NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant calibration.
From: Frank Reed CT
Date: 2007 Apr 20, 20:46 EDT
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From: Frank Reed CT
Date: 2007 Apr 20, 20:46 EDT
By the way, there was, in fact, a very good eighteenth century
method of calibrating sextants. I've seen it written up in the "Tratado de
Navegacion" by Jose de Mendoza y Rios, who was one of the great experts on the
subject in that period, especially after he transplanted to England. The way he
describes it makes it sound like a relatively well-known procedure.
The method depends on the existence of numerous church steeples as the
tallest bits of architecture in a city. Today, that would be slightly less
common, but it was not uncommon back then. You climb to the top of any suitable
tower or steeple and then you shoot horizontal angles between well-defined marks
on steeples in the distance around you, preferably at distances of a mile or
more (if the sextant can be placed on a table and re-positioned
repeatably to the nearest foot, then measuring angles between objects a
mile away will give you a calibration accurate to 0.1 minutes of arc). If
you have lots of steeples, you can do all sorts of pairings that will yield the
arc error for your instrument. Suppose, for example, I have steeples within a
few degrees of the cardinal points of the compass. Call them N, E, S, and W.
Suppose I measure the angle NE and find that it's 90d 10'. Then I measure angle
ES, SW, and WN and find that they are 89d 05', 91d 12', and 89d
31'. From these measurements, I can deduce that there is an arc error at 90
degrees of -0.5' of arc --because the measured angles don't add up to 360
degrees. Next suppose I see two other distant steeples, call them A and
B, in the southwest roughly evenly spaced between S and W. If I measure the
angle SA, AB, and BW, they must add up to 91d 12.5' of arc, the corrected angle
SW. If they don't add up, the difference gives me the correction for 30 degrees
on the arc. And so on for other angles. Note that none of this depends on a
calibration standard --you don't need a perfect sextant to start. Once you've
done the procedure a few times, and assuming no steeple fires (thanks to Ben
Franklin's lightning rods) or wandering churches, you eventually collect a set
of fixed, known angles that serve as your standard for other instruments.
This calibration method also works on the Connecticut coast at various
places since you can find lighthouses and other fixed markers. I've tried it.
It's simple, but time-consuming. I wish I had a set of those prisms we were
talking about a while back!
To me, the biggest problem with any of these outdoors methods of sextant
calibration is that you have to deal with the inevitable questions... "whatsat
thing do? are ya lookin at the Moon?" So I answer, "well, no, I'm just
calibrating it so that it measures angles accurately." And when I see that
look on their faces that says 'this guy must be crazy', I throw in "...so that I
can sell it on ebay." And then their eyes brighten and it all makes sense to
them --they walk away happy in the knowledge that they are seeing digital
commerce in the making.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
See what's free at AOL.com.
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To post to this group, send email to NavList@fer3.com
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