Dear Anselmo, > > We all know that we need just two points to draw a stright line (like the > solar time ones), five points to draw an hyperbola (like the datelines) > but...
Theoretically speaking, the hyperbola is a second-degree curve. Therefore three points should completely define its shape. > Which is the minimum number of points (ie., of dates) we need to draw > an analemma (in a flat dial) within a > worst-case precision of, say, 1 minute of time? The equation-of-time curve is (in good approximation, that is, within one minute) the sum of two sine waves, with frequencies of 1/year and 2/year. Knowing that, two parameters are necessary to describe each sine wave; for instance, amplitude and phase angle. So, four points should define the EoT curve. You didn't ask a recipe to actually construct the EoT curve for an arbitrary case, did you? Regards, Frans ===================================== Frans W. Maes Peize, The Netherlands 53.1 N, 6.5 E www.biol.rug.nl/maes/sundials/ =====================================
