They don't always occur every four years.
It might be common knowledge that a leap year occurs every four years, but what are they? And why do we have them?
As the year 2016 marks a special calendar year, it is a year we will be experiencing a leap year. To fully understand how a leap occurs, one has to consider the following. First, the Gregorian calendar system — also referred to the Western calendar — is the most widely used calendar system around the world, with a normal year that consists of 365 days. In a leap year, however, an additional designated day is added to end of the second month of the year, February 29th.
The addition of an extra day occurs to help synchronize the calendar year with the solar year, the length of time it takes Earth to complete an orbit around the sun. The duration of our orbit is actually 365 and a quarter day, to be exactly precise, slightly shy of a full quarter day by 11 minutes.
Therefore, a leap year with the addition of an extra calendar day occurs every 4 years to make up for the quarter of a day each solar year attains over the 4-year duration.
But sometimes 8 years pass between leap years. The leap year is omitted three times every four centuries to exclude the 11-minute short fall from every quarter day addition to our calendar.
Leap years have been around for more than 2000 years and were first introduced by Julius Caesar back in the Roman Empire time period. Statistics suggest that there is a 1 in 1500 chance of being born on a leap year, which some might see as great fortune or just bad luck. People that are born on February 29th are referred to as “Leaplings” or “Leapers”. Many leaplings choose to celebrate their birthdays on either February 28th or March 1st, while some precisionists insist on keeping the 29th as their special celebration date.
In the video below, Neil deGrasse Tyson, the American astrophysicist, cosmologist and science communicator explains the logic behind leap years, and gives a brief history on the origin of calendars.