A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2022 Sep 10, 07:13 -0700
David Pike, you asked:
"1. I downloaded 'TheNauticalAlmanac.com 2022 Lunar Distance Tables For the lunar distance method'. They offer Jupiter this month. Will these be OK?"
I have no specific opinion of those tables, but the creator is just "some guy", and more importantly they are ridiculously antique --and not in a good way! "They" offer "Jupiter this month"? Such built-in inflexibility is antique, and it's completely un-necessary in a 21st century product.
Speaking of the 21st century, I have had superior, flexible lunar distance tables available on my website, backed up by my skill and my knowledge of this subject for most of the 21st century. In fact, it's been over eighteen years since I first made these web apps available, witth real options relevant to lunars, both modern and historic, and genuine flexibility. So if you decide you want pre-computed geocentric lunar distances, get them here from me: https://clockwk.com/apps/predict/. Wow!! Where did that link come from?? Well, it's been available directly and conveniently at the link "Lunar Distances Almanac" under the "Data" menu on the NavList website since the NavList interface update which happened over eight years ago.
Now, having pointed out that my web apps are superior in every way, I would add that the tables you found previously are almost certainly just fine and quite adequate to the task. This ain't rocket surgery. There are other sources for lunar distances available, too. But I will assert this: when in doubt, trust mine. They have been vetted by many years of use, by myself and by other folks trying their skill at lunars.
On the other hand, maybe you shouldn't use a prepared/published resource for geocentric lunar distances. Why not compute your own? Indeed, it is not difficult to do so, and that sort of independence is part of the fun of this sport, right? You'll get slightly better accuracy if you produce them from raw ephemerides, which is how they're generated in my web app, but you can get excellent geocentric lunar distances directly from the published Nautical Almanac or any other high-quality almanac resource. The predicted lunar distances are simply great circle distances between the Moon and the other body in any coordinate system that is not topocentric. So if you want the predicted geocentric lunar distance at 18:00 UT, you can grab the ecliptic lat/lon, or the RA and Dec, or, better still, the GHA and Dec of the Moon and the other body at those times. Simply treat them as lat/lon pairs (which is exactly what they are in the case of GHA and Dec!) and work out the great circle distance. You can do it by hand, on a calculator, or using an almost endless number of apps. If you use an app, make sure it's doing the standard "sphere" problem and not some terrestrial globe-based analysis since the distances we want are angles on the "celestial sphere" (an imaginary, perfect sphere).
You also asked:
"2. When the Moon is almost round, which side do you aim at? Is it always the side facing the recently set Sun?"
Well, yes, it's the side facing the recently-set Sun or the soon-to-rise Sun, whichever is closer in angle. But that's not really a viable test that you could use by observation. So what do you do?? If the Moon is really, cleanly full, it doesn't matter. Take your pick. If the Moon is not quite full, grab a pair of binoculars and look. The sharper shadows and the slightly jagged edge will be apparent on the side that is further from the Sun. Avoid that "deficient" limb. You can also tell by looking at the run of geocentric lunar distances for the few hours around the time of your sight. Is the Sun-Moon lunar distance climbing towards 180° and descending from 180°? Note that the maximum distance may be as low as 175° (as it is this month, if I'm not mistaken) and will only peak close to 180° in the event of a lunar eclipse, but you can still get the direction of change right by simple examination of the predicted LDs.
"One thing I learned last night, it's best to estimate the rough angle, aim at Jupiter with my other eye and swing my Hughes Mates 6" until the Moon looked at through the eyepiece sits alongside it."
Yes, that works. If your other body is relatively close to the ecliptic, you can also use the "horns of the Moon" trick. Look at the Moon directly (through the horizon glass), and then rotate the sextant about the axis through the horizon view (which is also the axis of the telecope) until the horns are horizontal in that field of view and thus the line through them is perpendicular to the frame of the instrument. Presuming you preset the sextant to the nearest degree of distance, which is usually easy, the other body will pop right into view. This is especially preferred with Sun-Moon lunars.
" Then look at both through the eyepiece and start twiddling. I'm not expecting great accuracy, just enough to prove the system."
Yes. That's exactly what you'll be doing, and the process will work. There's no doubt that it works, and it works well. In the end you're testing the limiting capabilities of your sextant as fitted out and also, though much less so, the limits of your own observing skill. I say that this is "much less" of an issue because I have seen many times in my classes and workshops that rank beginners can get genuinely excellent results with lunars after a sufficient practical introduction. As far as your sextant is concerned, it may be limited by a certain amount of arc error (and in the absence of a recent certificate, that is one of the best reasons to shoot lunars in the 21st century!), and any sextant tested by lunars is limited by magnification. Your own eyes have a limiting resolution, assuming either perfect vision or proper correction, of just about one minute of arc, slightly better under excellent lighting conditions. But with manual sextant observations, we want lunar arcs observed to an accuracy at least five times, even ten times higher, and that means you need a good scope. I contend that this is the primary limiting factor for modern lunar experiments with reasonably decent metal sextants. If you only have a three-power scope, you will be limited by that. Nonetheless, that is what you are testing. If you know that going in, you won't be disappointed if you can't get results down to a couple of tenths of a minute of arc accuracy.
Speaking of practical introduction, be sure to read and pay attention to recent tips from Mordris Fersters in this post. And you may also benefit from some of the advice in my easy lunars essay here. The latter also includes a simple calculator based approach that is very close to the standard series methods that were popular historically (including the methods of Witchell, early Mendoza Rios, Lyons, Lacaille, Bowditch, Turner, Ward, Norie and more --whose numberous names reflect not genuine difference but "marketing"). As I say in the essay, "lunars are easy". But I should add that it is also "easy" to make lunars monstrously difficult through poor choices of tools and technique. Lunars easy if we let them be easy. :)