Contrary to popular belief, there is not a leap year every four years. While most years can be checked to see if they are a leap year by dividing by four, this is not true for century years - for a century year to be a leap year, it must be divisible by 400. Therefore, 2000 was a leap year, but 1900 was not, and 2100 will not be either.
History of the calendar
The first incarnation of our calendar was devised by the first Roman king, Romulus, in 735 BC. Unlike previous solar calendars, Romulus's calendar had only ten months rather than the usual twelve, and only 304 days. This was because the Roman number system, like our own, was based on the number ten. Romulus named the ten months as follows:
- Martis, after the Roman god of war Mars
- Aprilis, from the Latin aperite 'to open', as flowers open during this month
- Maius, after Maia, mother of Mercury
- Junius, after the goddess Juno, queen of the gods
From the fifth month onwards, the names were simply derived from the Latin numbers.
This calendar was far to inaccurate when compared to the true solar year, so in around 700 BC two extra months were added to the calendar by King Numa, and the total number of days brought to 355. These new months were:
- Januarius, after the two-faced god Janus
- Februarius, from Februa, a Roman feast
This new calendar still did not keep in step with the sun, causing problems for farmers. There were several attempts to put this right by adding extra months every few years, but they were largely unsuccessful and resulted in confusion.
In 46 BC, Julius Caesar brought the calendars back into line by adding two extra months of a total of 67 extra days to the year. He then brought in the best philosophers of the time to correct the calendar permanently. They calculated that the year should last 3651/4 days, and was achieved using a cycle of three years with 365 days, followed by a 'leap year' of 366 days. To make the year up to 365 days, Caesar made consecutive months alternate between 31 and 30 days, so that March had 31, April 30, May 31 etc. with the exception of February which had only 29 days, and 30 days in leap years. Finally, Caesar moved the start of the year to January, so that it was nearer the winter solstice.
The Roman Senate renamed Quintilis as Julius in his honour.
The Roman calendar was now the most accurate in the world, but after Caesar's death in 44 BC, the priests in charge of the calendar mistakenly began to add leap years every three years. This was corrected by Emperor Augustus in 8 BC.
The Romans also honoured Augustus by renaming Sextilis as Augustus. However, Augustus was dissatisfied with his month having fewer days than Julius Caesar's, so August was changed to have 31 rather than 30 days. This was accounted for by taking a day from February, making it 28 days long, or 29 in a leap year. To avoid having three months in a row with 31 days, the remaining months of the year were switched between 31 and 30 days (making September 30 days, October 31 days, etc.). This is how the months became as we know them today.
This new version of the Julian calendar remained unchanged for centuries, but still was not perfect. By the sixteenth century, it had once again drifted away from the solar year. In the 1580s, Pope Gregory XIII reformed the calendar so that century years were only leap years when divisible by 400. Therefore, 1700, 1800, 1900, 2100 etc. would not be leap years.
The Gregorian calendar was adopted in Great Britain in 1752, and is now used almost universally. It is so accurate that the difference between it and the solar year is about 26 seconds (although difference this is slowly increasing as the solar year is becoming shorter). Leap seconds are occasionally taken off the world's atomic clocks to allow for this difference.