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Vernal Equinox

March 19, 2016

Spring arrives in the northern hemisphere at 11:30 p.m. Central Daylight Time tonight, 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.

“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 equinox. 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 — the projection of Earth’s equator on the sky — on the vernal equinox is designated as zero hours. It’s the equivalent of zero degrees longitude — the 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 cycle through the stars.

Tomorrow: the Moon and a couple of bright companions.

Script by Damond Benningfield

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