I presume that you know what a leap year is. A year is the amount of time the Earth takes to orbit the Sun, and is measured as 365 days, or 365 rotations of Earth. It is, in reality, a little bit longer. Each orbit of the Sun takes 365.25 days, or 365 days and six hours. So what do we do with that six hours? We ignore it for 3 years. On the fourth year we add up those extra hours and we get 24 hours, an extra day! So we add in February 29th every four years.
This may sound ridiculous but if we didn’t correct for the extra orbit time, eventually the calendar would shift and we would see seasons shift over time. We have become very good at measuring time, in many ways. Generally Earthlings have two main standard systems of time. The first is called Universal Coordinated Time (UTC), and Universal Time 1 (UT1).
UTC is measured in France by an extremely precise atomic clock, which works by counting extremely predictable electromagnetic transitions in Cesium Atoms. Atomic clocks lose only one second in 1.4 Million Years. Using UTC, a day is always 86,400 seconds.
UT1 on the other hand is the time measured by the rotation of the Earth, and is measured from several places throughout the world using Very-Long-Baseline Interferometry (VLBI). VLBI measures very accurately the positions of distant bright objects (Quasars generally) to determine the precise position and orientation of the Earth. If you get enough data, you can precisely determine the instantaneous rotation speed and orientation of the Earth.
You may be surprised to find that the Earth’s rotation speed is not constant. The Earth is constantly altered by earthquakes, weather, and its interactions with the Sun and Moon. I was amazed the first time I considered the fact that real life is a lot more irregular and random at the smallest scales (Quantum Mechanics anyone?). On average, the Earth rotates in 86,400.002 seconds. This number also changes year to year.
The 0.002 extra seconds seems like it has no influence, but if you add that up each day, it becomes a lot more significant over time, and causes UT1 to fall behind UTC. So how do we fix the discrepancy?
Whenever the difference between UTC and UT1 reach a threshold of 0.9 seconds, a leap second is added to UTC so it slows down to match UT1. The leap second is added on December 31st or June 30th, and it’s done by sending the clock from 11:59:59 pm to 11:59:60 pm and then to 12:00:00 pm. From 1972, when leap seconds were first implemented, to 1999, leap seconds were added at a rate close to one per year. Since 2000, we have only added three.
The fourth will be added on June 30th, 2015. You won’t notice a darn thing, but it’s a good opportunity to learn about time, the Earth’s behaviour, and how we have mastered our measure of time, at least on our local planet.