Ah today is the Spring Equinox or Autumn Equinox depending on your location. If you are in northern hemisphere it is of course the autumn equinox and spring in the south.
One of the word origins has equinox coming from Medieval Latin equinoxium “equality of night (and Day)”. It’s easy to see why people think the day and night are the same length everywhere on Earth on the equinox. Problem is, this really isn’t quite true.
Here is how I like to think of it:
On the equinox (either the Northward or Southward) the sun’s rays are perpendicular to Earth as depicted on the image above, The tilt of the Earth is seemingly absent at and for the moment of equinox (but ONLY for the moment because of course the tilt is in reality not gone at all).
The lengths of light/dark periods towards either pole is not the same as it is on the equator, but for this day the lengths of light/dark periods for a particular latitude above and below the equator ARE pretty much the same. Each hemisphere is illuminated equally so the length of daylight and darkness for say 40 degrees north AND south latitude are the same, but the same cannot be said for mis-matched latitudes like say 10 degrees north and 40 degrees south.
So when does that moment of Southward Equinox occur? It varies a wee bit from year to year, this year the Southward Equinox occurs today (September 22, 2013) at 20:44 UTC (16:44 EDT).
Edit: Alan in his comment pointed out some confusing language on my part having to do with the 40 degree north and south example. I am not meaning those particular latitudes have equal light and dark times of 12 hours each, rather the light and dark periods they do have will match and remember there is some discrepancy.
The actual sunrise and sunset values for 40 deg N and S are below:
40 North Sunrise is 1048 UTC and /Sunset is 2257 UTC
40 South Sunrise is 1047 UTC and /Sunset is 2258 UTC