Huffington Post - 9/21/13, Mike Wall/Space.com
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?
More: HuffingtonPost.com
