PLANET EARTH TO CATCH UP WITH THE RED PLANET MARS
Document Actions
THE VERNAL EQUINOX AND THE DATE OF EASTER
(March 10, 2010) – Astronomers at the Pisgah Astronomical Research Institute point out that at 1:32 p.m. EDT on Saturday, March 20 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. 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 first day of spring has always held great significance for calendar makers. In fact, until recent times, the year actually began, not on January 1, but on March 25, which was an approximation to the first day of spring. (Ask your family genealogist what this does in determining the date of birth of great-great-grandpa or grandma.) 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 2010 the vernal equinox is on March 20 and the first full moon after that is on Monday, March 29. Thus, Easter this year is Sunday, April 4.
While this rule works in most years, things are not always so simple. 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. (In 2011 Easter Sunday occurs on April 24.) 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.
About PARI
PARI is a not-for-profit foundation established in 1998. Located in the Pisgah Forest southwest of Asheville, NC, PARI offers educational programs at all levels, from K-12 through post-graduate research. The institute is affiliated with the 16-campus University of North Carolina system through PARSEC, a UNC Center hosted at PARI, and is a member of the NC Grassroots Museum Collaborative. For more information about PARI and its programs, visit www.pari.edu.