Sep 22, 2009

The Phantom Bridging of the Autumnal Equinox

Today, is one of only two days in the entire year when the sun rises due east and sets due west. Today marked our calendars as the 'Autumnal Equinox', yet strangely enough, it is not truly a day when day and night are of equal time or value, as many of us have been led to believe.

According to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C., the true days of day-night equality always fall after the autumnal equinox and before the vernal, or spring, equinox. The difference is a matter of geometry, atmosphere, and language.

Day and night would each be exactly 12 hours long on a spring or fall equinox only if the sun were a single point of light and Earth had no atmosphere.But the sun, as seen from Earth, is nearly as large as a little fingertip held at arm's length—a size known to astronomers as half a degree wide.

Sunrise is defined as the moment the top edge of the sun appears to peek over the horizon., while sunset is when the very last bit of the sun appears to dip below the horizon.

The vernal and autumnal equinoxes, meanwhile, occur when the center of the sun's disk crosses what's known as the celestial equator, an imaginary line that projects outward from Earth's Equator. And because Earth's atmosphere bends sunlight when it's close to the horizon, it makes the sun appear to rise a few minutes earlier than it actually does.

"Those factors all combine to make the day of the equinox not the day when we have 12 hours [each] of light and darkness," Chester said. Never the less,the equinoxes are the only days of the year when a person standing on the Equator can see the sun passing directly overhead.

On the Northern Hemisphere's autumnal equinox day, a person at the North Pole would see the sun skimming across the horizon, signaling the start of six months of darkness. And on the same day, a person at the South Pole would also see the sun skim the horizon, beginning six months of uninterrupted daylight.

And strangely enough, the equinoxes aren't even midway between the solstices, which are the days of greatest and least light of the year, like many of us believe.

The Earth's orbit is not a true circle, it is actually slightly elliptical orbit.The elongated orbit means that Earth goes faster around the sun in January, when it is closest to the star, than it does when it is farthest away from the sun in July.In effect, we arrive at the September equinox a day late, because we were going a little bit slower in July, and then we arrive at the March equinox a day earlier.

Another equinox oddity: A rule of the calendar keeps spring almost always arriving on March 20 or 21—but sometimes on the 19th, and in the fall, Autumn can arrive on September 21 or 22, or even 23rd; depending on astronomical conditions, the calendar year and whether you live north or south of the equator.

Nowadays, according to the U.S. Naval Observatory, equinoxes migrate through a period that occurs about six hours later from calendar year to calendar year, due to the leap year cycle.

The system resets every leap year, slipping a little bit backward until a non-leap century year nudges the equinoxes forward in time once again.

So with all of the magic and mysticism and symbolic representations in nature (and my dear friend, Junie Moon has done a lovely job of discussing those already) and in spite of major changes to our calendars over the years, we're still forced to bridge the different affects of space, time, geometry, atmosphere and language!

Time to go wildcrafting and gather up the magical gifts of nature's bounty!

shown above:

Oregon's amazing 'Phantom Bridge'...a naturally arching stone structure which divides a lovely forested canyon and one which my husband, son, and my visiting brother crossed just as one season crosses over into another. The mist and the fog of this day might have hid the magesty of the moment...and you have to look closely to see the true depth of the canyon...but walking along the bridge's edge made for enough excitement...all on its own! A wonderful excuse to 'keep looking up and forward' instead of down and back ;)


Anonymous said...

Thanks for such an informative post! Great read.

Dena said...

Recently, I watched a documentary on the Colorado River and in it they featured a national park called Arches National Park in Utah. The park is filled with naturally created arches. It was awesome to see. Someday I hope to see it in person.

Janet said...

All I can see is the bridge and it freaks me out, lovely photos but heights are not for me. Really it is facinating.