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
    Name or NavList Code:
    Re: Bowditch Table 15
    From: Bill B
    Date: 2005 Jan 26, 19:44 -0500

    George wrote:
    > In fact, working on those simplifying assumptions, the "simple
    > trigonometry" would be to use
    > distance in feet = height in feet / tan angle
    > or distance in miles = height in feet / (6080 tan angle)
    > or (as near as dammit) for 106 ft height at a mile distant you will see an
    > angle of 1 degree. Smaller angle, then proportionately bigger distance.
    > But then, to follow that text directly with the formula for Table 15,
    > without further comment, is just crazy. Because that formula DOES allow for
    > the fact that the Earth isn't flat, and it DOES allow for refraction!
    > That's its whole point.
    In reading Jim's quote from chapter 22, it indeed looked like the
    explanations for table 15 and table 16 somehow been mixed together, but did
    not have time to explore it in depth so deleted that paragraph from my
    reply.  But you are spot on George.  What a proof reader!
    > In my edition, such problems are assisted by using table 41, "distance by
    > vertical angle; measured between waterline at object and top of object".
    > Does the newer edition carry that table, perhaps with a different number?
    Not sure I understand.  For your table 41 is the whole object visible down
    to where it meets the body of water?  If so, this is table 16 in the 1995
    There are three tables in the 1995 edition for finding distance by vertical
    Table 15, where one can see the top but not the water/base and knows the
    objects height above sea level.  Angle measured from top to visible horizon.
    Table 16, where one can see the top and the base/waterline and know the
    height above waterline.  Angle measured from the top to base/sea level.
    This is the one that claims to use geometry, flat earth, etc.  No mention of
    subtracting dip in the explanation.
    Table 17, where one does NOT know the height of the object but can see the
    base/waterline intersection and horizon beyond it (above the intersection of
    course).  Angle is measured from horizon to base/waterline.  Again no
    mention of dip correction but height of eye is accounted for when entering
    the table.  It is an odd table, as the instructions claim one should enter
    the table with "the height of eye of the observed in nautical miles,"
    corrected sextant angle, and the output is in yards.  Just to certain you
    don't miss the "nautical mile" reference, it is italicized.  Ah, but the
    legend on actual table calls for "Height of eye above the sea level, in
    feet."  Go figure!!

    Browse Files

    Drop Files


    What is NavList?

    Get a NavList ID Code

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

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

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

    Email Settings

    NavList ID 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