http://www.space.com/spacewatch/050318_equinox.html
More support for Vernal Equinox slowly getting earlier.
....
While it's true that we've traditionally celebrated the beginning of
spring on March 21, astronomers and calendar manufacturers alike now
say that the spring season starts one day earlier, March 20, in all
time zones in North America. Unheard of? Not if you look at the
statistics. In fact, did you know that during the 20th Century, March
21 was actually the exception rather than the rule?
The vernal equinox landed on March 21, only 36 out of 100 years. And
from 1981 to 2102, Americans will celebrate the first day of spring no
later than March 20.
In the years 2008 and 2012, those living in Alaska, Hawaii and the
Pacific, Mountain and Central time zones will see spring begin even
earlier: on March 19. And in 2016, it will start on March 19 for the
entire United States.
There are a few reasons why seasonal dates can vary from year to year.
1. A year is not an even number of days and neither are the
seasons. To try and achieve a value as close as possible to the exact
length of the year, our Gregorian Calendar was constructed to give a
close approximation to the tropical year which is the actual length of
time it takes for the Earth to complete one orbit around the Sun. It
eliminates leap days in century years not evenly divisible by 400,
such 1700, 1800, and 2100, and millennium years that are divisible by
4,000, such as 8000 and 12000.
************************************************
2. Another reason is that the Earth's elliptical orbit is changing
its orientation relative to the Sun (it skews), which causes the
Earth's axis to constantly point in a different direction, called
precession. Since the seasons are defined as beginning at strict
90-degree intervals, these positional changes affect the time Earth
reaches each 90-degree location in its orbit around the Sun.
************************************************
3. The pull of gravity from the other planets also affects the
location of the Earth in its orbit.
The current seasonal lengths for the Northern Hemisphere are:
Winter
88.994 days
Spring
92.758 days
Summer
93.651 days
Autumn
89.842 days
As you can see, the warm seasons, spring and summer, combined are
7.573 days longer than the colder seasons, fall and winter (good news
for warm weather admirers).
However, spring is currently being reduced by approximately one minute
per year and winter by about one-half minute per year. Summer is
gaining the minute lost from spring, and autumn is gaining the
half-minute lost from winter. Winter is the shortest astronomical
season, and with its seasonal duration continuing to decrease, it is
expected to attain its minimum value 88.71 days by about the year
3500.
Another complication revolving around the vernal equinox concerns the
length of day versus night. We have been taught that on the first days
of spring and autumn, the day and night are equal to exactly 12 hours
all over the world.
Yet, if you check the calculations made by the U.S. Naval Observatory
or the sunrise/sunset tables in any reputable almanac, you will find
that this is not so. In fact, on the days of the spring and fall
equinox the length of daylight is actually longer than darkness by
several minutes.
.....
One factor is that the moments of sunrise and sunset are considered
when the top of the sun, and not its center, is on the horizon. This
alone would make the time of sunrise and sunset a little more than 12
hours apart on these days. The Sun's apparent diameter is roughly
equal to half a degree (0.50 degrees).
But the main reason that this happens can be attributed to our
atmosphere; it acts like a lens and refracts (bends) its light above
the edge of the horizon. In their calculations of sunrise and sunset
times, the U.S. Naval Observatory routinely uses 34 minutes of arc for
the angle of refraction and 16 minutes of arc for the semi diameter of
the Sun's disc. In other words, the geometric center of the Sun is
actually 0.83 derees below a flat and unobstructed horizon at the
moment of sunrise.
Or, put in another way, when you watch the Sun either coming up above
the horizon at sunrise or going down below the horizon at sunset, you
are actually looking at an illusion the Sun is not really there, but
actually below the horizon.
As a result, we actually end up seeing the Sun for a few minutes
before its disk actually rises and for a few minutes after it has
actually set. Thus, thanks to atmospheric refraction, the length of
daylight on any given day is increased by approximately six or seven
minutes.
For Europe, spring will begin on March 21 in 2007. That, however, will
be the last time until 2102.
For places much farther to the east, such as Tokyo, Japan (9 hours
ahead of Greenwich), spring will fall on March 21 in two out of every
four years from 2006 through 2023 (2006, 2007, 2010, 2011, etc.), then
once every four years from 2027 through 2055. But then that's it until
2101.
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