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May 2008
Celestial
Navigation
Plotting the noon curve
by David
Berson
This is
the third article in a series concerning the use of the noon sight as a
means of finding longitude. Although most often associated with the
determination of latitude, the meridian passage of the sun — if it can
be timed precisely — can be utilized for ascertaining longitude. The
timing, however, is the tough part. As any navigator who has ever done
a noon sight knows, at the moment of its maximum altitude, the sun
appears to hang in the sky. This makes it very difficult to mark the
exact time that meridian passage occurs. If, on the other hand, we
could know the exact time of meridian passage then we can enter that
time — in GMT — into the Nautical Almanac and find the GHA of
the sun. Then we can find our longitude since the Local Hour Angle of
the sun at Meridian Passage is 0°. In this case the GHA is actually the
longitude of the observer. Of course, there are qualifications for this
assertion, but for all intents and purposes, an accurate longitude can
be found using a noon sight. The proviso being that we need to know the
exact time of the meridian passage and can shoot it at that moment.
This brings us to an idea known as the “noon curve.” What we do in
this case is graphically plot the rise and fall of the sun beginning at
about 20 minutes before calculated noon, and continuing for another 20
minutes after the meridian passage. The graph is constructed with the
sextant altitude on the left side of the page and the time at the
bottom. Perhaps 10 shots of the sun are taken as it rises and another
10 as it descends. At each shot time, a pencil mark is made on the page
that corresponds to the altitude at that moment. When completed, and
the points joined and faired, they should look like a sine curve. The
top of the curve is the maximum altitude of the sun and should be
marked with an exact time. Actually the top of the curve will be
flattened a bit, this is where the sun hangs before losing altitude.
The exact time and altitude of the sun can thus be found for
meridian passage. It is true that this method requires a great deal of
time, and a calm sea helps as well. In one sense it is almost a
contradiction of the raison d’etre of the noon sight which was
developed as a method that was quick and simple and required very
little time or math. Still it is important to know how to wring every
bit of information from the sight. There might come a time when you may
need it.
More Ocean Navigator Articles
About the Writer
Contributing Editor David Berson writes the Nav Problem
page in every issue of Ocean Navigator. He is also the owner
and operator of Glory,
an electrically powered excursion boat, in Greenport, N.Y.
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