The Earth orbits the sun every 365.2422 days (0.2422 days is equal to 5 hours, 48 minutes and 46 seconds, roughly 1/4 of a day). Every 365 1/4 days (not exactly 365 days), the Earth returns to the same exact spot in its orbit. That is why we have a leap year roughly every 4 years, to synchronize the seasons with our calendar; if we had a 365-day calendar every year, the seasons would drift around the calendar.

Why Do We Have Leap Years?
A year is defined as the time it takes for the Earth to orbit around the sun once. It takes the Earth about 365 1/4 days to make one entire orbit around the sun (a day is one rotation around the Earth's axis). By adding one extra day about every four years, the Earth is in the same point of its orbit at the same time of the calendar year each year.

Before a leap-year calendar was used, the seasons drifted around the calendar (the drift is about 1/4 day or 6 hours each year). For example, over three hundred years, July would go from summer to spring. Our current calendar is called the Gregorian calendar; this calendar was devised by Aloysius Lilius (an Italian physician) and named for Pope Gregory who decreed in 1582 that it be used in Catholic areas. This calendar wasn't adopted in Britain and the American Colonies until 1752.

What Years are Leap Years?
Usually, there is a leap year every four years -- but once in a long while, a leap year has to be skipped (this is because the Earth's orbit is 365.242 days, a bit less than 365 1/4).

To determine if the year is a leap year:

• Non-century years (a century year is a year ending in 00) are leap years if they are divisible by four. For example, 2004, 2008 and 2012 are leap years; 2009, 2010 and 2011 are not.
• Century years are leap years if they are divisible by 400. For example, 1600, 2000, and 2400 are leap years; 1700, 1800, 1900, 2100, 2200, and 2300 are not.

Leap Day Birthdays
People born on leap day (February 29) can celebrate their birthday on March 1 (or February 28) on non-leap years. A person born on a leap day is sometimes called a leapling.

What percentage of the population is born on leap day? Assuming an equal distribution of birthdays throughout the year, the fraction of people born on leap day is the fraction of people born on one day out of four years, or 1/[(365 x 4 ) + 1] = 1/1,461 = 0.068% (less than one-tenth of a percent of the population).

Calendar Rhyme (Two versions)

Thirty days hath September, April, June, and November, All the rest have thirty-one, Excepting February alone, And it has twenty-eight days time, But in leap years, February has twenty-nine.

Thirty days hath September, April, June, and November, All the rest have thirty-one, Excepting February alone, And that has twenty-eight days clear, And twenty-nine in each leap-year.

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