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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lewis and Clark lunars: more 1803 Almanac data
From: Frank Reed CT
Date: 2004 Apr 22, 17:00 EDT
From: Frank Reed CT
Date: 2004 Apr 22, 17:00 EDT
Ken M, regarding the misidentification of Betelgeuse as Aldebaran, wrote:
" But it's just way too close to the moon; 15°, in fact. Exactly 15°. Now I can't quite figure out what error would lead to the captains recording distances that were off by 15°, but for some reason the fact that there are no minutes or seconds in the error makes me think it's possible."
Now I think you've found it!
And I can suggest a scenario that would lead to this error. The recommended procedure for shooting lunars was (and is) to preset the sextant to the estimated angle based on the best guess of GMT. It would be easy to mistakenly preset the instrument by an angle that is in error by some multiple of the scale's gradation. I've done similar things myself on occasion. After presetting, you point the sextant at the Moon and rotate it until you sweep up a bright star. The horns of the Moon should be more or less perpendicular to the plane of the instrument, but since the Moon was gibbous it would look about right in this case. Betelgeuse could easily be mistaken for Aldebaran under these circumstances. Reading the distances, the observer would likely make the same error since he already knows the approximate expected distance from his preset calculations.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
" But it's just way too close to the moon; 15°, in fact. Exactly 15°. Now I can't quite figure out what error would lead to the captains recording distances that were off by 15°, but for some reason the fact that there are no minutes or seconds in the error makes me think it's possible."
Now I think you've found it!
And I can suggest a scenario that would lead to this error. The recommended procedure for shooting lunars was (and is) to preset the sextant to the estimated angle based on the best guess of GMT. It would be easy to mistakenly preset the instrument by an angle that is in error by some multiple of the scale's gradation. I've done similar things myself on occasion. After presetting, you point the sextant at the Moon and rotate it until you sweep up a bright star. The horns of the Moon should be more or less perpendicular to the plane of the instrument, but since the Moon was gibbous it would look about right in this case. Betelgeuse could easily be mistaken for Aldebaran under these circumstances. Reading the distances, the observer would likely make the same error since he already knows the approximate expected distance from his preset calculations.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
