NavList:

   

Reply

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