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Time keeps on slipping Earth's faster rotation eliminated the need for a leap second in 2003. by Michael E. Bakich (This article was originally published on the Astronomy Magazine web site but is no longer available there. It is reproduced below, updated to 2015.) Most of us have at least heard of leap years, but did you know about leap seconds? Like leap years, which help keep track of the 365.24219 days Earth takes to revolve around the Sun, leap seconds adjust for the uneven amount of time our planet takes to rotate on its axis. However, no leap seconds were added from 1999 to 2004 because our planet's rotation has been speeding up. A leap second is a second added to a time scale called Coordinated Universal Time (UTC) to make it agree (within 0.9 second) with astronomical time, which is based on Earth's rotation rate. UTC is derived from atomic clocks. Such clocks do not vary as Earth does, so leap seconds are needed to keep the two time scales in agreement. The difference between astronomical time and UTC changes at a rate of about 2 to 3 milliseconds per day. This makes a leap second necessary, usually at an average interval of slightly more than one year. The first leap second was added on June 30, 1972. Historically, leap seconds have been inserted on either June 30 or December 31. The sequence of events is:
Timekeepers at the National Institute of Standards and Technology (NIST), which introduced the world's first atomic clock in 1949, coordinate leap seconds. No leap seconds were added from 1999 to 2004. The reason is an ever-so-slight increase in Earth's rotation rate. Physicist Tom O'Brian, chief of NIST's Time and Frequency Division in Boulder, Colorado, suggests changes in the motion of Earth's core, the effect of ocean tides and weather, and changes in Earth's shape all may be affecting the rotation rate. In general, he notes, the long-term trend has been for Earth's rotation to slow down, but not from 1999 to 2004. "Earth's rotation rate has been the primary clock for nearly all of human history," he says. "Only in the last 50 years have we had clocks accurate enough to measure changes in the Earth's spin." For additional information on leap seconds click on the following links: https://maia.usno.navy.mil/products/leap-second https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/leap-second https://en.wikipedia.org/wiki/Leap_second |