This is a great question since equinox means "equal night". However, if you look at the sunrise and sunset times for our first day of Spring (March 20th), you will see that the sun rises at 7:23am and sets at 7:33pm. This is not equal at all.
First, you have to realize that sunrise and sunset times are based on the top of the sun rising and setting on the horizon. When it comes to the sun crossing the equator to signal the beginning of either spring or fall, that is based on the center of the sun.
Second, the Earth's atmosphere bends and refracts the incoming light. This creates the appearance of the sun above the horizon when it really is not.
These two factors added up, make for an "unequal equinox". However, there are a few days around the time of the equinoxes that end up being "equal" when it comes to amount of day and night.
From the U.S. Naval Observatory (The primary source for sunrise/sunset times) website:
"In the northern hemisphere, at latitude 5 degrees the dates of equal day and night occur about February 25 and October 15; at latitude 40 degrees they occur about March 17 and September 26. On the dates of the equinoxes, the day is about 7 minutes longer than the night at latitudes up to about 25 degrees, increasing to 10 minutes or more at latitude 50 degrees."
--http://aa.usno.navy.mil
To help localize the quote above, Erie's latitude and longitude is 42.1N -80.12W.
Ray Petelin,
WSEE Storm Team Meteorologist