Thirty days hath September…but why on earth does February have 29 every four years?
This year is a leap year, making the length of the 2012 calendar 366 days, instead of the normal 365. Every four years an extra day is added to the month of February, but have you ever wondered why this happens? In celebration of 2012 being a Leap Year, we invite you to hop back in time with us for a brief history of our modern day calendar and discover the FUN science of telling time!
The calendar is supposed to match the solar year, in other words, the length of time it takes for the Earth to orbit the Sun once. But things aren’t quite that simple. It actually takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to complete its orbit (about 365 1/4 days). Those extra hours gradually add up so that after four years the calendar is out of step by about one day. Adding a day every four years allows for the calendar to match up with the solar year again.
However, because the solar year isn’t exactly 365 days, even adding a leap day every four years means that the calendar is still out of step by 11 minutes and 14 seconds each year. Over the course of 400 years this would add up to three extra days. In order to solve this problem it was decided to leave out the leap year three times every 400 years. So the new rule was, a century year (1600, 1700, 1800, etc..) would only be a leap year if it was evenly divisible by 400. This means that the year 2000 was a leap year, but 2100 will not be.
Phew! So, who figured all this out?
The Egyptians were the first people to think of adding a leap day to the calendar. Later, the Romans copied the idea and in 1582 Pope Gregory XIII reformed the Julian Calendar (introduced by Julius Caesar in 45 BCE). By Pope Gregory’s time, the calendar had drifted 14 days off track. He neatly solved this by wiping ten days off the calendar, telling everyone that the day after October 4th was going to be October 15th. Bad luck for people with birthdays during that time, including famous scientist Sir Isaac Newton. His birthday according to the Julian calendar, used during the year of his birth, was Christmas Day, 1642. However, once the Gregorian Calendar went into effect,Newton’s birthday changed to January 4th, 1643.
The early roman calendar originally began the year in the month of March. It consisted of ten months, each lasting about 30 days, ending with December. This ten month calendar completely left out the winter months. It is thought that the two extra months, January & February, were added sometime around 715-673 BCE. This would have made February the last month of the year, which might explain why a leap day was added to that month. Later, it was decided to start the year with January.
Other nations have different leap year rules and different methods of keeping their calendar in line with the solar year. Countries may have a day, or in some cases a month, that gets added every few years in order to balance the time. The Chinese, for example, add a month about every three years, whereas in the Islamic Hijri Calendar a day is added 11 times during a 30-year cycle.
It can be pretty confusing keeping track of our modern day calendar, but just remember…
Thirty days hath September,
April, June and November;
All the rest have thirty-one
Save February, she’s alone
Hath eight days and a score
Til leap year gives her one day more!
Did You Know?
There is a tradition that women are allowed to propose marriage to men on leap days! One day in the 5th century, St. Bridget complained to St. Patrick about the unfairness of the system which only allowed men to propose, so he decided to let women do the asking once every four years! Today, we refer to this special day as Sadie Hawkins Day!
Learn more about Leap Years & Leap Day with these Resources!