New Year will be a second late

Monday, 22 December, 2008

As the world counts down to the New Year this 31 December, some of us will all have to wait an extra second.

The International Earth Rotation and Reference Systems Service (IERS), the international authority that monitors the rotation of the Earth, has announced that a leap second will be added to the last minute of 2008.

Dr Bruce Warrington, Section Manager for Length, Time and Optical Standards in Australia’s National Measurement Institute (NMI), said that this extra second is being added because the Earth’s rotation was slowing. The Earth's rotation is variable but has been gradually decelerating, so the day has been getting longer by around 2 thousandths of a second every century.

Time is one of the quantities of measurement that NMI is responsible for establishing and maintaining to ensure Australian industry operates competitively in a global environment.

"Thinking of the Earth as a clock, it is running slow compared to the time kept by atomic clocks," Dr Warrington said.

"The discrepancy between these two ‘clocks’ needs to be adjusted so that the world operates on an agreed and consistent time basis. The most accurate and stable time comes from atomic clocks, but for navigation and astronomy purposes, atomic time is synchronised with the Earth’s rotation."

IERS keeps track of time by comparing the Earth’s rotation, which varies, against an international network of atomic clocks, including several housed at NMI, which is unchanging. IERS schedules a leap second as needed to keep the time difference between atomic clocks and the Earth’s rotation to below 0.9 seconds.

"Historically, the second was defined in terms of the Earth’s rotation. Since the 1950s, atomic clocks have been used to keep time and are so accurate and stable that they have revealed the variations in the Earth’s rotation rate," said Dr Warrington.

In Australia we will see in the New Year on time as the extra second will be added 11 hours later. Leap seconds must be inserted at the same instant all over the world, and are added as the last second of either June or December immediately after 23:59:59 UTC (Coordinated Universal Time). The additional second is the 61st second of the last minute of the month, and is written as 23:59:60 UTC. UTC, based on atomic clocks, is the modern successor to Greenwich Mean Time.


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