A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Jan 24, 11:40 -0800
Bob Crawley, you wrote:
"Correction for irradiation."
I don't know dates on editions of Dutton's, but the presence of this topic alone suggests it's from circa 1970. Just so you know, the fascination with the idea of "irradiation" and the corrections for it which were all the rage in the 1960s eventually faded away. It's a perceptual effect and proper use of shades renders it irrelevant. The inclusion of an "irradiation correction" in the Sun altitude corrections in that period is now generally regarded as a mistake.
"Correction for air-sea temperature difference."
Doesn't work. It would be nice if it did, and in theory it should. Unfortunately, the variable driving anomalous dip is the air temperature gradient in the shallow layer between the sea surface and the observer's height of eye, and it turns out that this is only loosely correlated with the air-sea temperature difference. This idea, of correcting for unusual refraction based on simple air-sea temperature comparisons, has come and gone several times in the history of celestial navigation. Chauvenet thought it would work as early as the 1860s. It's good physics, in principle, and the math is solid in simple cases. Nature is not a simple case.
"Astigmatising shades - where can I get these?"
They were a standard feature on some sextants in the 1960s and 70s. The argument in their favor was founded on the idea that it was "hard" to swing the arc in some circumstances. Or was it? I believe that this was a reflection of the "bad habit" of swinging the arc around the horizon axis which developed in mid-century, apparently originating in USN practice. If you swing the arc properly, keeping the Sun or star centered in the field of view, then swinging the arc always works, and an astigmatizer becomes superfluous.
All that said, I don't want to diminish the value of the book. An older copy of Dutton is a great thing! Both as a historical document and as a textbook on celestial from an era when people still depended on it. That mission-critical dependence --quite naturally-- promoted better pedagogy in many textbooks.
Conanicut Island USA