River Wissey Lovell Fuller

Leap years

February 2004

An explanation of ehy every four years we have 29 days in February!


Since the earliest civilisations man has taken a keen interest in astronomy and the seasons. The ancient Greeks and Egyptians, even the ancient Brits may not have fully understood the Solar system, but they had a very good understanding of the cyclic nature of the behaviour of the Sun and the Moon. Before recorded history it was known that the Sun took 365 days to complete its annual cycle, and it was more than 2000 years ago that the Romans realised that, more accurately, it took 365.25 days. To adjust for this extra quarter of a day the Emperor Julius Caesar, in 45BC, introduced the concept of an additional day in every fourth year and produced a calendar much as we know it today with regular leap years.

Subsequently, more accurate measurements revealed that the annual cycle was a little less than 365.25 days and it was realised that, continuing with the Julian calendar would eventually produce a drift in the seasons, ultimately leading to Christmas in summer time. Pope Gregory XIII proposed that the error should be allowed for by foregoing leap years at the end of each century, except for those centuries divisible by 400. This established the Gregorian calendar which was introduced in Europe in 1582. As a consequence, since 2000 is divisible by 400, we have a leap year this year whereas in 1900 there was no leap year.

The British, in their characteristically perverse attitude towards Europe, initially refused to accept the Gregorian calendar, and it was not until 1752 that they finally succumbed. So, in September 1752, by decree, the date was moved forward 11 days. This generated a range of problems, there were strong protests; some argued that they had actually been robbed of some days of their life. If there were any who were paid monthly they would probably have kept very quiet whilst their employers probably joined the protesters.

The Gregorian calendar produces an average year of 365.2425 days. More accurate measurements of the annual cycle show that the average year is slightly less so that an error of 0.0003 days arises each year and over every 3000 years, assuming that things were to remain constant, a slippage of roughly one day would occur.

Ron Watts

Editor's note. This item was first published in the Village Pump in January 2000, before I assumed the role of editor. I think, however, it is well worth a repeat run to explain the reasons behind every fourth year being deemed a leap year.

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