Last night at 1:48 a.m. EST the sun passed directly over the equator, marking the first day of spring in the Northern Hemisphere, and the first day of fall down under.
Today is one of only two days in the calendar year when the sun spends an equal amount of time above and below the horizon everywhere on Earth, meaning night and day are the same length.
In reality, the day is longer than the night at an equinox. Daytime is commonly defined as the amount of time sunlight hits the ground. From the Earth, the Sun of course appears round rather than a single point of light, meaning the upper edge of the sun remains visible when its center dips below the horizon.
But who cares? The point is the days are getting noticeably longer, baseball season is set to open next weekend (Go Braves!) and most of us can comfortably switch from winter coats to sweaters.