Hi Frank, At the 2007 NASS conference, Bill Gottesman had several comments about calculating sunrise/sunset for the purpose of viewing these observations. He was consulted in the building of an observation area. These were some of his comments if I recall correctly.
(1) Get a panoramic view of the area because hills, buildings, and trees do effect sightings. (2) Use the last quarter of the sun setting, not the center, to calculate the azimuth at sunrise and sunset. (3) Refraction of the air does alter the observation time and azimuth. This needs consideration in calculations. I am sure he will say more when he sees this topic being discussed. Warren Thom ----- Original Message ----- From: "Frank King" <[EMAIL PROTECTED]> > Dear Jack, > > There is another little thing about sunrise and > sunset at the solstices that you might enjoy > knowing. It was, I believe, known in prehistoric > times so this isn't a recent research result... > > The point on the horizon where the sun rises at > the winter solstice is directly opposite the point > on the horizon where the sun sets at the summer > solstice. [And vice versa of course.] > > To see this from the formula you need to use the > two-argument inverse-tangent function. In Excel > the formula for the azimuth at sunrise is: > > ATAN2(sin(dec), +sqrt(cos(phi+dec)*cos(phi-dec))) > > The formula for the azimuth at sunset is: > > ATAN2(sin(dec), -sqrt(cos(phi+dec)*cos(phi-dec))) > > [Everything is in radians not degrees by the way.] > > If you reverse the sine of dec the argument of the > square-root function doesn't change but sin(dec) > does change sign. Of course you have to reverse > the sine of the square-root result when going > from sunrise to sunset. > > Accordingly, going from sunrise at one value > of dec to sunset at the negative of that value > of dec takes you 180 degrees round the compass. > > This is because you are going from: > > ATAN2(arg1, arg2) to ATAN2(-arg1, -arg2) > > I hope you aren't wishing you had never asked > the question!!! > > Best wishes > > Frank King > Cambridge, U.K. > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
