OK I am trying to write my own solar system simulator. Knowing that Saturn and Jupiter are nearly in opposition (or whatever you call being 180° apart) gives a good test of whether of not my ephemeris calculations are working correctly. But they don't look so good. I end up with J-S angular difference of ~260° for today's date. Jupiter at longitude 348° and Saturn at 92°. I bang my head against the wall for a few days and then I realize I can use HORIZONS to see what their values of mean anomaly etc are. HORIZONS returns (nearly :) exactly the same values as mine (so all that head banging was for naught). So can someone tell me why the mean (and true) anomalies do not place J-S 180° apart using todays date?
Looking at the values for the two planets I see that the calculated longitudes for both planets on todays date are 180 degress apart, but the Kepler formulae start with mean anomaly = longitude - longitude of perihelion. Is that incorrect. Or am I misunderstanding something?
Lastly, I was using the jpl orbit diagrams as a visual standard and it states at the bottom that the vernal equinox is along the horizontal x-axis to the right, and yet, on Oct 1, 2101 it looks to me like the Earth is darn near right on the vernal equinox.
Is it just me?
Thanks for any help you can give me with this.