Jimmy Westlake: Easter and Equinoxes
March 19, 2012
Ah, springtime. The early signs are all here: the mud, the blackbirds, the return of the Big Dipper to our early evening sky, more mud and the gradual lengthening of our daylight hours.
This year, the season of spring officially arrived in the northern hemisphere Monday at 11:14 p.m. in Colorado. That's the instant that the sun crosses the celestial equator on its way north, what we call the vernal equinox. With each passing day, the sun rises a little bit earlier and sets a little bit later, increasing the number of daylight hours that we enjoy. Our daylight hours will continue to increase until the summer solstice on June 20 at 5:08 p.m., the longest day of the year and the first official day of summer in the northern hemisphere.
Thanks to the 23.5 degree tilt of the Earth on its axis, the sun spends half of the year shining down on the northern hemisphere and the other half of the year shining down on the southern hemisphere.
Separating these extremes are two days called the equinoxes, six months apart, when the sun shines straight down on the Earth's equator. Equinox is a word that means "equal night" and is used to describe these two special days when every location on Earth experiences exactly 12 hours of sunlight and 12 hours of darkness. These also are the only two days of the year when the sun rises exactly in the east and sets exactly in the west.
The other equinox in 2009, the autumnal equinox, occurs on Sept. 22 at 8:49 a.m. when the sun again crosses the equator but this time heading south.
Equinoxes have been revered by many cultures throughout the course of history. The ancient Druids went to great lengths to align the gigantic stones at Stonehenge to point out the position of the rising sun on the dates of the equinoxes and solstices. The Great Sphinx of Egypt also faces the direction of the rising sun on the date of the equinox. A coincidence? Probably not.
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For Christians, the celebration of Easter is tied directly to the vernal equinox. Easter Sunday is the Sunday that falls immediately after the first full moon of spring. Easter in 2008 fell on March 23 and was the earliest Easter that anyone alive today will ever experience. This year, the first full moon after the vernal equinox happens on April 6, making Sunday, April 8 the date of Easter in 2012.
Also, lest we appear narrow-minded, remember that the seasons are reversed in the southern hemisphere. As we celebrate the vernal equinox and the arrival of spring north of the equator, our friends down under are celebrating their autumnal equinox and the arrival of fall. Happy Equinox!
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. His "Celestial News" column appears weekly in the Steamboat Today newspaper and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website at http://www.jwestlake.com.