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Subject: How did Sumner navigate in 1837?
From: Jim Thompson (jimt@XXX.XXX)
Date: Tue May 13 2003 - 05:17:08 EDT
I am a adult celestial navigation student (Power Squadron course). I've
become completely fascinated by how Sumner "discovered" the celestial LOP on
the morning of December 17, 1837. I still have these key questions at this
point in my understanding:
1. Am I right in assuming how he navigated?
2. Was the celestial LOP completely unknown in 1837, or were academicians
aware of the concept, but nobody had been able to operationalize it at sea?
3. Exactly how would he have determined his longitude in 1837?
4. How aware were navigators in his day that their longitude calculation was
dependant on latitude? They must have been. It must have been part of the
sight reduction?
Putting together some information on 18th century celestial navigation from
www.lunardistances.com, I assume that he probably had done this:
1. Determined latitude by DR from his last fix, 900 NM to the west, at 21
deg W longitude (he was now at 6 deg W). Is there no way he could have
determined latitude from the sextant altitude of the sun and his chronometer
time?
2. Determined longtitude by using these known variables: DR latitude, his
chronometer GMT time, the altitude of the sun from his 1000 shot, and tables
showing declination of the sun. From those he could determine local time,
from which he could determine the difference in time between his local
meridian and Greenwich. I still don't understand the steps he used, but I
think that's the basic process he would have used. Am I right?
The critical point is that his longitude estimate was dependant on his
latitude. He seemed to understand that, which is why he reworked the 10 AM
sight with two presumed latitudes. How common was that practice in 1837?
If that old DR latitude was way off, then his longtitude was too -- which
was one of the points that navigators in those days might not have
appreciated, because they did not commonly understand the concept of a
celestial LOP. He was pretty damn gutsy to have sailed ENE in poor
visibility toward the rocks, assuming that his longtitude was west of
Small's Light. When Small's Light popped out of the mist, he must have been
both immensely relieved and incredibly gratified.
I have posted my limited-understanding version story at
http://jimthompson.net/boating/CelestialNavigationNotes.htm
in two places:
Finding Position in the 1700s and 1800s
and
Sumner Discovers the Celestial Line of Position From a Single "Time Sight"
It is a brilliant, amazing story. It will make an interesting lecture, if I
can reduce the elements sufficiently to lay terms and spice it up with
information and graphics about a shipboard navigator's life in those days.
Jim Thompson
jimt@XXX.XXX
www.jimthompson.net
Charlottetown, Prince Edward Island, Canada
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