair. For those stars, lunars could still be effective for determining time if the star was within about 30° of the leading point or trailing point of the Moon's motion. This would only happen when the lunar distance was fairly large, typically greater than 40° distance.
Working lunars for time also benefited from linear, steady changes in the lunar arc angle with time, and this was not reliable (over the typical three-hour interval in the tables) when lunar distances were below 20°. That's the usual lower limit for historical limits. So those times when we see the crescent Moon nestled up close to a bright star, and it looks so amazing would ordinarily be bad times to shoot lunar for the determination of absolute time.
Certain geometric factors come into play for lunars near 90°, and this is really the sweet spot. If you want to practice historical lunars, I recommend sticking to distances from about 60° to about 100°.
We don't need lunars for absolute time, GMT, anymore, and most modern lunarians are testing their skill or testing their sextants. In that case, you can throw out the rules above, and nearly all lunars can be valuable. This implies that we don't need to worry about the traditional lunars stars and can shoot any star that we can identify at almost any angular distance. Want a Polaris lunar? It would have been pointless historically, but in the context of modern lunars, it's no problem. Even at its most southerly Declination, the angular arc from the Moon to Polaris would never exceed 118°, just inside the limit for most sextants. And when you experiment, you'll find that the worst cases historically --very short distance lunars-- can be the most enjoyable today since it's relatively easy to hold the star stable against the limb of the Moon and get an accurate observation when the distance is short.