Happy Spring Equinox 2012

The Earth's doesn't tilt towards or away from the Sun during the equinox (ill. not to scale).

March Equinox: March 20, 2012, 05:14 UTC

There are two equinoxes every year – in March and September – when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the equator, so the equinox in March is also known as the "spring equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "autumnal (fall) equinox".

The March equinox 2012:
March 20, at 05:14 UTC (or 5:14am).

Why is it called equinox?

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night". 

However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight

The March equinox occurs the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens either on March 19, 20 or 21 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis doesn't tilt neither away from nor towards the Sun, like the illustration shows. 

The length of day and night may not be equal on the vernal equinox, but that doesn't make the first day of spring any less special.

 

The fall and spring equinoxes, for starters, are the only two times during the year when the sun rises due east and sets due west.

The equinoxes are also 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 vernal equinox day, a person at the North Pole would see the sun skimming across the horizon, beginning six months of uninterrupted daylight.

A person at the South Pole would also see the sun skim the horizon, but it would signal the start of six months of darkness.

Equinox Calendar Oddity

A spring equinox oddity: A rule of the calendar keeps it so the first day of spring is almost always March 20 or 21—but sometimes on the 19th.

Pope Gregory XIII Responsible for Gregorian Calendar

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 spring equinox would have come a full calendar day earlier—eventually putting Easter in chilly midwinter.  This is because there is not an exact number of days in a year.

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 fall and spring 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.

Under the Gregorian calendar, the year is 365.2425 days long. Which gets close enough to the true fraction tht the seasons don't drift.

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.

Nowadays, according to the U.S. Naval Observatory's Chester, 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.

Mayan Sacred Tzolkin

Perhaps 2012 would be a good year to simply adopt the Mayan Calendar which calibrates on natural time...?  They say it is the end of the old and the beginning of the new....