In the Northern Hemisphere spring officially begins at 7:21 p.m. ET on Sunday, March 20, 2011—the vernal equinox, or spring equinox.
But don’t be fooled by the old rumor that on the vernal equinox the length of day is exactly equal to the length of night.
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.
Geometry, Atmosphere, Language of the Vernal Equinox
If the sun were just a tiny point of light and Earth had no atmosphere, then day and night would each be exactly 12 hours long on a spring equinox day.
But to begin with, as seen from Earth, the sun is nearly as large as a little fingertip held at arm’s length, or half a degree wide.
Sunrise is defined as the moment the top edge of the sun appears to peek over the horizon. Sunset is when the very last bit of the sun appears to dip below the horizon.
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,” Chester said.
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