NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Old style lunar
From: Ken Muldrew
Date: 2004 Dec 13, 10:00 -0700
From: Ken Muldrew
Date: 2004 Dec 13, 10:00 -0700
On 12 Dec 2004 at 0:55, Alexandre Eremenko wrote: > On Sat, 11 Dec 2004, Fred Hebard wrote: > 1. To decide anything on the source of Thompson errors, > it is desirable to pull the almanac data for the date of his > actual observations. I am a bit confused: who of us has a 1800 > (and prhaps 1801 almanac? Can we ask this person to post the data? Bruce Stark has the old almanacs but they're locked away in storage to protect them from further deterioration. If the opportunity arises and Bruce is able to photocopy any of the relevant pages then I will post the almanac data. The nice thing about this series of lunars at Rocky Mountain House is that we know exactly where he was (and now with the help of Frank's online almanac we know exactly where the moon was as well). The dates for his lunars at Rocky Mountain House are: 1800 17-Apr, 18-Apr, 22-Dec 1801 17-Feb, 28-Feb, 28-Feb, 1-Mar, 18-Mar, 17-Feb, 24-Feb, 24-Feb, 25- Feb, 25-Feb > 2. > > Errors for stars east and west of the moon often cancel each > > other out. > > Not the errors in the almanac! > It is the measurement errors that cancel, not the errors in almanac. I think what Fred meant here is that if Thompson took two lunars on a particular night, one on each side of the moon, then his average of the two resulting longitudes would tend to be close to the true position but the errors of each lunar individually would be unexpectedly large. > Let us consider an idealized situation when the stars are > exactly on the Moon's path and positions of the stars are known > precisely. Consider two stars, Moon in between. > Then the error in Moon's position will lead to the errors > in distances which are exactly opposite to each other. Yes. > Now suppose we measured both distances EXACTLY with our sextant > and want to deduce longitude (or chronometer corr., does not matter) > Averaging the two reduction results will give you nothing. > Both will give the same error in time/longitude. I don't see how that follows. One error puts you too far West of your true position, the other error puts you too far East. Averaging the two cancels the systematic error of the moon being in the wrong place. > To decide whether the main reason of his errors was the > almanac, we > need the almanac errors > on the specific dates of his observations. That would indeed be very helpful. I do have a couple of lunars within the October-November time period of 1800 for which I have almanac pages. I'll try to post at least one of these soon. Unfortunately I can't tell you his true position for these lunars as he is on the trail (though perhaps from his DR account and any landmarks that he describes I can get a reasonable position from a topo map (I know the terrain that he's travelling over pretty well as it's not too far from where I live)). Ken Muldrew.