On Spring Equinox, Day and Night Equal in Name Only

Updated March 20, 2006

In the Northern Hemisphere spring officially begins today, the vernal equinox, at 1:26 p.m. ET. Soon trees will bud, snows will melt, and for the next six months daylight will tick more minutes off the clock than darkness.

The reverse is true in the Southern Hemisphere. But whether you are entering the season of light or darkness, don't be fooled into thinking that on the equinox the length of the day is exactly equal to the length of the night. It's not.

The day of light and dark equality always happens before the spring and after the fall equinoxes, 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 Earth's 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

Chester explains that if the sun was just a tiny point of light and the Earth had no atmosphere, then the day and night of the equinox would each be exactly 12 hours long.

But, to begin with, the sun is bigger than a point—it appears nearly as large as a little fingertip held at arm's length, or half a degree wide, as seen from Earth.

As such, sunrise is defined as the moment the top edge of the sun appears to peek over the horizon, and sunset is when the very last bit of the sun appears to dip below the horizon. The equinox, however, is when the center of the sun crosses the Equator.

Additionally, the Earth has an atmosphere that bends the light cast by the sun when that light is close to the horizon. 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 longer than it really is. Therefore, on the equinox, 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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