Artwork showing a map of the south polar regions of Earth during the late Ordovician Period (480 MYa). Scientists have identified two distinctive cyclic episodes of glaciation, each due to a different cause and having different effects on ice distrib...

Artwork showing a map of the south polar regions of Earth during the late Ordovician Period (480 MYa). Scientists have identified two distinctive cyclic episodes of glaciation, each due to a different cause and having different effects on ice distrib...

... gins in the northern hemisphere when the planet's north pole is pointed most directly at the Sun (summer solstice). When the south pole is pointed at the Sun, it is summer in the south and winter in the north (winter solstice). Spring and autumn occu ...

... gins in the northern hemisphere when the planet's north pole is pointed most directly at the Sun (summer solstice). When the south pole is pointed at the Sun, it is summer in the south and winter in the north (winter solstice). Spring and autumn occu ...