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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar-distance almanac errors [was; The Online Nautical Almanac- beware!]
From: Frank Reed CT
Date: 2004 Jul 7, 17:34 EDT
From: Frank Reed CT
Date: 2004 Jul 7, 17:34 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
"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
