ESS200A
Prof. Jin-Yi Yu
Earth’s Orbit and Its Variations
qFirst, Earth spins around on its axis once every day č The Tilt.
qSecond,  Earth revolves around the Sun once a year č The shape of the Orbit.
qBoth the tilt and the shape of the orbit have changed over time and produce three types of orbital variations:
q     (1) obliquity variations
q     (2) eccentricity variations
q     (3) precession of the spin axis.
(from The Earth System)