Concerning Leap Years

Who Introduced the Leap Year?

Julius Ceasar, believe it or not. The Roman calendar had got seriously out of date with the solar calendar, even though Roman officials were supposed to introduce an extra month every so often. So Caesar consulted his top astronomers who worked out that the solar year—the time it takes for the Earth to do exactly one rotation around the sun—was 365.25 days. So, by maintaining a calendar with 365 days, and adding one extra day every four years, everything should stay synchronised. Rome moved on to the Julian Calendar with the first leap year occurring in AD 8.

What Went Wrong?

The Julian Calendar was pretty accurate but not accurate enough. A year is in fact slight less than 365.25 days (nearer 364.2422), and over the centuries the error increased by about 11 minutes per year, about 8 days every 1,000 years. In the 16th Century the Catholic Church, with an eye on the long term, calculated that within 12,000 years Christmas would fall in Autumn and Easter in January. Pope Gregory XIII consulted with his officials and, in 1582, introduced the Gregorian Calendar. This addressed the error by stating that every 100th year should not be a leap year, although divisible by 4. Thus a century has 75 ordinary years of 365 days, 24 leap years of 366 days, and one ‘non-leap leap year’ of 365 days. This gave a mean year length of 364.24 years—which they realised was just slightly too short. So to compensate for this, it was decided that every 400th year (such as AD 2000) would have its leap day reinstated. At a mean length of 364.2425 it was now too long, but with the discrepancy amounting to just one day every 3,322 years, it was decided to leave it to future generations to sort out.

What Happened Next?

Pope Gregory had one more problem to sort out. Since the introduction of the Julian calendar some 10 days had been lost, and his simple solution was to jump straight from October 4th to October 15th in 1582. It was a hard sell since many people felt that they had lost 10 days of their life and would die sooner! The new calendar was introduced in the majority of Catholic countries but non-Catholics were not keen to follow the Pope. It was not until 1752 that the UK and its colonies (notably America) followed suit, and they then had to strike 11 days from the calendar. The Russians stuck to their calendar until the revolution when they had to cross out 13 days (with the interesting result that the October revolution was actually in November)! The last European country to adopt the Gregorian calendar was Greece in 1923, but although this calendar is used worldwide for trade and communication, many countries and religions continue to use different calendar systems.

Any More Problems?

Yes. The rate of rotation of the Earth is decreasing very gradually—and rather unpredictably—over time, so days are getting longer. With the advent of incredibly accurate atomic clocks, a new standard called UTC (Coordinated Universal Time) was introduced in 1961 and is used for international timekeeping. To keep it synchronised with solar time, a ‘leap second’ is occasionally added—agreed in advance and programmed in to all atomic clocks. Since 1972 there have been 27 leap seconds, the most recent on December 31st, 2016. In the middle of the night, at 23:59:59, the clocks didn’t move on to 00:00:00 but added in 23:59:60! Was the world famous Sydney fireworks display on time—or a second early?