Dan Schrag and Paul Hoffman have revived the theory that the Earth was frozen over. Microscopic fauna could have survived in oceans with thin ice cover and local ice-free patches. But macroscopic life could not get going until after the return to a warmer climate. Non-members, click here to subscribe. This animation illustrates three variables affecting the Earth's orbit about the Sun.
Eccentricity refers to change in the elliptical shape of the orbit.
Note how the orbit becomes almost circular at minnimum ellipticity. Precession is the change in azimuth of Earth's tilt axis relative to the ecliptic plane. Non-members, click here to subscribe. This is a time-lapse movie spanning 24 hours, showing a view of the night sky over a silhouetted horizon. Note the dense band called the Milky Way.