CRI听力:Leap Year, the Roman Calendar, the Earth's Orbit and Odd Birthdays

Meet actor and musician, Paul Kissaun. At the end of February this year, he'll turn the tender age of 13, although he'll actually be 52.

That's because Kissaun was born on 29 February 1964. As such he's become accustomed to celebrating his 'real' birthday every four years.

"On the 1 March, it feels like it's March - I'm a February 28th, 29th. And then people go; 'Oh, you don't really have a birthday this year do you?' they always say things like that and you go; 'Yeah, yeah, I do, I do.' But then, when you don't want to have a birthday it's really convenient, like say when you're coming up to your big 3-0, in leap year terms you can just duck right out of it and go; 'Yeah, my big 1-0 isn't for ages, it's not for another ten years."

This February, those who follow the Gregorian calendar - also known as the 'Western' or 'Christian' calendar - will add an extra day to the end of the month to account for the Earth's orbit around the Sun.

Dr. Louise Devoy, Curator at the Greenwich Royal Observatory explains how this works.

"Every year the Earth takes 365 and a quarter days to make one complete orbit around the Sun, and so, after four years, that little quarter of a day is added up to make an extra day. So, every four years we add in an extra leap day."

It actually takes 365 days, five hours, 48 minutes and 46 seconds for the Earth to complete an exact orbit of the Sun.

According to Devoy, Earth's annual extra quarter day perplexed people for almost as long as calendars have existed.

"For thousands of years really, people have noticed this little extra day and tried to account for it in their calendars. One of the most significant dates is around 46 B.C. when Julius Caesar decided to reform the Roman calendar from a lunar-based one to a solar-based one. And he adopted the Egyptian calendar that had 360 days plus an extra five feast days. And to compensate for that extra quarter day he added in leap years every four years."

Devoy says the reason February 29th was chosen to be that extra day was because back in Roman times the year started in March, therefore it made sense to add the extra day onto the last month.

He says keeping the calendar synced with the Earth's orbit is important.

"If we didn't have leap years then the calendar would start to move out of sync with the seasons. So it's an important way of keeping in sync with the natural rhythms of the Earth's orbit around the Sun."

Other calendars aside from the western Gregorian calendar also observe leap years.

The modern Iranian calendar adds eight leap days into a 33-year cycle.