Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Lewis and Clark lunars: more 1803 Almanac data
    From: Frank Reed CT
    Date: 2004 Apr 17, 21:46 EDT
    Ken M, you wrote:
    "From this data, we see an almost constant relative motion of the moon of 25"/min over the period of 11:30PM to 1:30AM local time. Despite the scatter of the L&C data, they measure a relatively constant motion of 18"/min over the same period. I really think that this indicates that they were measuring the wrong star, and that it was well wide of the moon's path."

    Another possibility (which I looked into) was that the local time we've been assuming was incorrect for some unknown reason. A few hours later in the night, the rate of change of the measured Aldebaran lunars falls to a level that would be reasonably consistent with that 18 seconds or arc per minute of time. But it doesn't help. There's no way to line up the rates of change and the three sets of measured distances for *any* value of the local time at this location.

    It is certainly possible that they were observing a different star --I agree with that--, but the fact that no bright star fits the measurements (as George H pointed out in his original message) doesn't let us reach a conclusion. I'm inclined to think that these were simply poor observations. We don't know who took them. Maybe Lewis was sleeping in his warm bunk and had told an assistant to make the observations. Would you want to be up shooting the Moon in the middle of a cold December night? Or maybe the observer's hands were shaking badly, and he chose to write down numbers that he knew were sloppy to get the task over with and go back to sleep. And after all, this area had already been surveyed. There was no necessity to produce quality observations at Kaskaskia.

    Regarding the speculation that they might have observed the wrong limb, you wrote:
    "... and they had to be better at this than me."

    I don't think you should necessarily assume that. These folks had had only basic training in navigational methods. Everything was new to them, from handling and adjusting a sextant, to reading a vernier, to identifying the stars. I would *expect* errors at this early stage in the journey that would strike us today as shocking or amateurish.

    In another message, regarding my comment that they might not have known the stars, you wrote:
    "This comes as quite a surprise. I would have thought that everyone would have at least a passing familiarity with the constellations of the zodiac and a few of the more obvious ones (such as Orion, Ursa Major, etc.) in the era before streetlights. "

    It surprised me, too. As I've looked through 19th c. ships' logbooks, I've found numerous references to star patterns, but many of them are remarkably ignorant statements, and professional seagoing navigators rarely used the stars (except for lunars). I am sure that far, far more people knew the basic constellations in 1804 than know them in 2004. I would venture a (wild) guess that 25% of people could readily identify a dozen constellations in 1804 while perhaps 0.1% can do that today. Now although that earlier number is dramatically bigger than today's, I'm suggesting that it could still be much less than a majority. You can see that it might still leave the Lewis and Clark expedition with no one capable of identifying stars correctly. Of course, these things can be learned, and they had books and time to kill. I would imagine they knew the stars better at the end of the trip than at the beginning..

    And you wrote:
    "If what you say is true, then I think it increases the likelihood of the hypothesis that another star was misidentified as Aldebaran."

    The hypothesis is certainly viable. But fine-tuning a hypothesis unfortunately doesn't help to prove it. Until someone can point to a star that fits all the data, it remains a worthy speculation, but that's all.

    Incidentally, in case anyone would like to duplicate my calculations of observed lunar distances, these are the steps I used:
    I grabbed the data from Omar Reis's "Online Nautical Almanac" here http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isa
    This gave me GHAs, SHAs, Decs, etc for Greenwich date Dec. 3, 1803. I set up a spreadsheet to parse out the values from the text that the web page creates and then calculated LHAs for the night using the longitude of the camp (89.98 W). Next I calculated altitudes and azimuths for every half-hour (all of this in the same spreadsheet). To the altitudes, I added refraction (increased a bit since it was probably cold) and subtracted lunar parallax. From those altitudes corrected for refraction and parallax combined with the difference in azimuth, it was a simple matter to generate the predicted "observed" lunar distances. Adding or subtracting semidiameter is the last step.

    You can also use the same almanac data to confirm George H's calculation of the time of LAN (required to interpret the chronometer times). All you have to do is look for the GMT when the Sun's GHA is equal to the camp's longitude.

    Frank E. Reed
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)