What makes a leap year? by Adam Spencer

Every four years, maths nerds, astronomers and calendrists alike get pretty excited about today's date, 29 February. But why do we have leap years and why are they not actually every four years?

Let's go back over 2,000 years to Julius Caeser and the then Roman calendar - more specifically what we now know as 46 BC (which the Roman's knew as the 'Year of the Consulship of Caesar and Lepidus' or if you were more old school, the 'Year 708 Ab urbe condita'!

At this time the Roman year was a mere 355 days long; made up of four 'full' months with 31 days, seven 'hollow' months with 29 days, and one month with 28 days.

It didn't take long for people, (especially farmers!), to realise that the actual year ran longer than 355 days. Seasons soon fell out of sync with the weather. Some summers were hot. Some were freezing. In general, chaos ruled. Until along came a chap called Julius Caeser.

Using the best brains of the time, quite possibly a mega-geek called Sisogenes, Caeser knew that the Earth did not rotate around the sun in exactly 365 days. A full rotation of the sun, a 'tropical' year, was actually very close to 365 and 1/4 days. So after 365 days the Earth would be 1/4 day of orbit behind where it had been one year earlier. In other words, every four years our Earth would be a full day's worth of orbitting behind where it had started. Caeser did three things.

Firstly he dragged out 46BC for 445 days, earning it the name 'annus confusionus' or the 'year of confusion' so that the new year would start when he wanted.

Secondly, he changed the lengths of the 29 day hollow months to 30 or 31 days, to get the standard 'Julian' year of 365 days.

Thirdly, and most importantly for today, he added a day to February every fourth year (or 'leap year') to make up the extra time the Earth needed to stay in place for each new year.

And that gave us the Julian calendar, which ran until 1582 when Pope Gregory XIII made one more minor tweak…

You see the tropical year is NOT exactly 365 and 1/4 days. It's more like 365 days, five hours and pretty close to 49 minutes - that's 11 minutes shorter than precisely 365 and 1/4 days. Over time these 11 minutes add up, so the Gregorian Calendar scraps leap years for three of every four century years. 2100 will not be a leap year. Nor will 2,200 or 2,300. But 2,400 will be, as was 2000.

So for those of you reading this who were alive 24 years ago, you got to experience something truly special - a leap day in a century year!

Happy 29 February.