Earth’s Orbit and Its Variations
		q First, Earth spins around on its axis once every day è The Tilt. q Second,  Earth revolves around the Sun once a year è The shape of the Orbit. q Both the tilt and the shape of the orbit have changed over time and produce three types of orbital variations:      (1) obliquity variations      (2) eccentricity variations      (3) precession of the spin axis.