Friday, March 20, 2009

Why Does Spring Start on the 20th?

We celebrated the arrival of spring this morning at 7:44, prompting a listener to ask, "years ago spring came on March 21st!!! Why the change to March 20th???"

Mark Breen writes: 

"Calendars are funny devices...they try to make time even and steady, but the Earth's motions and orbits are not. It actually starts with leap years.  The orbit of the Earth around the Sun takes 365 and a quarter days.  We "save up" those quarters, so that every four years, we add that extra day to a year.  But the extact time of the Earth's orbit is 365.256366 days, a bit more than a quarter day.  Over a century, the extra 0.006366 days is more than a half day, so every year that ends in "00" we skip a leap year.  Of course, it isn't quite a full day, so after 4 centuries we've lost 1.5 days, so if the "00" year can be divided by 400, we add a day back in.  This keeps going, and takes into account other minor adjustments in the Earth's orbit and rotation.   Getting back to your question, because we added an "extra" day by having a leap year in 2000 (skip a leap year if it ends in "00", but NOT if it is divisbile by 400), this shifted the calendar back, so that all of the season dates are about a day earlier.  They will gradually creep back over the next 400 years.   I hope that makes sense.  It is an interesting question, and I am glad you noticed the difference."

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