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: Technique question
    From: Fred Hebard
    Date: 2003 May 19, 16:00 -0400

    I hope you can tell I talk from little experience, from the perspective
    of a beginner.
    With that in mind, I think one way you could test my hypothesis
    regarding observer error being the source of discrepancy between your
    observation and the known distance would be to try some semi-diameters
    of the moon.  It may not be too late this month to do this; you need to
    have opposite sides still fully illuminated.  You bring the moon across
    itself and take the reading when the two edges just coincide, then pull
    it away from itself until the two edges just coincide.  Half the
    difference in the two readings is your index error while one-fourth of
    their sum is the semidiameter.  Three repetitions should give you an
    idea of the repeatability of your measurements.  Bruce Stark has some
    similar determinations for the sun in his book, to give you a feel for
    the precision attainable for this measurement.  It can be quite high.
    It was clear from your altitudes and the closeness of the computed
    position to the known position that there wasn't any systematic error
    in your sextant, such as a 5' index error!
    Another possibility that might be involved is having the objects off
    center in the telescope when you take your reading.  The old Bowditch
    cautioned against this.  I don't know what effect this would have.
    Perhaps others could enlighten us.
    Hope this helps,
    On Monday, May 19, 2003, at 15:30 US/Eastern, Royer, Doug wrote:
    > I have a couple of technique questions about Lunar observations for you
    > guys.Mr. Hebard wrote something that got me wondering.Let me give you
    > the
    > observation conditions of the last 2 attempts.On 05-11 and 05-17 there
    > was
    > an atmospheric condition (high,thin layer of fog) that fogged the
    > images of
    > the bodies under observation.The edges of the Moon were clear but had a
    > corona type of fuzziness about it.The images of the planets and stars
    > observed were the same but because of the smallness of the bodies not
    > as
    > pronounced.I used the 6x scope on the sextant and it looked as if the
    > images
    > were in coincidence.However,maybe they weren't after reading the posts
    > this
    > morning.The altitude observations taken at these same times appear to
    > be
    > close to what they should be.The horizon was uneffected by the fog as
    > it was
    > high in the atmosphere and when the bodies were brought down to it
    > sharp in
    > the scope.I have confidence in the altitudes because I am used to
    > doing them
    > and feel there is little observer error in them.However,the sights of
    > the
    > distances may have an observer error due to inexperiance in useing the
    > sextant in the positions needed to take the angles of the distances.My
    > question is mainly about the images under the conditions and if that
    > could
    > lead to further reduction of accuaracy not including the observer
    > error?Under what atmospheric conditions do you guys usually attempt
    > Lunar
    > observations?
    > I have data from 05-17 but will try clearing it before posting it to
    > compare
    > the final output with what you guys will come up with.I also have to
    > clear
    > the data from last week and post my times for critique.

    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)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site