NavList:

David, you asked:
"Did I get you right, that in location with various structures that would
allow the drawing of lines of perspective, as an artist would draw them,
the "true horizon" for purposes of using a sextant can be derived that
simply?"

Well, yes. In principle. The accuracy is a matter for details. Isn't it always... :) But the KEY point is that this finds the "true" horizon. The converging lines of perspective from well-constructed buildings leads to that great circle that's exactly 90 degrees away from the zenith. There's no dip, of course, for the true horizon. In fact, if you can look at converging lines from buildings with a sea (or large lake) horizon in the distance, you can directly see the dip --the difference between the sea horizon and the true horizon.

And you wondered:
"Would the ghost of George Huxtable (God bless his soul) say
anything contrary?"

For that, I can give you a direct answer. The first time I brought up this idea of converging perspective lines was in a little essay called "Easy Lunars" which I posted just about nine years ago. It's hard to believe it's been that long. Here's a link that will take you to that message and follow-ups:
http://www.fer3.com/arc/sort2.aspx?y=200404&y2=200406&subject=easy+lunars

George Huxtable replied on that specific point of 'converging lines' as follows:
"I liked his clear explanations, and his simple approach to estimating a
land-bound rough-altitude of the Sun. In a flat and gridded city such as
Chicago, with many shoebox buildings, there will be no shortage of parallel
horizontal lines to converge at a vanishing-point on the horizon. Perfectly
good enough in accuracy for estimating Sun altitude, to allow for the small
correction for Sun refraction."

The 'good enough' in this case, for the altitudes in a lunar observation, would imply within +/-5 minutes of arc or so. Can one do better than that with converging lines? Yes, sure. But it would take some detailed work.

-FER

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