Question: why is there leap years

Ee Lyn Lim answered on 28 Jun 2013:

Paul explained it brilliantly, so I’ll just add a slightly random point – did you know that, in addition to leap years, we also have leap seconds?!

Our calendar breaks up the ‘year’, which is the amount of time the Earth takes to move once around the Sun, into 365 ‘days’, which is the amount of time the Earth takes to rotate once around its axis. But actually by the time the Earth has made its way around the Sun, we’ve done 365 and a quarter turns around our axis, so New Year’s Day should technically start just before 6am rather than at midnight! Of course we don’t do that – which means we’re cutting each year short by 6 hours, so we have to make up for that by putting in a full extra day at the end of February every four years.

But what about seconds? Where did they come from? Earth doesn’t do anything special in a second – that is quite literally us dividing the length of day up into 24 hours, each of which has 60 minutes, each of which has 60 seconds. So how long a second is will change depending on how long the Earth takes to turn around on its axis! Now, because of the gravity that’s pulling on Earth from both the Sun and the Moon, this amount of time this rotation takes can actually change from day to day, and overall it is gradually slowing down. That didn’t bother us before, because the only way we had to measure time was by looking at the Sun; but nowadays we measure time using incredibly precise atomic clocks, and can have seconds that are exactly the same length – that’s the kind of clock that computers follow. But because our days are actually getting longer, if we ignored the Sun and followed just the time on the atomic clock, we’d eventually be running so far ahead that midnight would be in broad daylight!

So we’ve come up with the idea of leap seconds – every time our atomic clocks get out of sync with the Sun by more than one second, we’ll insert an extra ‘leap second’ at the end of the day – so the minute between 11:59pm and 12:00am will have 61 seconds instead of 60! It doesn’t happen very often, just once every one or two years, but the last time they did it was last June – if you pay attention next time it happens maybe you’ll see something strange happen on your computer clock!