On Earth, one solar year is usually 365 days. This is the amount of time that it takes Earth to travel once around the Sun. When the Gregorian calendar was introduced in 1582, a year was defined as 365 days to make the calendar year coincide with the solar year.
Long, long ago, though, smart astronomers figured out that it actually takes Earth a little bit longer than 365 days to travel around the Sun. Measuring precisely, they determined that one trip around the Sun actually takes 365.242 days (or 365 days, 5 hours, 48 minutes, and 46 seconds, to be exact).
That means a year should actually be 365 and almost one-fourth of days long. That would be very difficult to manage on a calendar, so the ancient Egyptians came up with the idea of combining four one-fourth days into one day and adding that one day to the calendar every four years to keep the calendar year on track with the solar year.
The ancient Romans under Julius Caesar decided to do the same as the Egyptians when they developed our modern calendar. In fact, they were the first to designate February 29 as the extra day.
This extra day is called Leap Day. The year in which it occurs is called a Leap Year since it has 366 days and helps the calendar leap one day forward to stay on track with Earth’s journey around the Sun.
Is all this really necessary? An extra day every four years may not seem like a big deal now, but it adds up over time. For example, after a century, the difference between the calendar and solar years would be 25 days. That would mean that seasons would start almost a month later than they’re supposed to. Leap years help correct this problem and keep the calendar on track.
Unfortunately, the math turned out to be just a bit more complicated than merely adding a day every four years. Astronomers figured out that the solar year is really 11 minutes and 14 seconds less than 365 and one-quarter days. That means adding a day every four years would lead to three more days than necessary being added every 400 years.
To solve this problem, the ancient Romans decided to skip leap years three times every 400 years. The rule they put in place is this: only one out of every four “century” years is considered a leap year. For example, 1700, 1800, and 1900 were not leaped years, but 2000 was.
The easy way to know whether a century year is a leap year is to determine whether it’s evenly divisible by 400. So, 2100, 2200, and 2300 will not be leap years, but 2400 will be leap years.
People born on February 29 are often called “leaplings” or “leapers.” In non-leap years — called common years — they usually celebrate their birthdays on either February 28 or March 1. Some “leapers” take advantage of their special birthday when they get older. For example, an 84-year-old “leapling” might claim he’s only 21, since he’s only celebrated 21 birthdays (every four years)!