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    Re: CN and Mason-Dixon line
    From: Brad Morris
    Date: 2013 Nov 27, 01:15 -0500

    Hi Bruce


    I found reference to this article on historical refraction.  The abstract of the paper indicates some key points.... 


    Astronomical refraction has had a long and fascinating history. Cleomedes (100 A.D.) and Ptolemy (200 A.D.) were aware of its existence and understood in a qualitative way some of its properties. Alhazen (1100 A.D.) quite correctly suggested that the flattening of the sun's disk near the horizon was due to astronomical refraction. Tycho Brahe in 1587, however, was the first to make direct measurements of the magnitude of the refraction. The first theory of astronomical refraction based on Snell's law was that of Cassini, who in 1656 looked upon the earth's atmosphere as being of constant refractive index up to its upper limit at which all the refraction took place. Extensive investigations of the physical properties of the atmosphere modified these ideas and led to the “concentric spherical shell model” and the “plane parallel layer model,” the latter being an approximation for the former. Attempts to evaluate the “refraction integral” for the concentric spherical shell model have led to the theories of Bessel, Bradley, Gylden, Ivory, Laplace, Mayer, Simpson, Young, and others. At the beginning of the twentieth century, investigations on the physical properties of the atmosphere extended to higher elevations, and new physical properties appeared which were not anticipated by the earlier workers. This led to the celebrated work of Harzer, who for the first time was able to compute the astronomical refraction purely from meteorological measurements. Harzer also made the first detailed investigation of the applicability of the spherical shell model, which is used almost universally by workers in this field. Harzer's work has served as a source of inspiration, but much remains to be done. This paper attempts to give a brief chronological description of some of the more significant theories of astronomical refraction for the purpose of pointing out the types of problems which have existed, how these problems were attacked, and the problems which still remain. It also gives adequate references for those wishing additional information.

    On Nov 26, 2013 10:32 PM, "Bruce J. Pennino" <bpennino.ce@charter.net> wrote:


    I recently stumbled on and now reading "Drawing the Line " by Edwin Danson. The book is meticulous and fascinating for anyone interested in boundary surveys, geodesy, surveying and navigation instruments, time keepers etc. Basically the book describes how two English Surveyors (Astronomers, navigators and mathematicians) surveyed and resolved a boundary dispute between Pennsylvania and Maryland. Mason and Dixon used many CN skills. But the book also brings together so many familiar CN topics and characters. Harrison,Maskelyne, astronomer royal Bradley. Briefly, Mason worked under Bradley, and Dixon was an assistant to Mason. Maskelyne, Mason and Dixon were all involved with developing instruments and taking data for the 1761 transit of Venus.

    I've always been curious about the historical background of dip measurement data, and assumed the basic work was done by British astronomers. I guess you know if you are a close student of "dip", as Danson states, James Bradley published in 1720 a table of refraction "for the correction of astronomical measurements for the light bending effects of the atmosphere". That is really early! Amazing to me. Maskelyne assisted Bradley in preparing a table of refractive indexes that were published in the 1767 Nautical Almanac. I checked the 1802 Bowditch & Kirby Table VII Depression Dip of the Horizon (and also Bowditch 1837), and the dip table is basically the same as present. I conclude the original astronomical work and calculations for dip were done in early to mid 1700s. Really astounding the excellence of the early astronomers and mathematicians.

    Best regards

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