NavList:

   

Reply

Peter Hakel wrote-

" ...experience has apparently shown that doing a running fix is preferable
to getting longitude from the time of LAN.  This is very interesting.  Does
anyone know how this rule was established?  Has anyone tried both methods
and compared their accuracy?  It is also conceivable that nobody has really
tested this and the reasons are historical: i.e. the LAN is used to give
latitude ONLY and thus the running fix is the next best thing to establish
position."

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

Peter has it right. Although much attention has been given on this list to
the determination of longitude from observations around Local Apparent Noon,
mentions of its use at sea in actual chronometer-navigation are rare. That
is because its defects were recognised, as a method which would require
extended and precise observational data, to achieve a mediocre result; there
were better ways of doing the job. The moment of Local Apparent Noon was
indeed calculated, but that was to provide the best monent to take an
altitude of the Sun to determine latitude, which would (unlike the maximum
observed altitude) be unaffected by the vessels course and speed.

Before Sumner / St. Hilare position lines were adopted,
Chronometer-navigation under a clear sky, using only the Sun, involved the
following procedure-

1. Several hours before noon, take an altitude of the Sun. On that basis,
and the best guess of latitude, based on dead-reckoning from previous
observations, compute morning longitude.

2. From that longitude, using dead reckoning, compute when the moment of
Local Apparent Noon will be, and at that LAN moment take another Sun
altitude, to provide noon  latitude. From that latitude, using dead
reckoning, work backwards to check whether the latitude assumed at the time
of the morning observation was reasonable. If it was out, recalculate that
morning longitude on the basis of the newer latitude.

3. From that morning longitude, using dead reckoning, work out the longitude
at noon. Announce noon lat and long.


4. Take another Sun altitude, several hours after noon. Use dead-reckoning,
from noon, to estimate latitude at that moment. On the basis of that
altitude and latitude, compute afternoon longitude.

In that why, whenever a clear sky served, a navigator would interleave
observations for latitude and longitude, bridging the gaps between them by
dead reckoning, and keeping the errors within bounds..

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.

   

Reply