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Spring is here! March 21, 2007

Posted by grhomeboy in Science.

The vernal equinox and the traditional start of spring arrives at 8:07 p.m. this evening, explain astronomers at the Pisgah Astronomical Research Institute.

The Sun, in its annual path around the sky will cross the celestial equator on its way north for the spring. “Of course, we realize it is not really the Sun that is moving but rather the Earth as it orbits the Sun each year. But from our vantage point on the Earth it appears to us that the Sun is moving along a path through the zodiac constellations,” said astronomer Bob Hayward.

This path, called the ecliptic, crosses the celestial equator at two points on opposite sides of the sky. The point at which the Sun crosses heading north for the spring is called the spring or vernal equinox.

The vernal equinox historically has been used to determine the date of Easter. In simple terms Easter occurs on the first Sunday following the first full moon that occurs on or after the day of the vernal equinox. In 2007 the vernal equinox is on March 20 and the first full moon after that is Monday, April 2 at 1:15 p.m. EDT. Thus, Easter this year is Sunday, April 8.

While this rule works in most years, things are not always so simple, Hayward said. The western Christian churches don’t actually use the precise astronomical dates of the vernal equinox and full moon. Rather they use what is called the ecclesiastical moon, determined from tables, and fix the vernal equinox at March 21. Under these rules Easter can occur as early as March 22 or as late as April 25. Not only that, the eastern churches have a variety of rules some going back to the ancient Julian calendar rather than the Gregorian calendar developed in 1582 under the direction of Pope Gregory XIII. Thus, it is possible that Easter could occasionally occur on other than the date determined by the astronomical vernal equinox and full moon.

For more information on this topic and future dates of Easter visit the Astronomical Applications Department of the US Naval Observatory at http://aa.usno.navy.mil/data/docs/easter.html.

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