From Sky and Telescope:
Spring begins in the Northern Hemisphere at the equinox, 6:45 p.m. EDT (3:45 p.m. PDT). This is when the Sun crosses the equator heading north for the season.
The word "equinox" comes from night and day supposedly being equal on this date, but that's not really true. It would be true if Earth had no atmosphere and the Sun were a point rather than a disk. But with an atmosphere, daylight continues long after the moment of sunset. Second, sunrise and sunset are counted not from when the Sun's center is on the true horizon, but when its top edge is on the horizon; this adds a couple minutes to daytime. And third, atmospheric refraction at the horizon elevates the Sun by about its own apparent diameter, adding a few more minutes before sunset time.ring
( But Jack Frost decided to stick it to us ...again )
