NavList:

From: George Huxtable <george@hux.me.uk>
Subject: [NavList 7677] Re: Transcription of Worsley's Log
To: NavList@fer3.com
Date: Wednesday, March 18, 2009, 8:51 PM

I was just getting round to replying to Henry Halboth's posting about
Worsley's navigation, when a second posting appeared, stating-

"I believe that I have deciphered Worsley's numbers for April 25th. He is
using the old style format for a Longitude time sight at Latitude 61.4 S,
which I presume to mean 61-04 S. The numbers in the right hand column,
although separated by decimal points are actually the logarithmic values
applicable to the formula employed. I will recalculate and let you know the
results asap. Depending on the sun's azimith at the time of observation, the
Latitude used may not be that important to the correctness of the Longitude
determined.

By the way, until quiet recently at least, there appears to be an
international difference of opinion as respects positions on Elephant
Island."

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

That's interesting, on several counts.

Worsley is turning his back on "New Navigation", and instead, just as Henry
says, deriving longitude from an old-style time-sight of the Sun, away from
Noon.

Brad and I, between us, have worked out how he implemented this calculation,
following exactly Norie's "first method" for time-at-place. This appeared in
posting [7468], with the relevant section from Norie's Navigator (1900)
attached. I attach it again, and ask Henry if this corresponds with the
method he was familiar with, in his days on the bridge. There were many such
methods in use, and navigators had their own favourites. Brad has put
together an Excel spreadsheet that reproduces Worsley's method, logs and
all.

We're able to follow every detail of many of these calculations of local
hour angle of the Sun. This could  provide either a measure of chronometer
error if longitude was known (as for Elephant Island) or a measure of of
longitude, assuming the chronometer had been corrected. But these
calculations required a good knowlege of latitude, because at that latitude
and time-of-year, the Sun didn't appear anywhere near the prime vertical.

Worsley had serious problems in establishing his latitudes. He only had
occasional glimpses of the Sun, and an accurate latitude could be deduced
only when the Sun could be seen at or near noon, which happened rarely.. On
the day of departure, 24 April, the Sun made its first appearance since
their arrival on Elephant Island, allowing a morning Sun altitude for a
time-sight to be observed, but a noon observation for latitude was
impossible, as the Caird was loading-up and setting-off at around that time.
So the calculation depended on assumed values for lat and long of Cape Wild,
Elephant Island.. Because the time-sight was less than 3 hours before local
noon, it was very sensitive to the accuracy of that latitude.

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

It interests me that in the era when radio time checks were widely available
for a chronometer, navigators in Henry Halboth's days were still using
time-sights of the Sun. Was this just a matter of going through a ritual
that was by-then already ancient, I wonder, or were those results taken
seriously? I'm sure Henry has many insights of how things were done in days
before the War up his sleeve, if we could persuade him to tell us about
them.

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.