# A leap second

We added an extra second (called a leap second) to our clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC) on June 30. Why are we messing around with our clocks? Well, let’s try to understand how we keep “time”. What is a day on Earth? It is the time it takes our planet to make a complete rotation on its axis. We count it as 24 hours, where one hour is 60 minutes, and one minute is 60 seconds. So a day is 60X60X24 = 86,400 seconds. However, there is a slight catch. Today, it takes our planet 86,400.002 seconds to complete a rotation. So our clocks, which assume that a day is 86,400 seconds, are off by 0.002 seconds every day. If a year has 365 days, our clocks are off by 0.73 seconds (0.002 X 365) every year. We have extremely precise and accurate clocks, called atomic clocks, around the globe that keep time for us. So every few years, an international group of timekeepers decides that we need to add one second to these atomic clocks so that we don’t fall too behind. The last time we added a leap second was New Year’s Eve of 2008. The leap seconds have been added since 1972, and this was the 25th time we added a leap second. Most cell phone providers and computer systems check with the world’s atomic clocks and automatically add the second. There were some computer systems that struggled on July 1 with the extra second. Qantas Airways had problems with its reservation systems and could not check in some passengers. As a result, some of its flights were delayed. Internet sites such as LinkedIn and Yelp also experienced problems.

There was a year when a day was exactly 86,400 seconds long. Scientists estimate it to be the year 1820. How can that be? Well, the Earth is very gradually slowing down because of the braking action of the tides. The tides, caused by the gravitational pull of our Moon, slow Earth’s rotation. Before 1820, the day was shorter than 86,400 seconds. About 900 million years ago, the Earth was rotating so much faster that a day on Earth was only about 18 hours long! Incidentally, 2012 is also a leap year, so we have already had a whole day plus a second extra this year!

Here’s a joke:
The math teacher asked, “Class, how many seconds are there in a year?”.
Nathan replied “12”.
The confused teacher asked “Nathan, how is that?”.
Nathan said “Well, there’s January 2nd, February 2nd, March 2nd, …!”

### Did you know?

A leap year consists of 366 days, whereas other years have 365 days. The extra day is February 29. Every fourth year is usually a leap year. A year is the period of time during which Earth completes a single revolution around the Sun, which is 365 days, 5 hours, 49 minutes, and 16 seconds. Therefore, by adding 1 extra day once every four years is like adding a quarter day every year. The problem is that a “quarter day” is 6 hours, which would make one year 365 days and 6 hours – a bit more than it should be. So in addition to the rule that a leap year occurs every four years, a new rule was added: a century year is not a leap year unless it is evenly divisible by 400.

Image Credit: NASA for leap second image