NavList:

   

Reply

Scott, you wrote:
"Bought a copy of Bruce Starks, Tables for Clearing the Lunar Distance a few
weeks ago.  From the archives it looks as though Bruce was a regular
contributor but I did not see anything of late."

Something I've been meaning to say about Bruce's tables: while the
development of another clearing method holds some technical interest (how
much depends on your tastes for such things), Bruce Stark's book had a
really significant impact on modern navigation education that I don't think
is widely enough recognized. He brought back lunars almost single-handedly.
By publishing his book and spending cash to advertise it and making it
available to distributors, he helped make people aware of this once arcane
historical method and sparked a lot of curiosity among practical navigators.
Bruce Stark's book, combined with the generally too negative, albeit very
prominent, account of lunars in Sobel's "Longitude," generated most of the
modern interest in lunars among navigation enthusiasts. Of course, once the
Navigation list started generating significant traffic, everything changed,
and this is now "lunarian central." But Bruce Stark's tables started it all.


You asked:
"1)  Are these the tables most members use for reducing Lunar observations
or is there some other preferred method/tables?  [I know there are many
methods to do this, I'm just trying to get a feel for what method list
members are using and what may be considered the most accurate method.]"

Most? Probably not. There are as many answers to this as there are lunarian
enthusiasts. When you first start thinking about lunars, you have to ask a
number of questions (and everyone answers them differently).
1) Why shoot them?
Is it for historical interest (either serious historical analysis, perhaps
of old logbooks, or even just antiquarian re-enactment), or is it for modern
entertainment (just something new and challenging to do with your sextant),
or for modern practical use (as a method for testing your sextant), or for
modern disaster thinking (hard to imagine losing GMT but lunars will get it
back if you do)?
2) How will you observe them?
With an antique or traditional sextant (related to the question of
historical interest above), or a modern sextant (probably, but how much are
you willing to "cheat" with modern technology)? And after you've observed
them or acquired observations from a historical document,
3) how will you clear them? With a "historical" computer method (in other
words, use software but base the algorithms on traditional methods in order
to see how well they work), with a historical pen-and-paper method (this is
much more practical today than even five years ago since you can download
19th century editions of Bowditch, Norie, etc. and use the old tables
directly), or wih a modern computer method (the tools on my web site, for
example, which include the almanac data thus making very short work of the
whole process), or FINALLY will you use a modern pen-and-paper method (for
which the best, maybe only, option is Bruce Stark's tables)?

With so many options and possible ways of answering all these questions,
it's not hard to see that relatively few people end up using Stark's tables
exclusively. There have been a couple of list members, though, who really
enjoy them (Mike B, where are you?).

You asked:
"2)  Is anyone familiar with the trig/math that Bruce used to derive his
lunar tables?  If so, does anyone know if Bruce is amenable to posting
it on the list or has it been posted in the past and I just missed it in
the archives?"

You probably know this bit already, but at the basic level, it's really
simple. You set up the Sun-Zenith-Moon spherical triangle with the observed
altitudes (zenith distances actually) of the bodies and the observed
Sun-Moon distance. You use that to calculate, very exactly, the difference
in azimuth between the Sun and the Moon. Next you correct the altitudes for
refraction and parallax (just as you would clear normal altitudes --but use
the bubble sextant rules in the Nautical Almanac altitude correction tables)
and then since the difference in azimuth is NOT changed by the altitude
corrections, you reverse the triangle and calculate the cleared distance.
Compare that with known values of the distance at Greenwich for known values
of GMT and a simple interpolation gives GMT at your location. This is a
"direct" solution to the problem. Historically, and sometimes on this list,
this was erroneously called a "rigorous" solution ("series" solutions which
were morepopular historically were sometimes dismissed as "approximate").
Bruce Stark's tables are a clever, modern re-working of this "direct"
solution. There's an easy way to find posts describing his tables in the
archive. Go here: http://www.fer3.com/arc and in the search box, type
"Stark's tables" (quotes not needed in this case). Click search and you
should find about 87 matching pages. You can find all the details you want
regarding the underlying calculational method by browsing through those
messages. Somewhere along the way, you should find one from Bruce Stark
himself describing the mathematical tricks he used.

Follow the link after my initials for the online clearing tool on my web
site. If there's sufficient demand, I am prepared to make a stand-alone tool
with all the bells and whistles people ask for (historical clearing methods,
graphing tools, automatically generating arc error from long series of
observations, star-star distances, etc.), but I find that many lunarian
enthusiasts are graduates of the "roll-your-own" school. They would rather
build their own calculating tools carefully suited to their own personal
answers to all those questions I described above...

 -FER
http://www.HistoricalAtlas.com/lunars



--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply