Today is the last day of summer 2012, and fall is just hours away—that means another fall (or autumnal) equinox is coming.

But what is the autumnal equinox? 

Equinoxes fall on the halfway point between solstices and occur, according to the Washington Post, "when the sun crosses the equator and day and night are of roughly equal length, everywhere in the world."

The nearly equal 12 hours of light and darkness can be attributed to the Earth's lack of an axial tilt on the day of the equinox. The word equinox is derived from the Latin words aequus, meaning equal, and nox, meaning night.

In Western Pennsylvania, according to the National Weather Service in Moon, the sun will come up at 7:07 a.m. and set at 7:17 p.m on the autumnal equinox. This, however, is not a perfect 12 hours of light and darkness. As timeanddate.com reports, that occurs due to light refraction and other reasons.

See ya next year, summer. Hello fall, and happy autumnal equinox!

Here's the weekend forecast for the region, according to the National Weather Service:

• Friday night: Scattered showers, low of 69.
• Saturday: 50 percent chance of showers, high of 69, low of 45.
• Sunday: Showers taper off to a chance of 20 percent, high of 59, low of 40.