NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Point Venus, May 1774
From: Fred Hebard
Date: 2007 Apr 27, 17:29 -0400
From: Fred Hebard
Date: 2007 Apr 27, 17:29 -0400
Alex, It's a joy to read your posts. Fred On Apr 27, 2007, at 5:10 PM, alex wrote: > > This is the continuation of my messages of April 24 and April 25 > on the Point Venus observations of Cook's expedition on May 2-6 1774. > > 1. I fixed the "clock problem". The Clock was not so bad after all. > Except an unexplained leap of 1 minute on May 1 (see my message on > April 24) > it was slowing down by 1m 22.6s per day plus-minus a fraction of one > second. > The astronomers of the expedition checked its going against the Sun, > and they determined the rate perfectly, and this is confirmed (to 1s) > by my computations. > This was a Syderial Clock (showing GHA Aries, rather than Mean solar > time). > > 2. I can prove that what they call "Quadrant error" (for the > instrument used in measuring > the lunar distance) in their observation journal is in fact the > CORRECTION, not "error". > They don't say which instrument exactly was this. > To prove this, I reduce their own observation using their own almanac, > and this correction (which they call error) and obtain the same > longitude AS THEY COMPUTED. > > 3. The residual error of their lunars (after applying the correction > mentioned above in part 2), > was -0.5' on May 2, -0.6' on May 3 and -0.45' on May 6. These are the > averages of 5-10 > observations each time and sigma in each series is from 0.2' to 0.4'. > These errors are the errors of OBSERVATION, have nothing to do with > the > almanac. > These errors are SYSTEMATIC, every single shot is an overshot. > Exactly the same picture as I had for years with my own observations. > Averaging does not help at all in this situation. > > 4. The resulting error in longitude is a sum of the observation error > described in section 3, > and the error resulting from the almanac. On these dates these two > errors ADDED. > On May 2, the almanac contributed 30' and the observation error > contributed 14', > giving the total error in longitude of 44' in longitude. > On May 3, the almanac error contributed 32' and observation error 17' > to the total of approx. 50' > On May 6, the almanac contributed 20' and the observation error 23' , > and there was probably > some smaller reduction error. > Thus we see that the role of the observation errors was roughly of the > same > magnitude as the errors in almanac. > > 5. The only thing about these observations that I still don't > understand is the errors > in the Moon altitudes. They are very large and also systematic. > 6' on May 2, > 15' on May 3 (!) and > 8' on May 6. > For this I have no clue. > These numbers do not take into account their "quadrant error" which > was less than 3' > for this quadrant all the time. > > Alex. > > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---