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: Sun squash- was Green Flash and Longitude
    From: Fred Hebard
    Date: 2006 Jan 14, 22:24 -0500

    On Jan 14, 2006, at 9:58 PM, Ken Gebhart wrote:
    
    > Gentlemen,
    >
    > The comments on green flash and longitude reminded me of a project
    > I once undertook to establish longitude from estimating the percent
    > squash of the sun due to refraction after rising, or before
    > setting.  It all started during a flight I was making in a single-
    > engine Cessna airplane from Honolulu to Wake Is.  In 1973 we had no
    > navigation equipment except a radio direction finder which told
    > which way to go, but not when you would get there.  Also it could
    > fail.  Thus, I always used an aircraft sextant on such trips.
    > Around noon I actually crossed the subpoint of the sun.  Sextant
    > read 90 deg. no matter which way I looked.  This was handy since I
    > could immediately put a fix on my chart without using the 249
    > tables.  It otherwise took a long time to work out a sight while
    > flying with my knees, not having an autopilot.  Later as the sun
    > was beginning to set (this was a 15 hour flight), and I was still a
    > few hundred miles out, I planned to grab a longitude by observing
    > the sunset.  All was fine until it did set... on a cloud deck!  So
    > without a real horizon I couldn?t use the sight.  But I remembered
    > that the sun began to show a squash as it approached the clouds.
    > So I began to think about correlating the percent squash with the
    > actual altitude, so I could get around this cloud problem.  And I
    > thought it may be of use to people on land too, who had no accurate
    > horizon.
    >
    > For the next several months I took hundreds of photos of the sun as
    > it was rising or setting, noting the time and known geographic
    > position.  I projected each photo on the wall of a room, and
    > measured the height and width of the disk, thus getting the amount
    > of squash.  Working backwards from the almanac refraction tables, I
    > was able to correlate the percent squash not with just Hs, but
    > directly with Ho.  I even developed correction curves for
    > temperature and pressure (including altitude).  As it turned out,
    > maximum squash ever observed was only about 17%.  I tested this
    > method with my co-workers who while driving to work in the morning
    > would note the squash and the time.  We would work the sight out,
    > and to our surprise, they were never more than 3 or 4 miles off.
    > For a long time after that, I would note sun squash while driving
    > cross country, and work the sight upon return, with the same
    > apparent accuracy.
    >
    > I had in mind to publish a book on celestial navigation anyway, and
    > the sun squash chapter could be ?the hook?, that is, some
    > information that had never been published before to give it
    > intrinsic value.  There would be an insert with ellipses of 5, 10,
    > and 15 % printed on sun shade material, that could be held up to
    > compare with the actual sun.  Trouble was, I could not produce such
    > ellipses  of suitable quality for publication.  I needed a PC with
    > a desktop publishing program which had not yet been invented. So,
    > the whole project languished in a file cabinet all these years.
    > Now the issue is moot except for the interest some list members may
    > have in knowing about it.
    >
    > Ken
    
    Great post Ken.  I can imagine it would be a lot of fun, or
    something, to try to work out a sight by hand while steering with
    one's knees!
    
    How would you measure squash?  The altitude of the two limbs and
    assuming the SD for the width or the semi-diameter both ways?  I
    don't know how to do semi-diameter with an aircraft sextant (or
    anything else with an aircraft sextant!).
    
    Fred
    
    
    

       
    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