Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Measuring Dip in the 18th Century
    From: Brad Morris
    Date: 2014 Jan 1, 13:53 -0500

    Gary

    Thank you! 

    From the pages provided Wright asserts

    1) that the solstice altitudes of the sun were observed with a quadrant. 
    2) that the observations were recorded to 1/2 arc minute
    3) that the solstice observations measured the maximum excursion of the declination of the sun.  If I understand Alex correctly, then that declination excursion is good to 0.25 minutes of arc.
    4) that he used Copernicus' mighty work as a reference, a mere 54 years after its publication (!!)

    There are some points which must be deduced from the text

    A) the quadrant uses the horizon for the reference.  Other than specifying London, Wright does not tell us what body of water is used.
    B) Wright does not tell us a dip table was used.  We can assume that it was, because unless the HoE was 0, then the observation would have been in error by that amount.
    C) Wright does not inform us as to whether the observation was ship or land based.  There are problems with either one.  If land based, then the TIDES affect the measurement, unless one can guarantee the same tide height for minima/maxima observations 6 months apart.  If water based, the pendulum referenced as establishing vertical would be challenging at sea.

    One additional point can be made.  The assertion that a quadrant is only good to 10 arc minutes can be readily disposed of.

    Brad

    On Jan 1, 2014 11:42 AM, "Alexandre Eremenko" <eremenko---.edu> wrote:

    Gary,
    
    > I have also attached the June page showing Wright's
    > determination of the Sun's maximum declination as 23� 30'
    > north while the
    > modern value is 23� 26.3' a difference of only 3.7'.
    
    Did you take into account that obliquity of the ecliptic
    decreases at the rate about 0'.7 per century?
    More precisely, 46".8 per century.
    This implies that Wright wrote it correctly.
    
    I have not read the book yet:-)
    but I assume that he was using some astronomical tables available at that
    time, or did he determine all astronomical constants himself??
    
    Alex.
    
    
     I have also included
    > Wright's explanation of how he determined the Sun's declination.
    >
    > http://en.wikipedia.org/wiki/Gregorian_calendar
    >
    > gl
    >
    > gl
    >
    > Attached File: http://fer3.com/arc/img/126089.march declination table from
    > wrights book.jpg
    >
    > Attached File: http://fer3.com/arc/img/126089.june declination from
    > wrights book.jpg
    >
    > Attached File: http://fer3.com/arc/img/126089.explanation of declination
    > determine by wright_page_1.jpg
    >
    > Attached File: http://fer3.com/arc/img/126089.explanation of declination
    > determine by wright_page_2.jpg
    >
    >
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=126089
    >
    >
    >
    >
    
    

    View and reply to this message: http://fer3.com/arc/m2.aspx?i=126097

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    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:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    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