Vernal Equinox

StarDate: March 20, 2009

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Spring arrives in the northern hemisphere today at 6:44 a.m. Central Daylight Time, when the Sun crosses the celestial equator from south to north -- the vernal equinox. Over the next three months, the Sun will travel ever-farther northward, bringing longer, warmer days north of the equator -- and shorter, colder days south of the equator.

The word "vernal" comes from the Latin word for spring. And equinox means "equal nights." Theoretically, on the equinox all points on Earth should see equal amounts of daylight and darkness. But for several reasons, the interval between sunrise and sunset -- which should be exactly 12 hours -- varies by a few minutes.

The vernal equinox marks the starting point for the system that astronomers use to plot positions in the sky.

Astronomers measure the positions of astronomical objects using coordinates called right ascension and declination -- the equivalent of longitude and latitude here on Earth.

Right ascension is measured in hours. The point where the Sun crosses the celestial equator on the vernal equinox is designated as zero hours. It's the equivalent of zero degrees longitude -- the imaginary line that runs through Greenwich, England.

And just as Earth's equator marks zero degrees latitude, the celestial equator is designated zero degrees declination. So at the moment of the vernal equinox, the Sun stands at celestial coordinates zero-zero -- beginning a new circle through the stars.

Script by Damond Benningfield, Copyright 2003, 2009

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