Contrary to popular belief, the vernal equinox begins today, at exactly 1:26 p.m cst.
What's more, throughout the remainder of this century, spring in North America will often begin on March 19th.
"We haven't celebrated March 21 as the first day of spring (since 2002), but every year, it keeps coming up," says Joe Rao, a meteorologist who writes a column for Space.com.
Also known as the vernal equinox, spring begins in the northern hemisphere at the moment the sun is directly above the equator — in the southern hemisphere, that moment marks the beginning of autumn — and day and night are almost equal in length.
But why, exactly, has spring sprung earlier?
Actually, the planet's orbit is slightly less than 365-and-a-quarter days. So even with the leap-year system in place, over a long period of time, the arrival of spring would fall on an increasingly earlier position on our calendar.
In the late 16th century, Pope Gregory XIII and his astronomical consultants were displeased to see that the vernal equinox land on March 10th. They dealt with the problem by wiping 10 full days off the calendar to get spring caught up to the 21st.
They then tinkered with the leap-year system, adding a clause whereby leap years would be omitted from "century years" not divisible by 400.
In other words, century years — 1700, 1800 and 1900 — would not enjoy that extra leap day. The century year 2000 happens to be divisible by 400, and was therefore entitled to experience a leap-year day.
This allowed the vernal equinox to drop back to an earlier point on the calendar.
The effect will remain until the calendar reverts to its normal non-leap-year status in the year 2100. This calendrical game of tug-of-war between the primary and secondary leap systems produces a calendar that correlates nicely to celestial activity.
"It's a way of resetting the clock every 400 years," says Paul Delany, professor of Astronomy and physics at York University. "It fixed the calendar back in the 16th century, and it will keep us nicely accurate for the next several thousand years. It's still not perfect, but it's a really good correction. You can't tinker much more without driving people nuts."
Another reason that spring comes earlier: The Earth's route through space is changing. The planet doesn't travel around the sun in a perfect circle. Rather, its path is an irregular orbit affected by the gravitational pull of other celestial bodies.
Earth lingers in some areas longer than it does in others, making for uneven seasons. Winter, for example, is the shortest season, clocking in at a shade less than 89 days, while summer is about 93.5 days long. That means spring doesn't necessarily start on the 21st.
"The Earth's orbit is changing slightly as it goes around the sun, and the pull of gravity from other planets affects it," says Rao. "Our position in space is not exact. In a perfect world, we'd be going around the sun in perfect circles... in a perfect solar system, every year the vernal equinox would be occurring exactly six hours later on our clocks, but it's not."
These are strange days for time in general. At the end of 2005, a leap second was added to our global clocks to accommodate Earth's slowing rotation.
"Astronomists realize that the Earth is falling behind a little bit. As a result, they add an extra second to add Mother Earth catch up to the clock," says Rao. "Who says chivalry is dead?"
"The year 2000 threw the whole calendar system off," says Rao. "In years such as 1700, 1800, 1900 and 2100... there's no leap day in February. But there was one in 2000, quite contrary to previous years, which makes the 20th the first day of spring."
Now, about the first day of autumn...
― Laurel (Laurel), Monday, 20 March 2006 21:20 (twenty years ago)