Jun 29 2015 : The Times of India (Delhi)
LEARNING WITH THE TIMES - Why time will stop for a second on June 30
How is time measured?Our perception of time is based on the earth's revolu tion around the sun and its ro tation around its own axis.One complete cycle around the sun is a year, while a com plete rotation on its axis con stitutes a day. Based on these observations, a day has 24 hours while 365 days constitu te 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 as suming 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 de fined by the Interna tional System of Units is not based on astrono mical observation. The length of a second is de fined according to the vibrations of caesium atoms at various atom ic 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 accommo dition 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.
