|
|
|
|
|||||
|
||||||
From: Frank Reed (no email)
Date: Wed Jul 07 2004 - 17:34:10 EDT
George H wrote:
"Did Greenwich wait until 1848, I wonder, before instituting a comparison
between their own observed Moon positions and their own predictions?"
From what I've seen, the Nautical Almanac was significantly revised in 1834.
The planets were added to the lunars tables, and the Moon's position was
improved. Before that date, it is not uncommon to find lunar distance errors up to
1.0 minutes. After that date, they are occasionally as large as 0.5 minutes.
After 1883 (?) the Moon's position and the lunar distances derived from it are
accurate to less than 0.1 minutes of arc in almost every case.
And:
"It's easy enough, now, to calculate the precise positions, then, using
modern data and technology; but a comparison with the published data of that era
would involve a lot of dreary transcribing from old almanacs, so would have to
be something of a labour-of-love."
It IS interesting to do occasional spot checks, and you can do that very
easily using my online lunars predictions, but why bother with an extensive
analysis of the old almanac data at all?? If you're studying a set of lunar
observations from 200 years ago, what good does it do to compound the errors of
observation with errors in almanac data?
That said, if you would like to take a look at some cases, you can do that
easily with the online calculator I've assembled. Go to the lunars predictor
here:
http://www.clockwk.com/lunars/lunars_pre_v5.html
Enter Jan. 3, 1804 as an example. Select Gr. Apparent Time as the time
standard. Select "dd mm ss" for the angle format. And hit calculate. Now open
another browser window and look at the corresponding page of the 1804 almanac which
is online at the Mystic Seaport Library site here:
http://www.mysticseaport.org/library/initiative/ImPage.cfm?PageNum=8&
BibId=21382&ChapterId=2
[the above URL may be split in half in some mail readers. You may have to
patch it back together "by hand"]
Just a few moments of comparison reveals that these published lunars are all
off by about 50 seconds of arc compared with modern ephemeris data. The
almanac for the whole year is there, so you can do hundreds of these comparisons if
you're so inclined.
And:
"would require access to some rare volumes, closely guarded by jealous
librarians."
Just curious: anyone know what old Nautical Almanacs (c.1800? c.1850?) are
worth on the open market?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
|