In the Northern Hemisphere spring officially begins at 7:21 p.m. ET on Sunday, March 20, 2011—the vernal equinox, or spring equinox. An equinox occurs twice a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth’s equator.
Although the word equinox is often understood to mean “equal day and night,” this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are referred to as the “equiluxes” to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days.
The true days of day-night equality always fall before the vernal equinox and after the autumnal, or fall, equinox, according to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C. “Exactly when it happens depends on where you are located on the surface of the Earth,” he said. By the time the center of the sun passes over the Equator—the official definition of equinox—the day will be slightly longer than the night everywhere on Earth. The difference is a matter of geometry, atmosphere, and language.
The vernal equinox, however, occurs when the center of the sun crosses the Equator. Plus, Earth’s atmosphere bends the sunlight when it’s close to the horizon, so the golden orb appears a little higher in the sky than it really is. As a result, the sun appears to be above the horizon a few minutes earlier than it really is. Therefore, on the vernal equinox day, the daylight hours are actually longer than the length of time between when the sun crosses the horizon at dawn and when the sun crosses the horizon at sunset. “Those factors all combine to make the day of the equinox not the day when we have 12 hours of light and darkness,”
Sources: National Geographic, Wikipedia