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.