We observe the modern leap year because the Earth orbits the sun every 365.2421896698 days and of course that is not an easy number to account for in how we calculate our calendar of days. So the leap day, (today... Wednesday, February 29) is a day that is added onto the month of February every four years to keep the calendar in sync with the celestial cycles and our seasons, otherwise they would drift out of alignment and we'd be celebrating Christmas in January or March (eventually).
The extra quarter of a day that the leap year added was slightly longer than the 0.242 of a day in the actual solar year.
This seemingly small difference made the solar year about 11 minutes too long, resulting in an entire day of discrepancy every 128 years.
Because of this glitch, the Julian calendar had drifted ten days by the late 16th century.
"Finally it became so ridiculous that Pope Gregory XIII was convinced by his astronomers that basically all the Christian holidays were being celebrated on the wrong days," Duncan said.
The pope introduced his Gregorian calendar in 1582, which determined that only one out of every four "century years" would observe a leap year. Thus while the years 2000 and 2400 are leap years, 2100, 2200, and 2300 are not.
The Gregorian calendar was gradually, and sometimes grudgingly, adopted by much of the world and remains in common use.
Read more at National Geographic.
The human activity of keeping track of time has always fascinated me.
Here is the official US Time website and the world clock.
Interactive map of our solar system
Enjoy Leap Day!
How will YOU use your extra day?