Despite a common belief time it constant, the Earth's rotation rate generally slows down or speeds up throughout the year. Some changes are predictable. The gravitational pull of the moon, for example, is slowing the Earth's rotation by roughly two thousandths of a second for each day per century.
"Think of it as an ice skater doing a spin, said Tom O'Brian, chief of the Time and Frequency Division at the National Institute of Standards and Technology (NIST), which coordinates universal time. "As the ice skater puts her arms out as she's spinning, moving some of her mass farther from the center of her body, she slows down and when she pulls her arms in, she speeds up."
Unpredictable changes in rotation speed comes from the circulation of molten rock inside the Earth, and the climate on the planet's surface, such as the way water is distributed throughout the world and whether there is more snow and ice on the mountains farther away from the Earth's core.
"Extremely accurate time and synchronization is crucial to our modern technological society," O'Brian said. Synchronization of the Internet is an important use of accurate timing, as well as cellular phone networks that are required to be "synchronized better than a millionth of a second daily."
NIST distributes time over the Internet. It gets 1.6 billion automatic service requests daily. To put that in contexts it is more than 20,000 a second on average, O'Brian said. Software distributed in operating systems such as Microsoft Windows XP and Mac OS X automatically synchronizes computer clocks to NIST time.
Time is determined by atomic clocks that are synchronized with the Earth's rotation, and an international community of scientists has agreed the two should never differ by more than nine-tenths of a second. To keep accurate time, NIST has a complicated system that uses sophisticated software and many atomic clocks and determines averages to deliver the best time possible. In the software there is a prevision when a leap second is required. The first leap second was initiated in 1972. NIST has added one second a year from 1972 through 1998. This is the first instance in seven years scientists have agreed to add time.
At midnight Coordinated Universal Time (UTC) atomic clocks worldwide will read 23:59:60 before hitting 00:00:00. This year's leap second will add an extra second to atomic clocks at NIST in Boulder, Colo., and other sites around the world. Normally, the last second of the year is 23:59:59 UTC on Dec. 31, 2005, while the first second of the new year being 00:00:00 UTC on Jan. 1, 2006. The leap second added at 23:59:59 UTC (06:59:59 p.m. Eastern Standard Time) on Dec. 31, will make the atomic clocks read 23:59:60 UTC before changing to all zeros.