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.
q