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Hello Sundial-listers, I used to rely on Luke Coletti's Great Circle website's GROK calculator for a precise calculation of sun positions, but that page has been non-operative for about a year now. The National Renewable Energy Laboratory (NREL) in Golden Colorado has provided an excellent resource at http://www.nrel.gov/midc/solpos/spa.html, similar to GROK. Their MIDC Sun Position Algorithm utilizes Jean Meeus' modified VSOP87 algorithm, reportedly accurate to 0.0003 degrees between years -2000 to +6000. The NREL site also provides a link to an explanation of this algorithm as a pdf file http://www.nrel.gov/docs/fy08osti/34302.pdf. The program will output a table of sun positions over a range of dates, in as little as 1 second intervals. It allows many options for sun position measurements, but the most helpful to me are topocentric azimuth, geocentric sun declination, Topocentric (uncorrected) sun elevation, and topocentric sun elevation corrected for atmospheric refraction. These tables are essential to me in the field when I delineate true north using a theodolite, and lay out predictive sightings for sunrise and sunset. Beware; if you select "Topocentric Zenith Angle", the results are corrected for refraction, though they do not tell you this up front. You must select "Topocentric (uncorrected) sun elevation" if you want to exclude atmospheric refraction. Lastly, they report the time of sunrise/sunset as when the center of the sun is 0.8333 degrees below the horizon (uncorrected for refraction). This is to account for the width of the sun, and an average effect of refraction, and represents the moment that the advancing or receding limb of the sun (not the sun's center) crests the horizon. -Bill |
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