September 22, 2006—Every year in September the sun crosses the Equator and day and night fall into balance.

The event is known as the autumnal equinox, and it marks the beginning of fall in the Northern Hemisphere. The crossing happens only one other time, at the start of spring, when it is called the vernal equinox. Around the world, day and night are roughly the same length during an equinox.

See the celestial dance that creates this phenomenon, then step back in time to find out how ancient cultures—from the Druids to the Inca—incorporated the astronomical event into their beliefs.