"Leap Second" to be Added to World Clocks

By Nancy Atkinson - December 9, 2008 at 7:12 AM UTC | Physics
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If you ever feel like you need more time, here's some great news: you're actually going to get it. On December 31, 2008 a "leap second" will be added to the world's clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). This corresponds to 6:59:59 pm Eastern Standard Time, when the extra second will be inserted at the U.S. Naval Observatory's Master Clock Facility in Washington, DC. This is the 24th leap second added to UTC, a uniform time-scale kept by atomic clocks around the world, since 1972. Coincidentally, Fraser and Pamela's

most recent episode of Astronomy Cast is about time,

so if you want to know more about time and the atomic clocks used to provide precise timekeeping,

check it out.

Historically, time was based on the mean rotation of the earth relative to celestial bodies and the second was defined in this reference frame. However, the invention of atomic clocks defined a much more precise "atomic time" scale and a second that is independent of the earth's rotation. In 1970, an international agreement established two timescales: one based on the rotation of the earth and one based on atomic time.

Atomic clocks do not use radioactivity, but they use the exact frequency of the microwave spectral line emitted by atoms of the element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), and this provides the fundamental unit of time, which are measured by cesium clocks.

The problem is that the earth's rotation is very gradually slowing down, which necessitates the periodic insertion of a "leap second" into the atomic timescale to keep the two within 1 second of each other. The International Earth Rotation and Reference Systems Service (IERS) is the organization which monitors the difference in the two timescales and calls for leap seconds to be inserted or removed when necessary.

Since 1972, leap seconds have been added at intervals varying from six months to seven years, with the last being inserted on December 31, 2005. The U.S. Naval Observatory is charged with the responsibility for the precise determination and dissemination of time for the Department of Defense and maintains its Master Clock. The U.S. Naval Observatory, together with the National Institute of Standards and Technology (NIST), determines time for the United States.

Source: US Naval Observatory

Nancy Atkinson

Nancy Atkinson

Nancy Atkinson is a space journalist and author with a passion for telling the stories of people involved in space exploration and astronomy. She is currently retired from daily writing, but worked at Universe Today for 20 years as a writer and editor. She also contributed articles to The Planetary Society, Ad Astra (National Space Society), New Scientist and many other online outlets.

Her 2019 book, "Eight Years to the Moon: The History of the Apollo Missions,” shares the untold stories of engineers and scientists who worked behind the scenes to make the Apollo program so successful, despite the daunting odds against it. Her first book “Incredible Stories From Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos” (2016) tells the stories of 37 scientists and engineers that work on several current NASA robotic missions to explore the solar system and beyond.

Nancy is also a NASA/JPL Solar System Ambassador, and through this program, she has the opportunity to share her passion of space and astronomy with children and adults through presentations and programs. Nancy's personal website is nancyatkinson.com