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The solar year is just a little longer than a 365-day calendar year, and leap years correct the difference.
In the Gregorian Calendar, the civil calendar in use today, the rule is to add a day in every year evenly divisible by four. Years divisible by 100 are not leap years and no day is added.
This is to compensate for the fact that the year has an average of 365.2422 days.
The problem is not confined to years. The average time between two new moons is 29.5306 days, implying that there are 12.368 lunar months a year.
Every calendar is a compromise, and the least deviation mounts up over the centuries - a fact known to the Babylonians, who used leap months in the third millennium before the modern era.
In 238 BCE, the Egyptians added a day every four years, much like in the modern system.
The Julian Calendar dates back to Julius Caesar. It was introduced in 45 BCE, specifying an extra day every four years.
This still left 11 minutes a year to be accounted for, adding up to 10 days by 1582, when Pope Gregory XIII simply dropped these days and modernized the system of introducing additional days.
The Gregorian Calendar is now used throughout the world for non-religious purposes.
The new system resulted in 36,524.25 days a century - a number close to the time earth needs to go around the sun. The remaining deviation is corrected by the addition of a leap second from time to time.
In the Gregorian Calendar, the civil calendar in use today, the rule is to add a day in every year evenly divisible by four. Years divisible by 100 are not leap years and no day is added.
This is to compensate for the fact that the year has an average of 365.2422 days.
The problem is not confined to years. The average time between two new moons is 29.5306 days, implying that there are 12.368 lunar months a year.
Every calendar is a compromise, and the least deviation mounts up over the centuries - a fact known to the Babylonians, who used leap months in the third millennium before the modern era.
In 238 BCE, the Egyptians added a day every four years, much like in the modern system.
The Julian Calendar dates back to Julius Caesar. It was introduced in 45 BCE, specifying an extra day every four years.
This still left 11 minutes a year to be accounted for, adding up to 10 days by 1582, when Pope Gregory XIII simply dropped these days and modernized the system of introducing additional days.
The Gregorian Calendar is now used throughout the world for non-religious purposes.
The new system resulted in 36,524.25 days a century - a number close to the time earth needs to go around the sun. The remaining deviation is corrected by the addition of a leap second from time to time.
