A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Aug 31, 12:34 -0700
Jaap vd Heide, you wrote:
"I used the data from USNO online (http://aa.usno.navy.mil/data/docs/celnavtable.php) and for some reason I tended to get the values as listed in Stven Wepster's precomputed distances, rather than those on Frank Reed's website. I computed the distances using the all-haversine formula (ZD), on a Casio FX-4500P. Any thoughts? Might it be a rounding issue?"
I don't want to over-emphasize the small offsets in Steve Wepster's tables. They're a tenth of a minute of arc typically, which is clearly not a lot to worry about. I would be happy to go over some specific cases to see how things compare. I compared a few just now (just to make sure I wasn't nuts!). I checked Sun-Moon lunar distances for every three hours on September 5, 2015. If I use the displayed data from the USNO online tool (which you mentioned above), I find that the LDs from my tables are offset on average 0.041 minutes of arc, while Steve Wepster's tables are offset -0.034. So that seems like a tie --even a slim edge to Steve Wepster's tables. But the trouble is that the USNO site only lists the GHA and Dec values to the nearest tenth of a minute. If you go one digit further (you'll need to find another source of almanac data), then I find that my values are offset by an average of near zero and Steve Wepster's are offset by -0.07' (which usually rounds to an error of one-tenth of a minute of arc).
The advantage of Steve Wepster's tables is that he tells you which bodies to shoot on any given day. But arguably some might consider this a disadvantage. Since he's liberally selecting stars from the modern navigational stars list, this is not a historical selection. So the idea that the tables are representing historical practice is not supported. And since the tables only list a few stars, it doesn't work well as a modern sextant test. If we're shooting lunars to test our skills and/or our sextants or to emulate modern lunars, like the lunars observations performed in spacecraft forty years ago, then we don't have to limit ourselves to stars that are along the ecliptic at all. His selection of bodies is an interesting hybrid of old needs and new, but it doesn't really match either.
By the way, don't forget that you can use my tables to compare against (or supplement) historical almanac data going back to the beginnings of the use of lunars in the 1750s. Like the early nautical almanacs, you can choose to tabulate them as a function of GAT instead of GMT. For a modern lunarian, if you're trying to appreciate historical practice, you should generate predicted lunar distances by GAT for any years before 1834.