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Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Sat Oct 12 2002 - 13:34:38 EDT
Good posting from Arthur and George. Anyone interested in the navigation of
that era should go over that posting several times to absorb not only what
they've said, but the implications.
The procedure Arthur suggested will work today for someone who's lost
Greenwich time and is uncertain of the dead reckoning. I'll lay that out in
more detail:
Take and work a time sight in the morning. You won't get accurate time from
it, not unless the sun was due east, or the DR latitude correct, but you'll
get it close enough to know, within a few minutes, when to start monitoring
the sun's altitude for the noon latitude. Anyone who's tried to get noon
latitude with only vague idea of when the sun will "dip" will appreciate the
value of this first, approximate, working of the time sight.
Once you've got the correct latitude, work the time sight again to find
exactly how fast or slow the watch is on local apparent time. Be sure to
write that down because, until you get the next time sight, it will be the
basis of your calculations.
That's all the regular "sight reduction" you'll have to do, and the only
thing you took from the Almanac was the sun's declination. At the very most,
that never changes more than 1' per hour. Your estimated GMT would have to be
a long way from the truth to get you in trouble.
Now let's say you get a set of distances of the sun from the moon. If you are
able to take the altitudes before and after the set of distances you can go
ahead and clear it, find, from the Almanac, the GMT that fits, and apply the
equation of time to convert GMT to GAT.
To the average watch time of the lunar observation apply the correction you
found with the time sight: so many hours minutes and seconds to be added or
subtracted to convert watch time to local apparent time. The difference
between the LAT and GAT of the lunar is the longitude of the place where you
took the time sight.
Keep that in mind. The time you're using is specific to the meridian where
you took the time sight, so the longitude you find is specific to that
meridian also.
You've found latitude and longitude, and the lack of accurate GMT was no
hindrance whatever in working the observations. Besides the noon latitude and
lunar, which took no more work than if you'd had accurate GMT, you've worked
a time sight twice, using different latitudes. That's exactly what was
required in order to plot one Sumner line, using an accurate chronometer.
Modern navigators find this hard to swallow. In the system they've been
taught, everything is founded on, and must begin with, accurate GMT. They've
come to accept, as a bedrock truth, that to work observations successfully
you have to have accurate GMT. If you don't have it the only hope, in their
view, is to flounder toward it by iteration.
So much for now.
In case a list member is wondering what to do when he can't measure altitudes
for the lunar, I recently stumbled on a way of calculating them that fits
present procedures, but is no more dependent on accurate GMT than the method
posted under "It Works." Given time, I'll explain it soon.
Bruce
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