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Gary wrote-

"There is nothing magic or special about an LOP taken at noon. It is only
due to its simple computation that made it useful in the past to navigators
without the necessary math skills to do the trig for a Sumner line of for
the St. Hilaire method that gave it any usefulness."

Gary has omitted the most important reason why it was historically so
useful. It didn't call for any knowledge of time.

"Since the normal use of these tables allow you to choose an AP within 30 NM
of the DR this allows a 4 minute period that you use the tabulated Hc, two
minutes to the east and two minutes to the west. So certainly using the time
to one one minute falls into the normal level of accuracy for celnav LOPs."

Gary is referring to the normal use of an observation at LAN for determining
latitude. There is, indeed, much leeway in timing that observation, just as
he says.

But Bowditch, rather unexpectedly, read "To calculate latitude AND LONGITUDE
at LAN, the navigator seldom requires the time of meridian passage to
accuracies greater than one minute." It's in calculating longitude from
around local apparent noon that a much better time of the moment of noon
than to the nearest minute is called for.

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.


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