Yes, 2008 Really Did Seem a Little Longer Than Normal
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 marks the 24th leap second to be added to UTC, a uniform time-scale kept by atomic clocks around the world, since 1972.
The spin of the Earth is slowing down. Not by much -- only about 0.002 seconds a day (it varies), relative to our modern definition of the second. The varying rotation of the Earth is due to the cumulative effect of friction from the ocean's tides, the Moon’s orbital momentum, snow (and the lack thereof) at the polar ice caps, the 23-degree tilt of the Earth, the atmosphere, solar wind, space dust, and magnetic storms. In any case, the Earth does not rotate exactly once every 24 hours (or 86,400 seconds).
This fact was of little consequence and went unnoticed until the highly accurate atomic clocks developed by the National Institute of Standards and Technology (NIST) and others around the world arrived on the timekeeping scene in the late 1960s. Thanks to precise measurements of the position of the Earth relative to the stars made possible by the atomic clock, the Earth’s rotation was found to be gradually slowing down. But the miniscule slowing of the rotation is not the primary reason for adding a leap second. The definition of the second itself—and its cumulative measurement of minutes, hours and days—does not match the Earth’s rotation.
Rather than periodically changing the definition of the second to match the Earth (as had been done prior to the era when the definition of the second was tied to atomic clocks), it was decided by international agreement in 1972 that operators of atomic clocks around the world adjust the time of day by adding one second to the world’s official time when needed. This keeps Coordinated Universal Time (UTC) in sync with astronomical time—that being the position of the sun above the Earth—and the leap second was born.
When the Earth gets sufficiently out of sync with UTC, based on observations from the International Earth Rotation Service (IERS) in Paris, a leap second is scheduled. After several months notice, the leap second is added at 18:59.59 Eastern Time (corresponding to 23:59.59 at the zero medidian in Greenwich, England) on either Dec. 31 or June 30. This year, New Year’s Eve celebrations in the United States won’t feel a thing because the adjustment is made just before 7 p.m. Eastern Time. Those interested can watch the leap second occur by logging on to www.time.gov before midnight, London time, and clicking on their time zone.
There have been 24 leap seconds added since the practice was begun in 1972, the last being in December 2005. No special adjustments need be made by us humans because most of the gadgets we use—computers, cell phones, GPS units, etc.—adjust themselves automatically.
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. Modern
electronic navigation and communications systems depend increasingly on the dissemination of precise time through such mechanisms as the Internet-based Network Time Protocol (NTP) and the satellite-based Global Positioning System (GPS).
The U.S. Naval Observatory is the largest single contributor to the international time scale, which is computed in Paris, France, at the International Bureau of Weights and Measures. This international prominence in atomic timekeeping is due to the sheer number of atomic clocks operated by the Observatory and the fidelity to which they are maintained. Moreover, the U.S. Naval Observatory’s principal role in
keeping track of changes in the “Earth clock” (i.e., Earth rotation) and its dissemination of this information attests to the fact that globally, as well as nationally, the U.S. Naval Observatory remains the leader in precise time.
For more information:
http://www.usno.navy.mil/pao/press/2008_Leap_Second_Release.pdf
http://www.nist.gov/public_affairs/techbeat/tb2008_1223.htm#leap
http://www.time.gov
http://www.astromart.com/news/news.asp?news_id=802
http://www.astromart.com/news/news.asp?news_id=417
Astromart News Archive:
http://www.astromart.com/news/search.asp?search=.+&search_btn=Go
The spin of the Earth is slowing down. Not by much -- only about 0.002 seconds a day (it varies), relative to our modern definition of the second. The varying rotation of the Earth is due to the cumulative effect of friction from the ocean's tides, the Moon’s orbital momentum, snow (and the lack thereof) at the polar ice caps, the 23-degree tilt of the Earth, the atmosphere, solar wind, space dust, and magnetic storms. In any case, the Earth does not rotate exactly once every 24 hours (or 86,400 seconds).
This fact was of little consequence and went unnoticed until the highly accurate atomic clocks developed by the National Institute of Standards and Technology (NIST) and others around the world arrived on the timekeeping scene in the late 1960s. Thanks to precise measurements of the position of the Earth relative to the stars made possible by the atomic clock, the Earth’s rotation was found to be gradually slowing down. But the miniscule slowing of the rotation is not the primary reason for adding a leap second. The definition of the second itself—and its cumulative measurement of minutes, hours and days—does not match the Earth’s rotation.
Rather than periodically changing the definition of the second to match the Earth (as had been done prior to the era when the definition of the second was tied to atomic clocks), it was decided by international agreement in 1972 that operators of atomic clocks around the world adjust the time of day by adding one second to the world’s official time when needed. This keeps Coordinated Universal Time (UTC) in sync with astronomical time—that being the position of the sun above the Earth—and the leap second was born.
When the Earth gets sufficiently out of sync with UTC, based on observations from the International Earth Rotation Service (IERS) in Paris, a leap second is scheduled. After several months notice, the leap second is added at 18:59.59 Eastern Time (corresponding to 23:59.59 at the zero medidian in Greenwich, England) on either Dec. 31 or June 30. This year, New Year’s Eve celebrations in the United States won’t feel a thing because the adjustment is made just before 7 p.m. Eastern Time. Those interested can watch the leap second occur by logging on to www.time.gov before midnight, London time, and clicking on their time zone.
There have been 24 leap seconds added since the practice was begun in 1972, the last being in December 2005. No special adjustments need be made by us humans because most of the gadgets we use—computers, cell phones, GPS units, etc.—adjust themselves automatically.
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. Modern
electronic navigation and communications systems depend increasingly on the dissemination of precise time through such mechanisms as the Internet-based Network Time Protocol (NTP) and the satellite-based Global Positioning System (GPS).
The U.S. Naval Observatory is the largest single contributor to the international time scale, which is computed in Paris, France, at the International Bureau of Weights and Measures. This international prominence in atomic timekeeping is due to the sheer number of atomic clocks operated by the Observatory and the fidelity to which they are maintained. Moreover, the U.S. Naval Observatory’s principal role in
keeping track of changes in the “Earth clock” (i.e., Earth rotation) and its dissemination of this information attests to the fact that globally, as well as nationally, the U.S. Naval Observatory remains the leader in precise time.
For more information:
http://www.usno.navy.mil/pao/press/2008_Leap_Second_Release.pdf
http://www.nist.gov/public_affairs/techbeat/tb2008_1223.htm#leap
http://www.time.gov
http://www.astromart.com/news/news.asp?news_id=802
http://www.astromart.com/news/news.asp?news_id=417
Astromart News Archive:
http://www.astromart.com/news/search.asp?search=.+&search_btn=Go
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