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In the Fact File section we bring you a new collection of quick facts each week. (Click on the links below for more facts)

 
 

Special Atomic Clocks Fact File

2661/ / An early pioneering atomic clock from Hewlett Packard (HP 5060A cesium-beam atomic clocks) gained worldwide recognition in the 1960s as the 'flying clocks' after they were flown from Palo Alto to Switzerland to compare time.

2662/ Atomic Time replaced Earth Time as the world's official scientific time standard in 1972.

2663/ The measurement of time is currently determined by an international consortium based in France which averages the time from approximately 220 atomic clocks in over two dozen countries.

2664/ Radio-controlled (atomic) time was invented by scientists at the National Institute of Standards & Technology, an agency of the U.S Department of Commerce. Established in 1901, NIST's main objective was to develop & apply technology, measurments & standards. After much trial and error through the 1940s and 1950s, Nist completed it's first cesium atomic beam device in 1957.

2665/ The second according to atomic time is defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's frequency. This replaced the old second that was defined in terms of the earth's motions.

2666/ Here’s how an Atomic Clock gets to send its signals:

The Atomic clock sends out a signal to a unit called a time code generator which produces the time code.

The signal is then amplified by powerful radio transmitters at several radio stations.

The high power signal is now sent to an antenna using a transmission line. This thick cable connects the transmitters to the antenna towers.

The antenna radiates the time signal through a network of wires connected to several antenna towers.

Finally, the radio signal travels through the atmosphere and is received by millions of people every day using equipment ranging from low cost clocks & watches to sophisticated weather stations.

2667/ NIST-F1, the cesium atomic clock at NIST's Boulder, Colorado., laboratories, is referred to as a fountain clock because it uses a fountain-like movement of atoms to obtain its improved reckoning of time. Introduced in 1993 it is reckoned to be three times more accurate then the model it replaced (the NIST-7).

2668/ The NIST-F1 will neither gain nor lose a second in nearly 20 million years.

2669/ High-accuracy timekeeping is critical to a number of important systems, including telecommunications systems that require synchronization to better than 100 billionths of a second and satellite navigation systems such as the Defence Department's Global Positioning System where billionths of a second are significant. Electrical power companies use synchronized systems to accurately determine the location of faults (for example, lightning damage) when they occur and to control the stability of their distribution systems. In the domain of space exploration, radio observations of distant objects in the universe, made by widely separated receivers in a process called long-baseline interferometry, require exceedingly good atomic reference clocks. And navigation of probes within our solar system depends critically on well-synchronized control stations on earth.

2670/ Steven Chu (Stanford), Claude Cohen-Tannoudji (College de France), and Bill Phillips (NIST) shared the 1997 Nobel Prize in Physics for their work on laser cooling - a key technology for modern atomic clocks.

2671/ The most accurate clocks in the world are the new atomic fountain clocks, in which thousands of extremely cold atoms are tossed gently into a vacuum chamber, where they fall under gravity's pull. Before atomic fountain clocks, the most accurate timekeepers were clocks that measured the vibrations of atoms in a beam flying through a vacuum chamber. Although they are extremely accurate, such clocks suffer from errors caused by the speed of the atoms flying through the chamber. Atomic fountain clocks measure the atoms' vibrations at the top of the fountain, where they are practically motionless for a fraction of a second before they fall back down. As a result, the time measured by such clocks is accurate to within one second in more than thirty million years.

2672/ The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error between a pair of such clocks, i.e., the difference in indicated time if both were started at the same instant and later compared, was typically about one second in three thousand years.

2673/ An atomic clock powered by a hydrogen atom is accurate to 1 part in 2 quadrillion.

2674/ Atomic clocks are quite complex, but the basic theory is simple. Like all clocks, they are intended to make the same event happen over and over. The repetition of this event produces a frequency, which is intended to be as stable as possible. For example, the pendulum in a grandfather clock swings back and forth at the same rate, over and over. The swings of the pendulum are counted to keep time. In a cesium oscillator, the transitions of the cesium atom as it moves back and forth between two energy levels are counted to keep time. The best cesium oscillators (such as NIST-F1) can produce frequency with an uncertainty of about 1 x 10-15, which translates to a time error of about 0.1 nanoseconds per day.

2675/ Leap years are years with 366 days, instead of the usual 365. Leap years are necessary because the actual length of a year is 365.242 days, not 365 days, as commonly stated. Basically, leap years occur every 4 years, and years that are evenly divisible by 4 (2004, for example) have 366 days. This extra day is added to the calendar on February 29th.

2676/ A leap second is a second added to Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second.

2677/ The first leap second was added on June 30, 1972, and they occur at a rate of slightly less than one per year, on average. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the earth's rotation, it is unlikely that we will ever have a negative leap second.

2678/ Since a millennium is 1000 years, and the first millennium began at the start of the year 1, it ended at the end of the year 1000. The second millennium then began with the year 1001 and concluded at the end of the year 2000. Therefore, the current millennium technically began with the year 2001.

2679/ PARCS is an atomic-clock mission scheduled to fly on the International Space Station (ISS) in 2008. The mission, funded by NASA, involves a laser-cooled cesium atomic clock, and a time-transfer system using Global Positioning System (GPS) satellites. PARCS will fly concurrently with SUMO(Superconducting Microwave Oscillator), a different sort of clock that will be compared against the PARCS clock to test certain theory. The objectives of the mission are to:

- Test gravitational theory
- Study laser-cooled atoms in microgravity
- Improve the accuracy of timekeeping on earth

2680/ In 1958 Commercial cesium clocks became available, and cost $20,000 each.

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