Why leap years

Leap years happen every four years, except if the year is a multiple of But then years that are a multiple of are also leap years. This complex arrangement has a year history. The reason for an extra day in a leap year is that there are two fundamental time cycles both to do with rotation.

One is the time it takes for the Earth to rotate once on its axis — that's 24 hours, which is a day. The second is the time it takes the Earth to orbit the sun — which is The pope decided to drop 10 days from that year in October. So the day after 4 October was 15 October.

Too bad if your birthday fell on one of the days in between! To do this, leap years began more than two millennia ago, when the scientific advisers to Roman emperor Julius Caesar spotted that the years were not properly aligning with the seasons. For example, say that July is a warm, summer month where you live. If we never had leap years, all those missing hours would add up into days, weeks and even months. Eventually, in a few hundred years, July would actually take place in the cold winter months!

This is true of almost every other planet in our solar system. Mars, for example, has more leap years than regular years! A year on Mars is sols, or Martian days.

However, it takes So, you would sometimes have to add a sol to help the calendar catch up. In a 10 year period, four of the years would have sols and six of the years would be leap years with sols. For more information on leap years, visit this NASA page. Intercalary months, however, were not necessarily regular. Historians are still unclear as to how the early Romans kept track of their years, mostly because the Romans themselves may not have been entirely sure.

It appears that the early Roman calendar consisted of ten months plus an ill-defined winter period, the varying length of which caused the calendar to become unpegged from the solar year. Eventually, this uncertain stretch of time was replaced by the new months of January and February, but the situation remained complicated.

They employed a day intercalary month known as Mercedonius to account for the difference between their year and the solar year, inserting it not between months but within the month of February for reasons that may have been related to lunar cycles. To make matters even more confusing, the decision of when to hold Mercedonius often fell to the consuls, who used their ability to shorten or extend the year to their own political ends.

As a result, by the time of Julius Caesar , the Roman year and the solar year were thoroughly out of sync. The Mercedonius-when-we-feel-like-it system apparently irked Caesar, the general-turned-consul-turned-dictator of Rome who drastically altered the course of European history. In addition to conquering Gaul and transforming Rome from a republic into an empire, Caesar re-ordered the Roman calendar, giving us the blueprint off of which much of the world still operates to this day.

During his time in Egypt, Caesar became convinced of the superiority of the Egyptian solar calendar, which featured days and an occasional intercalary month which was inserted when astronomers observed the correct conditions in the stars. Caesar and the philosopher Sosigenes of Alexandria made one important modification: instead of relying on the stars, they would simply add a day to every fourth year. In keeping with the Roman tradition of messing with the length of February, that day would fall in the second month of the year—thus Leap Day was born.

By the 16th century, scholars had noticed that time was still slipping—Caesar's calculation that a year lasted This was a problem for the Catholic Church, as the date of Easter had drifted away from its traditional place, the first Sunday after the first full moon following the vernal equinox, by roughly ten days.

Pope Gregory XIII commissioned a modified calendar, one which kept Leap Day but accounted for the inaccuracy by eliminating it on centurial years not divisible by , , and were not leap years, but was.