Why time will stop for a second on June 30

2015 is not a leap year, but it does have a leap second, set to take place Tuesday (June 30) at 7:59:60 p.m. EDT (23:59:60 GMT).

"Earth's rotation is gradually slowing down a bit, so leap seconds are a way to account for that," Daniel MacMillan of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.

Ask any person the length of a day, and she'll say 24 hours, which equates to 86,400 seconds. But the time it takes for Earth to rotate on its axis relative to the sun, called a mean solar day (or the average length of a day) is roughly 86,400.002 seconds. This happens because Earth's rotation is slowing down, thanks to a kind of braking force caused by the gravitational tug of war among Earth, the sun and the moon, researchers at NASA said.

Because of these planetary forces, a mean solar day likely hasn't lasted 86,400 seconds since about 1820, NASA scientists said.

How is time measured?
Our perception of time is based on the earth's revolution around the sun and its rotation around its own axis. One complete cycle around the sun is a year, while a complete rotation on its axis constitutes a day. Based on these observations, a day has 24 hours while 365 days constitute a year. The day is further divided into hours, minutes and seconds. Time measured by the earth's rotation relative to the sun is called solar time. For any given point, there could be two values of solar time — apparent and mean. Apparent time is measured by direct observation of the sun by a sundial. Mean solar time, however, is measured by assuming that relative to the earth, the sun is at the same position after every 24 hours. Most clocks and watches are based on mean solar time. 

What is the most accurate measure of time?
Measurement of time based on the earth's rotation and revolution has its limitations and hence the unit of time defined by the International System of Units is not based on astronomical observation. The length of a second is defined according to the vibrations of caesium atoms at various atomic clocks. International Atomic Time is based on a system of about 270 atomic clocks. Signals from these clocks are transmitted to the International Bureau of Weights and Measures located in Sevres, France, which uses the signals to form the International Atomic Time. Seconds measured by atomic clocks are the most accurate. These clocks are predicted to be off by less than a second in 50 million years. 

How is this discrepancy corrected?
Today, the international basis is the Coordinated Universal Time (UTC). This system, introduced on January 1, 1960, is designed to accommodate the timekeeping differences between the atomic and solar time. Since January 1, 1972, the UTC is being modified by adding an extra leap second whenever it is necessary. This is done by adding one extra second in the last minute of June or December. The announcement of adding the leap second is done by the International Earth Rotation and Reference System Services. For the synchronization of these two measured values, time will have to stop for a second on June 30th as the last minute of the month will have 61 seconds.

(Picture source: Nasa)

How will the leap second affect computers? 

The leap year rule used in the Gregorian calendar — the most widely used international calendar — adds an extra day every four years. Addition of an extra day compensates for the extra time of 5 hours, 48 minutes and 46 seconds that the earth takes over 365 days to complete one revolution around the sun. This permanent rule is typically considered in all computer programmes and hence there is no problem. The leap second, on the other hand, cannot be predicted as the time taken by the earth to rotate around its axis depends on many factors like dynamics of the earth's core, variation in the atmosphere and oceans, ground water, ice storage and so on and can change from year to year. The irregularity of the leap second could cause problems in many computing systems. Some major web-based services suffered because of the addition of a leap second three years ago.