NavList:
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Re: Lewis and Clark lunars: a request for help.
From: Frank Reed CT
Date: 2004 Apr 8, 18:13 EDT
From: Frank Reed CT
Date: 2004 Apr 8, 18:13 EDT
Of lunars set B, George H wrote:
"Here is a discrepancy of 4.9', which would make sense only if the longitude was about 92.5 deg rather than 90deg. Again, this is a serious discrepancy "
Serious, yes. But seems to be about as good as Lewis & Clark were capable of doing from the evidence of the later parts of the trip. I assume you've read Preston's article about the lunar observations in 1804 and 1805. Probably there are people on the list who don't know about this though, so here's a link:
http://www.aps-pub.com/proceedings/jun00/Preston.pdf
In Preston's table comparing calculated longitudes with known longitudes, it's clear that the lunars they took were frequently off by two or three degrees in longitude. Further, the error is systematic --it's almost always in one direction. Preston speculates on causes. Here's one to consider: bad eyesight. If an oberver is nearsighted, bringing a star to the edge of the Moon's limb is tricky since the star appears to have a large "disk". Even within any given set (the sets A, B, C in yout post), the lunars jump around by 2 or 3 minutes around the distances that one would expect.
By the way, I did try working the longitude from "set A" and experimented with various possible sources of error/confusion. I don't see anything obviously wrong but I agree that the results are way off.
One point on identifying stars... Preston notes that their log work mentions at one point taking sights from alpha Pegasi and another star, too, since they weren't quite sure which star was the correct one. I don't think this sort of confusion is likely with Aldebaran since the Hyades are instantly recognizable, but it's something to keep in mind.
Myself, I'm currently working on a set of lunars from 1809 taken on a trip from Boston to Yemen and then Calcutta which are TOO accurate. I have a feeling the navigator was simply tossing out any lunars that disagreed with significantly with his dead reckoning.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"Here is a discrepancy of 4.9', which would make sense only if the longitude was about 92.5 deg rather than 90deg. Again, this is a serious discrepancy "
Serious, yes. But seems to be about as good as Lewis & Clark were capable of doing from the evidence of the later parts of the trip. I assume you've read Preston's article about the lunar observations in 1804 and 1805. Probably there are people on the list who don't know about this though, so here's a link:
http://www.aps-pub.com/proceedings/jun00/Preston.pdf
In Preston's table comparing calculated longitudes with known longitudes, it's clear that the lunars they took were frequently off by two or three degrees in longitude. Further, the error is systematic --it's almost always in one direction. Preston speculates on causes. Here's one to consider: bad eyesight. If an oberver is nearsighted, bringing a star to the edge of the Moon's limb is tricky since the star appears to have a large "disk". Even within any given set (the sets A, B, C in yout post), the lunars jump around by 2 or 3 minutes around the distances that one would expect.
By the way, I did try working the longitude from "set A" and experimented with various possible sources of error/confusion. I don't see anything obviously wrong but I agree that the results are way off.
One point on identifying stars... Preston notes that their log work mentions at one point taking sights from alpha Pegasi and another star, too, since they weren't quite sure which star was the correct one. I don't think this sort of confusion is likely with Aldebaran since the Hyades are instantly recognizable, but it's something to keep in mind.
Myself, I'm currently working on a set of lunars from 1809 taken on a trip from Boston to Yemen and then Calcutta which are TOO accurate. I have a feeling the navigator was simply tossing out any lunars that disagreed with significantly with his dead reckoning.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
