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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.