Monday, March 18, 2013

Vernal Equinox—It's Officially Spring

Equinox translates literally to "equal night."

On Wednesday, March 20, 2013, at precisely 7:02AM Eastern Daylight Time U.S., the sun crosses directly over the Earth's equator.  That moment is known as the vernal equinox in the Northern Hemisphere announcing the arrival of spring and the autumnal equinox in the Southern Hemisphere announcing the arrival of fall.  A second equinox in 2013, the fall equinox, will occur on Sunday, September 22, at precisely 10:49AM Eastern Daylight Time U.S.

The fact that the Earth has distinctive seasons is due to the 23.4 degree tilt of the Earth's axis.  The Earth receives more sunlight (longer daylight hours) in the summer and less sunlight (fewer daylight hours) in the winter.  The tilt of the axis makes the seasons opposite in the Northern and Southern Hemisphere.  At the north pole summer gives six months of daylight while at the same time the south pole is experiencing six months of darkness.  The closer you are to the equator, the daily hours of daylight and darkness become more equal.

The spring and fall equinoxes are the only two times during the year when the sun rises due east and sets due west.  Modern astronomy aside, people have recognized the astronomical connection to the season changes for thousands of years.  The ancients of various civilizations all over the world built structures that illustrate this—temples dedicated to their various gods that modern man recognize as observatories.  Not only the spring and fall equinox days, but also the summer and winter solstice days.

I think it's also interesting to note a connection between the spring equinox and Groundhog Day (another holiday derived from the practices and celebrations of the ancients).  If the groundhog sees his shadow on February 2, we have six more weeks of winter here in the Northern Hemisphere.  And by coincidence that six weeks takes us to within a few days of the spring equinox.

The spring and fall equinoxes are the only two times during the year when the sun rises due east and sets due west.  They are also the only days of the year when a person standing on the Equator can see the sun passing directly overhead.

Another equinox oddity: A rule of the calendar keeps spring arriving on March 20 or 21—but sometimes on the 19th.  In 1582 Pope Gregory XIII established the Gregorian calendar, which most of the world now observes, to account for an equinox inconvenience.

If he hadn't established the new calendar, every 128 years the equinox would have come a full calendar day earlier—eventually putting Easter in chilly midwinter.  Before the pope's intervention, the Romans and much of the European world marked time on the Julian calendar.

Instituted by Julius Caesar, the old calendar counted exactly 365.25 days per year, averaged over a four-year cycle. Every four years a leap day helped keep things on track.

It turns out, however, that there are 365.24219 days in an astronomical "tropical" year—defined as the time it takes the sun, as seen from Earth, to make one complete circuit of the sky.  Using the Julian calendar, the spring and fall equinoxes and the seasons were arriving 11 minutes earlier each year. By 1500 the vernal equinox had fallen back to March 11.

To fix the problem, the pope decreed that most century years (such as 1700, 1800, and 1900) would not be leap years. But century years divisible by 400, like 2000, would be leap years.

Are we confused yet?  :)

With an average duration of 365.2425 days, Gregorian years are now only 27 seconds longer than the length of the tropical year—an error which will allow the gain of one day over a period of about 3,200 years.  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 leap nudges the equinoxes forward in time once again.

And now we are officially confused?  :)

I keep track of this by the tried and true method of checking my calendar and looking for the date where the calendar company has printed Spring Begins or Autumn Begins.  :) 

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