What is a Leap Year

Leap Year was created to correct a discrepancy between the calendar year (or civil year) and the actual time is take for the Earth to travel around the sun.  The civil year is 365 days but the actual time is 365.2422 days which is also know as the tropical year.  With this discrepancy, the calendar year will eventually be out of sync with the seasons.  For example, after 100 years, the days will be 24 days out of sync.  After 1000 years, the difference would be 240 days.

Julius Caesar changed the calendar by adding an extra day every 4th year.  So, the average number of days in the civil year is now 365.25 days.  So, after 100 years, the out of sync is less than one day.  This calendar is know at the Julian calendar.  However, the seasons did get out of alignment.

In 1582, Pope Gregory XIII removed the days of October 5 through October 14, 1582 to fix the alignment.  He also removed three days every four centuries.  So, all centuries that are not divisible by 400, are not leap years.  This is called the Gregorian calendar.  The average number of days in the civil year is now 365.2425 days.  This will keep the civil year and tropical year in sync for many years because of leap year.  Additional information on Leap Year can be found on Wikipedia.

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Calculate a Leap Year

Background on Leap Year

Leap Year is calculated by the Astronomy Calculator first by determining if the year entered into the calculator is a Julian calendar year or a Gregorian calendar year.  If a Julian calendar year, if the year is divisible by four, then it is a leap year.  If a Gregorian calendar year, first the year is checked if it is a century year.  Next, if it is not divisible by 400, then it is a common year.  Lastly, the other years are checked if they are divisible by four.