Feb 25, 2008 - 04:05:31 CST
On Friday, we'll do something we haven't done since 2004 - celebrate Feb. 29.It will be leap day, the extra 24-hour period inserted into the Gregorian calendar once every four years (with some exceptions) to keep the calendar in sync with the universe.
Interestingly, because of leap day this year, February 2008 is a rare "five Friday" month - it started on a Friday and ends on a Friday. This hasn't happened since 1980 and won't happen again until 2036.
Why do we need a leap year? Because it takes the Earth 365.2422 days to complete one orbit around the sun (that is, complete one tropical year).
That's fine for the universe, but people generally like their years to be measured in whole numbers, not fractions, so we have a calendar year made up of 365 whole days.
Things would be great if the Earth would just cooperate and orbit the sun according to our calendar and not according to its own cosmological whim.
But it doesn't and that means our calendar year is shorter than the tropical year by .2422 days.
In four years, our calendar would be about a day ahead of where the Earth should be in its orbit.
Over time, real spring (based on where our planet is located in its orbit) would be starting later than calendar spring.
Ancient astronomers recognized this problem of "calendar creep," so when the Julian calendar was introduced in 46 B.C. (named after Roman emperor Julius Caesar), they stuck an extra day into the calendar every four years to make up the difference.
Nice solution. But still not quite right.
The average calendar year was now 365.25 days. But this made our calendar year a few minutes longer than the tropical year (365.2422 days).
Over time, real spring would be starting earlier than calendar spring.
Which is exactly what happened in history.
By the late 1500s, the calendar year and the tropical year were off by about 10 days. Pope Gregory XIII introduced the Gregorian calendar, which deleted 10 days between Oct. 4 and Oct. 15, 1582, to bring the calendar back in line with the tropical year. To improve the accuracy of the calendar, the rule for leap day was modified as follows: Add a leap day in years divisible by 4 and only in century years divisible by 400. Thus, 1900 wasn't a leap year while 2000 was a leap year.
This means there are 97 leap days every 400 years under the Gregorian calendar instead of 100 leap days every 400 years under the Julian calendar.
And that, over time, takes care of the extra few minutes difference between the calendar and the Earth's orbit.
Almost.
There is still a difference of 27 seconds per year between the calendar year and the tropical year.
But it will take 3,300 years before calendar spring falls a day behind real spring.
Which is not too shabby, all things considered.
Want to learn more about the history of leap year and the calendar in general?
The following Web sites can provide you with everything you need to become an expert in mankind's efforts to measure time and stay in tune with the universe:
Why Leap Years?
www.timeanddate.com/date/leapyear.html
Curious Gregorian Calendar
www.infoplease.com/spot/gregorian1.html
Gregorian Calendar
scienceworld.wolfram. com/astronomy/GregorianCalendar.html
Calendars Through The Ages
webexhibits.org/calendars/
Calendopedia
calendopedia.com
Calendar Converter
www.fourmilab.ch/documents/calendar
Calendar History
www.polysyllabic.com/ ?q=calhistory
National Maritime Museum
www.nmm.ac.uk
U.S. Naval Observatory
aa.usno.navy.mil/faq/ docs/leap_years.html
LeapZine
www.leapzine.com
Leap Year Festival
leapyearcapital.com
When to Celebrate
www.straightdope.com/ classics/a960209.html
(Keith Darnay is the webmaster and designer for bismarcktribune.com. His Web site, featuring this column going back to 1995, is at www.darnay.com.iec.)
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