What is a Leap Year?

Leap Year doesn’t happen every year – in fact, it’s only about once every four years – but sometimes February has an extra day: leap day.

What is a leap day? And why does it happen?

Usually, a year is 365 days long. This is about how long it takes for the Earth to travel once around the sun. However, the time it takes for the Earth to rotate once – one day – and the time it takes for the Earth to circle the sun once – one year – don’t actually have anything to do with each other. Because of this, a year works out to be just about 365 and one-quarter days long. That extra quarter of a day doesn’t seem like it would make much of a difference, but it can cause a lot of trouble! You see, the cycle of seasons matches up with the orbit of the earth around the sun.

Every four years the seasons would move by one day, which still doesn’t sound like a lot until you realize that after a few hundred years the days would be completely out of sync with the seasons. Halloween would be in spring and Christmas would be in the middle of summer! To stop this from happening, an extra day is added to every fourth year. This fourth year is called a leap year, and the extra day is called a leap day! The need for a leap year was known as far back as ancient Egypt, but the practice did not begin in Europe until the reign of Julius Caesar. In 46 BC the Julian calendar was introduced, with a year 365 days long and a leap year every fourth year. With the Julian calendar, the seasons would still shift, but the year only moved one day every 128 years instead of every four years. Still, by 1582 the calendar was ten days out of sync with the seasons, and Pope Gregory XIII introduced the Gregorian calendar, which is still used today. Every year that can be divided by 4 is a leap year – unless that year can be divided by 100, then it is not a leap year – unless it can also be divided by 400, and then it IS a leap year. The years 1700, 1800, and 1900 were not leap years, but the year 2000 was.

By using the Gregorian calendar, the year will stay in line with the seasons for almost 8,000 years! 8,000 years from now it’s possible that humans will be using an even more precise calendar system, but for now, a leap year every four years – well, mostly – keeps everything in place.