I was looking at sunrise and sunset time tables from the US Naval site (entering Hillsborough). It seems the day with 12 hours between sunrise and sunset will be March 17th, not March 21st, the vernal equinox. Why?

MIKE MOSS SAYS:       Mike,    You're noticing two effects that combine to push the occurrence of "twelve hours of daylight" to a few days before the Vernal Equinox and likewise a few days after the Autumnal Equinox.

The first reason is how we define sunrise and sunset. On a smooth earth with no atmosphere, we would see the center of the sun's disk cross the eastern horizon heading up in the morning, and then on the way down across the western horizon in the evening very close to twelve hours later. However, in the U.S., and most other countries, we define the time of sunrise and sunset not by means of the center of the sun's disk, but by the time that the upper limb (top edge) of the sun crosses the horizon instead. Therefore, on the equinox, there will be a couple of minutes more than twelve hours of "daylight" associated with the time it takes one-half the sun's disk to cross the horizon.

The other, larger effect is that of atmospheric refraction. Around sunrise and sunset, the rays of sunlight reaching a given location are following a path that crosses horizontally through the atmosphere, almost parallel to the ground. Since atmospheric density decreases rather rapidly with height, the rays of light are refracted, or bent, toward the layers of greater density, resulting in a downward curvature of the light rays that are reaching us as observers. Since the rays arriving at our location are bending downward, and we perceive objects to be located in the direction from which their light waves arrive, the sun appears higher in the sky than it really is, and in fact we see it rise above the horizon when it is actually still two or three minutes away from doing so. In effect we are able to "see around the corner," or over the hill if you like, because of the curving light rays. This refractive effect adds to the sunrise/sunset definition effect to make the "twelve hour day" occur even farther before the spring equinox (and vice versa in the fall when days are growing shorter).

There's a nice discussion of this, along with some helpful illustrations (including the one you see above) at

http://www.bedford.k12.ny.us/flhs/science/geoscience/sunrise.html