Stone Henge in the Autumn Equinox.  (Photo credit: Stonehenge Stone Circle)

The autumn equinox falls on September 22nd, the date autumn officially commences in the northern hemisphere.  For the second time this year the polar axis of our planet is at right angles to the Sun and all locations receive equal hours of daylight and darkness.  Having re-crossed the celestial equator the Sun is now retreating southwards, arcing ever lower above the southern horizon, its warming rays diminishing in intensity as it does so.