Hi Jos, Great idea!
You can do it without a clock: Eccentricity and phase were established in classical times (by Ptolemy) by timing as accurately as possible the solstices and equinoxes. Timing the equinoxes is relatively easy, as you need a nodus, or something equivalent, with which to observe when the sun is exactly in the equatorial plane. As the declination is changing quickly you should be able to judge it quite well. You need to judge it to within an hour or two, at least. Tough luck if that turns out to be during the night! Timing the solstices is much much harder. One suggestion is to observe the sun's altitude at noon for several days either side of the solstice, then calculate or draw a graph to interpolate to the moment of solstice. Once you know the lengths of the four seasons you can calculate the eccentricity and the dates of perihelion and aphelion. The Ptolemaic model, with the sun on a uniform circular orbit about a fixed point in space somewhere near the earth, is quite accurate enough for these purposes. By the way, Ptolemy seems to have cheated - he got more accurate figures for the eccentricity than his data warranted. You could go the whole hog and fit your observations to an ellipse, of course. If you allow the use of a clock as well as your sundial, you can make daily measurements of the Equation of Time. After a year you can perform a Fourier analysis on the data to find both the eccentricity and the obliquity components. What a great project. Let us know what results you get. Chris Lusby Taylor 51.4N 1.3W ----- Original Message ----- From: Jos Kint To: [email protected] Sent: Thursday, August 21, 2008 9:11 AM Subject: Earth eccentricity Hello all, I am looking for some help in observing the earth's orbital eccentricity, just by using my sun dial. Who gives me some hint? With my vertical 2,5 meter by 1,5 meter sun dial I can measure the local solar time with an accuracy of less than 60 seconds. Jos Kint, Belgium ------------------------------------------------------------------------------ --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
