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The seasons will change this Sunday (Sept. 22), with the Northern Hemisphere moving into autumn and the South emerging from winter into spring.

 

The celestial event that marks this transition is called an “equinox,” and it happens twice every year, around March 21 and Sept. 21. Just what is an equinox, and why does it occur?

 

The Earth moves in two different ways. First, the planet spins on its polar axis — a line through the north and south poles — once every 24 hours, causing the alternation of day and night. Secondly, it moves in its orbit around the sun once every 365.25 days, causing the annual cycle of seasons. The equinox occurs when these two motions intersect.

 

Autumnal Equinox 2013