I am disappointed and a little alarmed to find big discrepancies between simulator software. Following advice from this thread, I tried simulating the same date: May 5 1975 with three simulators: http://www.fourmilab.ch/cgi-bin/uncgi/Solar (Online) http://www.pwr-tools.com/simsolar/ (Download s/w) http://www.orbitsimulator.com/gravity/articles/what.html (Download s/w) I got some non-trivial discrepancies between all three, and one has Mercury WAY off. I double-checked the results. See attached file to show discrepancies. (I've rotated, zoomed and calibrated the charts for clarity.) What gives???