NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Point Venus, May 1774
From: Alexandre Eremenko
Date: 2007 Apr 25, 11:01 -0700
From: Alexandre Eremenko
Date: 2007 Apr 25, 11:01 -0700
This is a continuation of my previous message with the same title. My calculation outlined in the previous message implies that the going of the Clock from May 2 to May 6 was approx 22m33sec per day. What sort of "astronomical clock" was this, I don't know. And why did not they use chronometers instead. 2. Moon's altitude. Here something happens that I cannot understand. The average error in Moon's altitude was +6' on May 2 (average of 5) and +7'8 on May 6. The stated Index errors were +1' and +3', respectively. Which leaves us with unaccounted 5'. The only plausible explanation I have for this 5' is that they had a substantial dip, which they did not record in their account... 3. The Lunars. According to my computation, the average error of the Lunar on May 2 was -3' and on May 6 it was the same -3', with sigma=0.3. I think this is a decent sigma. (My average sigma over VERY many observations with modern sextant is 0.18). However their stated index ERROR was +2'45" and +2'17". I stress that this they call quadrant ERROR and they have a clear, visible + sign in front of it. While my computation gives the error in their lunar -3' (not corrected for IC). Of course it is tempting to assume that they confused "error" with "correction". But such assumption implies that their altitude error was actually wrong by 7' and 10', which is unbelievable, even if they used a very bad quadrant. Of course, it is possible that I am missing something. But what? Here is a complete computation for of one observation on May 6. Time by their clock: 0h9m54s (of May 7) Sun UL ZenithDist 52d20' I compute: Alt(UL)=37d40, minus UL/refr. corr'n =37d23'. (Correction is from modern almanac, I suppose Sun SD and refraction did not change from 1774). >From this I find with Frank's calculator GMT=19:11:52, May 6. Thus the error of the clock is 4h58m2s. Same calculator says that their Lunar distance of 42d55'1/4 has error -3.2. Same calculator says that Moon alt was 72d24.5 (Moon's center, as seen from the center of the Earth in the absence of air). They measured Moon's alt as 72d00' (LL) Using the HP from THEIR almanac I compute corrections from MODERN almanac and obtain the cleared alt of Moon center 72d32'3. So the error in the Moon alt is +7.8. What I am doing wrong here? So far I investigated 3 series of observations. All have small sigmas in all errors, but the systematic errors of unclear origin are much larger than I expected. Alex. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---