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From: Gordon Talge (no email)
Date: Sat Jun 04 2005 - 14:52:17 EDT
According to J.W. Norie's "Complete Epitome of Practical Navigation"
1844, there are several ways of finding Longitude at sea.
A short listing of the table of contents shows:
On finding Longitude by Observation
To find the apparent or mean time by an Altitude of the Sun
" " a star.
" " a planet.
" " the moon.
To find the error of a watch or chronometer by equal altitudes
of the Sun.
On Finding Longitude by Chronometers
On Finding Longitude by Lunar Observation
To find the apparent Altitude of the Sun, Moon, Star, or Planet.
So they had various ways to find their Longitude.
It seems to me that at that time (1844) and for years to come,
finding Latitude and Longitude were related, but separate problems.
There wasn't any "line of position" as we know it today.
The "Noon Sun Shot" was a staple. However, due to the fact that the
Sun tends to "hang" in the sky on the meridian with little apparent
change in altitude means that finding longitude by noting the time
of passage, while in theory should work, in reality is not practical.
One method has the observer, taking a shot at maybe 10 minutes before
LAN and noting the altitude and the watch time, doing a noon shot
and resetting the sextant to the altitude that was noted 10 minutes
before and watching the Sun until it is at this same altitude and
noting the watch time. Since the Sun must have been at LAN midway
between these two times, you then calculate that time and using the
equation of time from the NA get your longitude.
There is more to it then just time, but it is the basic idea.
In the tropics with the Sun at close to 90 degrees in altitude at
LAN is would seem to me to almost impossible to get an Azimuth or
decent altitude. Maybe a Moon, planet or Star shot would work
better.
I also tend to thing that unless you are close to land, that knowing
your exact position is really not that big of a deal. If you are
out in the middle of no where, an error of 20 or 30 miles would be
no big deal. Close to land, however would be another story.
Voyages took weeks and there were plenty of opportunities for
observations
and corrections if the DR was kept up.
My $.02
-- Gordon
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