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Wayne, via "Rebecca Lowry" , asked-
about "Longitude by meridian transit/ fishline sextant"


Cool,
A new toy to make....

Any idea what the numbers on page 2-109 for the year are?
And how do we find the numbers for the current year?
Wayne

===========

Those numbers sum up to provide GHA Aries, in arc-minutes. But for this
application, it's absurd to bother allowing for such a year-to-year
variation. If a constant value of around 35 was chosen, unchanging from year
to year, that would be perfectly compatible with the precision that the
method is capable of providing.

The claim is, as Mike Boersma repeats, a precision in precision of 10 miles,
which requires latitude and longitude to be within one sixth of a
degree.That would require the following quantities components to be known,
or set, within the appropriate accuracies, all within 10 arc-minutes.

GMT to 40 seconds (quite plausible)
A pendulum string, some few metres long, hanging steady and vertical in the
open air, with no shielding from wind.
Another string, a few metres long, set accurately horizontal, with no
guidance given as to how to achieve that.
That horizontal string to be set precisely in a North-South direction "by
using a compass". No mention of variation.
Another diagonal string, tensioned to be straight enough to measure an angle
from, attached part-way up the length of the vertical pendulum, which
doesn't kink that vertical string out of straight.
A paper protractor, for measuring the angle between two bits of string in
space.

All these are supposed to combine together to give a precision in the result
amounting to 10 arc minutes!. With such a Heath-Robinson arrangement, anyone
who got the answer right within a couple of degrees, or 120 miles, would be
doing well.

There's nothing wrong, in principle, with any component of this scheme. But
it's a procedure that looks as if it's been assembled by a committee, given
the job of knocking up some sort of emergency manual. Completely
unrealistic.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

=====================


--- On Mon, 2/15/10, Mike Boersma  wrote:


From: Mike Boersma 
Subject: [NavList] Longitude by meridian transit/ fishline sextant
To: NavList@fer3.com
Date: Monday, February 15, 2010, 12:48 PM



The US Airforce survival manual from the 1960's included a description of
how to make a fishline sextant and tables for determining longitude and
latitude from its use.
The claimed accuracy is 10 miles. Individual results may vary.
Mike Boersma
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