Vernal Equinox 2013 a day early?

Why the Vernal Equinox of 2013 (N. Hemisphere) falls on Wednesday, March 20 at 11:02 UT, or 7:02 a.m. EDT/4:02 a.m. PDT.

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. Earth's year — the length of time it takes to circle the sun once — lasts about 365.25 days. To try to achieve a value as close as possible to the exact length of the year, the 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 as 1700, 1800, and 2100, and millennium years that are divisible by 4000, 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 — a process 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:88.99 days
• Spring:  92.76 days
• Summer:  93.65 days
• Autumn:  89.84 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.

The situation for different parts of the world, being in different time zones and hemispheres, varies.

For places much farther to the east, such as Tokyo, Japan (nine hours ahead of Greenwich Mean Time), spring will fall on March 21 in two out of every four years from 2014 through 2023 (2014, 2015, 2018, 2019, etc.), then once every four years from 2027 through 2055. But then that's it until 2101.