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Why doesn't the equinox occur on March 21 anymore?

It doesn't seem right, does it? I mean, when many of us were growing up, the first day of spring, also known as the vernal equinox (in the Northern Hemisphere) was on March 21, not March 20. Now, all of a sudden spring is arriving on March 20. How did that happen?

During the 20th century, at the longitude of Greenwich, England, the vernal equinox landed on March 21 no fewer than 58 times (39 times between 1901 and 1951). Yet, in Europe and Asia only the years 2003 and 2007 will see the equinox arrive on March 21.

For North America throughout the entire 21st century, the equinox will arrive no later than March 20. And in 2004, for those in the Mountain and Pacific Time zones, spring will officially arrive on March 19!

There are several factors to account for the date shift, including variations in our Gregorian calendar, the precession or "wobble" of the Earth's axis and the pull of gravity from the other planets, which (ever so slightly) affects the location of the Earth in its orbit.

Interestingly, in the Northern Hemisphere, 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 a minute lost from winter.

Winter is the shortest astronomical season. With its seasonal duration continuing to decrease, it is expected to attain its minimum value of 88.71 days by about the year 3500.

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