Can someone please help me with this? Epoch J2000.0 refers coordinate systems to the mean equinox and mean ecliptic of January 1, 2000, noon TT. I have the following mean orbital elements for Earth with element date January 1, 2000 and refered to j2000.0 epoch. a := 1.00000011 - 0.00000005 *cy e := 0.01671022 - 0.00003804 *cy p := 102.94719 + 1198.28/3600 *cy L := 100.46435 + 129597740.63/3600 *cy i := 0.00005 - 46.94/3600 *cy O := -11.26064 - 18228.25/3600 *cy a - semi-major axis e - eccentricity p - longitude of perihelion L - longitude of planet O - longitude of ascending node, of the intersection of the orbital plane and the plane of the ecliptic i - inclination, angle between the plane of the ecliptic and the plane of orbit as I understand it, the earths orbital plane is synonomous with the ecliptic and as these planes do not intersect the zero direction is taken to be the vernal equinox I realize that these references move over time, hense epoch. However I'm confused about how at the time of epoch the last 2 elements can have a value, not equal to zero? Where is "O" measured from, and to? I'm not sure about the use of "longitude" either, is this figurative or literal for these 3 elements? thanks matt PS does anyone have the Osculating elements for the solar system plants at January 1, 2000.