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On 9/12/2014 10:11 PM, Samuel L wrote:
> Thank you for the method. I read Henry's comments on the Time Sight
> method but I know nothing about it other than that. Can you direct me to
> a clear exposition of it?

Seem to caught in a time warp. The time site clarification was sent
AFTER the longer message. BTW typo: a time sight uses Ho, not Hc.

For a complete answer, I would search the archives--I don't have a pointer.

Brief answer:

Prior to the current intercept method (St Hilaire method) longitude
could be calculated from a time sight. Lore has it a good sailor ALWAYS
knew his latitude. (Not too difficult with dead reckoning and a recent
LAN observation. Not so much in rough seas and bad weather if they did
not have a fix for several days.)

Input was:
--Ho derived from an observation at a given time with the usual
corrections for index error, dip and refraction.
--AP latitude
--Declination at the time of the observation.
The result was the Local Hour Angle (LHA).

If LHA (west lon) = GHA - AP longitude
Then GHA (at time of observation) - LHA = Longitude

(Memory is foggy on the following in this paragraph). Back in the day
they used the equation of time, perhaps converted from time to arc
coupled with a mean GHA and adjusted for time after the hour to arrive
at the GHA for the time of observation?

Unless you are a purist, with the current NA that's a waste of time. The
GHA in the almanac reflects the equation of time. So simply calculate
the GHA for the time of the observation and subtract the LHA from it for
longitude.

My sequence on the TI for azimuth is sin mem3 - sin mem1 x sin mem2
/(cos mem1 x cos mem2) = cos azimuth.
sin dec - sin lat x sin Hc / (cos lat x cos Hc) = cos azimuth

If you swap out Hc/Ho fand or dec in the above formula, it becomes a
time sight.
sin Ho - sin lat x sin dec / (cos lat x cos dec) = cos azimuth

For me 4 keystrokes swaps out Hc/Ho and dec, and I can proceed with my
usual Az mem... sequence with no further thought required.

For extra credit (Reader's Digest version), Sumner discovered the line
of position via time sights a bit by accident. He had not had a fix in
days and did not have enough sea room to go boldly forward. So he sat
there awaiting better conditions and figured his possible longitudes
based on an observation and using different latitudes. Connected the
dots and what he saw was a line that he was on somewhere along depending
on his latitude.

He subsequently wrote a book (a made for TV dramatization IMHO), became
a bit famous, and went quite mad.

And that is my Friday eve bedtime story :-)

   

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