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Earth's orbital parameters as viewed from Mars above the Earth's geographic North Pole (NP), in a configuration of northern summer solstice (NP pointed towards the Sun). The Earth's orbit is elliptical with (invariant) major axis a and minor axis b defining eccentricity e. The Sun occupies one of the two foci (f1,f2); variables e, P, I and W are the orbital elements. The plane of the Earth's orbit is the "ecliptic of date" and is inclined at an angle I relative to a fixed reference ecliptic (greatly exaggerated in this depiction from its actual magnitude of 1 to 2°), and intersects this fixed plane at a longitude W at point N, the ascending node, relative to a fixed vernal point go. The orbital perihelion point P is measured relative to go as the longitude of perihelion, P, and moves slowly anticlockwise. The Earth's figure is tilted with respect to the ecliptic of date normal n at obliquity angle e. Earth's rotation f is anticlockwise; gravitational forces along the ecliptic of date from the Moon and Sun act on the Earth's equatorial bulge and cause a clockwise precession y of the rotation axis. This precession causes the vernal equinox point g to migrate clockwise along the Earth's orbit, shifting the seasons relative to the orbit's eccentric shape; this motion constitutes the "precession of the equinoxes." The angle ϖ between g and P is the moving longitude of perihelion and is used in the precession index esinϖ to track Earth-Sun distance. Click here for 10 Ma of La93 orbital variations. Click here for a spectrogram.