I always understood that the vernal equinox was not only one of two days in the year in which the Earth wasn't tilted towards or away from the sun, but also a day in which the same amount of time elapsed between sunrise and sunset as between sunset and sunrise. When I checked the sunrise data on the US Naval Observatory webpage, they show that date as having happened on Saturday, March 17. I was about to celebrate the equinox with my class tomorrow when I realized the difference, and now I want to be sure before I tell them something that isn't correct.