Wednesday, 29 February 2012

Look before you leap

Today is a special day. It is the first time I am able to put up a post in this blog on a day that comes once in every four years. February 29, 2012.

We all know what a leap year is... but why does it have 366 days instead of the 365 that we get from a normal year? Nowadays, it is very easy to get an answer to this question... just google. But back when I was in primary school, I remember looking for the answer in a thick book in the school library.

It was 1972 and I was in Standard 4. My teacher told me that the month of February in that year has 29 days. When I asked why, she told me to look for the answer in a book that explains about our solar system. I remember looking at an illustration in the book showing the Earth with an imaginary line circling the Sun. The caption below the picture says that the time it takes for Earth to do one complete revolution around the Sun is 365 and 1/4 days. This then equals one year. But then, it would be impractical to have a quarter day at the end of each year... just imagine that there would be a December 32nd that lasts for only 6 hours, from midnight to 6am, and that New Year's Day (1st January) would then start at 6am. Everything would be out of sync.

It was Julius Ceasar who first introduced the concept of leap days when he invented his Julian calendar at around 45 BCE. It was not terribly accurate and was later improved by a new calendar created by Pope Gregory XIII around 1,500 years later. The Gregorian calendar now forms the basis of time tracking and measurement for most of the modern world today. But why do we need to add that extra day every four years? It is because of the seasons. If we don't add the extra day, we lose about 6 hours every year and winter in the northern hemisphere would start to move forward. The recoupment of that 6 hours each year for 4 years ensures that the seasons happen at roughly the same time every year.

While refreshing my knowledge on this subject earlier today, I found out that the Earth's orbit is not exactly 365 and a quarter days. It is approximately 365 days, 5 hours, 48 minutes and 46 seconds... and I say approximately because this duration can vary slightly depending on the relative position and influence of other planets. This means that even adding a day after every four years is not really that perfect if we look at the big picture. But the effect may only be significant in 8,000 years time. That's way ahead in the future for us to worry about... let the people living at that time solve it themselves.


Wan Sharif said...

So when you were in standard 4 this old man was in form 5. An 8 yera difference.. Since I started secondary school with remove class..
Interesting research you have there.. I have been doing physics up to tertiary education but did not even know the stuff you post.. A good teacher you used to have back then.. To push you to find out yourself.. Of course this could only happen to equally good student.. Non?.

Cara Lim said...

Very interesting post, Mr Oldstock. I knew that the leap year had something to do with compensation and accumulation of time; that's about all I can remember from school.

But I could tell you what I do remember about 29 February. It's the day when I got my first car. Yup! 29 Feb 1996. It was a 2nd hand red Mini - gift from my then (now, Ex) Husband. It used to slide all over the icy roads in winter :-)

Oldstock said...

Ayoh Wang,

Yes, I had very good teachers those days. And it doesn't hurt to have our own sense of curiosity. That's why I teach my own kids to ask `why?'...

Oldstock said...


You mention 2nd hand red mini... immediately brings to my mind the car in the 1st Bourne movie, where Matt Damon drove through the narrow streets of Paris to evade a police chase. Nifty car that...

Pat said...

And in 1972, I was in Form 4! Hahahaha!

Yes, history can be interesting, can't it? Did you also know that July and August were added to the calendar for Julius and Augustus Ceasar - pushing all the other months back?

December should be the 10th month, November the 9th, October the 8th, and... you get it lah! But, I'm sure you already knew this ;)

Al-Manar said...

So those born on 29th February will celebrate birthday on a four year cycle, a real big one and a big saving as well.

Oldstock said...


I knew about how some of the months got their name. Didn't know the part about July and August pushing the other months back :-)

Oldstock said...

Pakcik Al-manar,

29th Feb babies are quite special ones :-)

sabre23t said...

Actually the leap year algorithm is a little bit more complex than every fourth year. See .

Basically that means ..
(1) add a day when year can be divided by 4
(2) minus a day when year can be divided by 100
(3) add a day again when year can be divided by 400

Oh, even big software developers got the above wrong. See . Microsoft Excel incorrectly assumes 1900 is a leap year.

More info in ;-}

Oldstock said...


Yes, the leap year calculations are a bit more complex than what I wrote. I did some online reading but decided not to complicate my post with all those rules.

Thanks for dropping by and commenting.