Leap seconds have been added to atomic clocks since 1972. timeanddate.com examines in this article what a leap second is, why it is used and why the Earth is slowing down.
What is a Leap Second?
A leap second is a second, as measured by an atomic clock, added to or subtracted from Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. It compensates for slowing in the Earth’s rotation and is added during the end of June or December. The first leap second was added to atomic clocks in 1972, with the most recent leap second being added on December 31, 2008. To understand the concept of a leap second, we need to look at how seconds are used.
The second is the base unit for modern time keeping. The second was previously defined based on the Earth's rotation, but because modern atomic clocks are more accurate than the Earth's rotation the definition was changed in 1967. A second is currently defined as being the duration of 9,192,631,770 periods/oscillations of radiation from a Cesium-133 atom at the ground state. This is where ground state refers to a cesium (or caesium) atom at rest at a temperature of 0 K (kelvin) (coldest possible).
It is also possible to have a negative leap second, where one second is removed, in a case where the Earth is rotating faster, but such a negative second has never been used, and is rather unlikely to be used in the future.
Why are Leap Seconds Used?
Leap seconds are added to keep the atomic clocks synchronized with the Earth's rotation. This is because the Earth rotates at a slower pace over time while the atomic clocks do not slow down. The Earth is rotating slower and slower over time, while the atomic clocks are not slowing down. On one average day the difference between atomic clocks and Earth's rotation is around 0.002 seconds, or around 1 second every 1.5 years.
The time to do one rotation differs from day to day and from year to year. The Earth was slower than atomic clocks by: 0.16 seconds in 2005; 0.30 seconds in 2006; 0.31 seconds in 2007; and 0.32 seconds in 2008. It was only 0.02 seconds slower in 2001 (based on data from IERS).
The atomic clocks are occasionally instructed to add an extra second, known as the leap second, to synchronize the atomic clocks with the Earth's observed rotation. Leap seconds are inserted so that the difference between the UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) is kept below 0.9 seconds. Therefore, the leap second adjusts the clocks to ensure that civil time (used by clocks) is as close as possible to mean solar time (the mean sun’s hour angle).
The Earth's rotation is variable but is gradually slowing down therefore the days get longer by about two thousandths of a second every century, according to Dr Bruce Warrington, from Australia’s National Measurement Institute (NMI). The most accurate and stable time comes from atomic clocks but for navigation and astronomy purposes, atomic time is synchronized with the Earth’s rotation.
Why is the Earth Slowing Down?
According to Donald L Hamilton, author of “The Mind of Mankind” (cited in “On second thought” in the Cape May County Herald), the Earth loses its kinetic energy due to all forms of friction acting on it; tides, galactic space dust, solar wind, space weather, and geo-magnetic storms.
“While the rotation has slowed a seemingly insignificant amount, it has caused mountains to rise, earthquakes to occur, volcanoes to erupt and the Earth’s vast mountain ranges to rise,” Hamilton said. He believed that the earthquake that caused the devastating tsunami in south-east Asia was another “minor” adjustment that the planet had to make. It has made millions of these adjustments over its lifetime.
What Will Happen in the Future?
There have been many discussions and proposals for and against the future of leap seconds. A vote to stop leap seconds is currently being planned.
More information
- About Leap Seconds – December 2008 Leap Second
- The Future of Leap Seconds
- Why and When Leap Years are Used
- Time Zone News
- List of countries that observe Daylight Saving Time in 2009
- Time Zone Abbreviations
Related links
- The World Clock – Current time all over the world
- Personal World Clock
- Meeting Planner
- Time Zone Converter
- Fixed Time Calculator – If it's 3 pm in New York, what time is it in the rest of the world?
