Spring Equinox 
Spring arrives in the northern hemisphere at 12:14 a.m. Central Daylight Time tonight. That’s the moment the Sun crosses the celestial equator -- the projection of Earth’s equator on the sky -- from south to north -- a moment known as the vernal equinox. Over the next three months, the Sun will travel ever-farther northward, bringing longer, warmer days to northern climes.
“Vernal” comes from the Latin word for spring. And equinox means “equal nights.” Theoretically, all points on Earth should see equal amounts of daylight and darkness on the equinoxes. 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 the sky.
They measure the positions of astronomical objects using coordinates called right ascension and declination -- the equivalent of longitude and latitude. 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 2012