Did You Miss the Spring Equinox?

Spring officially began on Friday, March 20, 2009. Many recognize this day as the spring equinox, or vernal equinox.  If you happen to be standing on the Equator, this means the exact center of the sun will pass directly over your head, which only happens twice a year – once on this day and once in September on the Autumn Equinox, which is an interesting bit of trivia if you are into sun-watching.

Here’s some more.  It is only the Spring Equinox if you are in the northern hemisphere.  On the other side of the world it is the Autumn Equinox. Due to the tilt of the Earth’s axis, only one side can get warmer at a time.  Equinox means ‘equal night’, and at these two times the sun rises exactly due east, sets exactly due west, and the day is almost the same length as the night.  Unless, of course, you happen to be at the North Pole, in which case the sun will indeed rise in the east – and stay there for six more months.  Or if you are on the South Pole, which means six months of darkness has just started for you.

Wherever you live, the equinox signals a change in season.  One is always on March 21, 22, or 23.  The other is always September 21, 22, or 23.  How is this possible?  Well, Julius Ceasar set up our calendar to make sure it was possible.  And hundreds of years later, a pope made a little correction to make doubly sure.

Ceasar set up the Julian calendar based on the fact that there are 365.25 days in each year.  Every four years he added one day to the year – leap year – to keep the calendar correct.  But by 1582 Pope Gregory XIII realized that the world was still backtracking.  He figured out that each year is actually 365.24 days, which means that every 128 years the equinox moved back a full day.  That is a problem; sooner or later Easter would be in mid-winter.  So, to correct Ceasar’s oversight the Pope came up with a solution.  Most century years, like 1700, 1800, 1900…) would not ordinarily be leap years.  But any century year that is divisible by 400 – like the year 2000 – are also leap years.  That happens just enough times to keep the equinoxes on the right days.  This is called the Gregorian calendar, and most of the world now uses it instead of the Julian.  All hail the Pope, and thank God he had enough time to spare so as to spend on this problem!