# Accuracy of the time

## Measurement of time

Automatic translation Updated May 16, 2015

Historically, humans measure time passing. The rotation of the Earth allowed him to divide the time in days between two sunrises or sunsets. Today the movements of the Earth are constantly monitored using techniques more and more advanced. With sundials day was divided into hours, and the clocks were divided in hours minutes and seconds. But it is not enough to avoid the drifts of the time, scientists have found mechanisms more and more accurate to measure the time. The vibration of the quartz crystal has helped improve the time precision to thousandth of a second but it was not yet accurate enough for technological research demand more and more precision. So we used the natural frequency of certain atoms and in particular the isotope cesium-133 atom whose oscillations emit pulses of light. The oscillation frequency of the cesium atom is incredibly high, that is what scientists seeking for a high frequency allows time measurement with high accuracy. This is the cesium atom which currently guarantees the correctness of the time on our planet. If we bombards with energy the cesium atom, using a laser, it vibrates emitting light pulses at a frequency of 9,192,631,770 Hz. With the frequency of 9,192,631,770 light pulses per second, the precision is such that the clock loses or gains a second, that every 100 million years.
Today the definition of the second, made ​​by the International System of Units is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom 133.
A clock that uses the continuity and immutability of the frequency of electromagnetic radiation emitted by an electron during the transition from one energy level to another is called "atomic clock".
It is with these atomic clocks that we keep accurate Time International (TAI) and we distribute the reference time scale (UTC).

NB: Cesium is an alkali metal, its name "caesius" means "blue sky" in Latin. Its spectrum gives two characteristic lines of light blue. It was discovered in 1861 by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff by spectroscopy of the lepidolite (mineral belonging to silicates).

## Clocks increasingly accurate

Gnomons, water clocks, hourglasses were used for centuries to measure time. After the bells that religious operate every hour, come the clocks pendulums. It is Christiaan Huygens (1629 − 1695) in 1657 which designs the first pendulum clock that has a relatively constant oscillation frequency.
This mechanism, slightly improved over time, will be used for nearly 300 years, until a precision on the order of the second.
Electric clocks, electronic clocks and quartz watches will gradually replace the pendulums.
In the twentieth century, the properties of the material will come significantly improve the accuracy of the measurement of time.
Today it is the atomic clocks that get the highest precision (second 10-10 with the Cs133). But already at the beginning of the XXI century light replaces the material in the measurement of time and accuracy increases further (10-12 second with optics).

## Atomic Fountain Clocks

An atomic clock is a quartz oscillator slaved to the natural frequency of the transition between two energy levels of an atom. The first atomic fountain worked in 1989 with sodium atoms. Today is the stable isotope of cesium, 133Cs, which is used in atomic clocks. This is the reference atom in the definition of the second. Everything happens in a heated chamber containing cesium is in this place that we will create a stream of atoms. After some manipulations through a magnetic field, the beam of atoms passes through a resonant cavity called Ramsey, Norman Foster Ramsey physicist who proposed it for the first time in 1950.
After preparing the atoms in one of two levels, basic or excited, the atoms pass more or less from one level to another. The macroscopic oscillator frequency is adjusted so as to remain permanently slaved around the maximum of the atomic transition, it is the response of the cesium atom to the excitement.
The frequency is then 9,192,631,770 Hz This value defines the exact second.

The measurement of time is then provided by a division of the oscillations of the crystal oscillator in combination with an electronic circuit which will output the value or control equipment requiring an operating frequency stable and high.

NB: Norman Foster Ramsey is an American physicist, born August 27, 1915 in Washington and died on 4 November 2011. He has won half of the Nobel Prize for Physics in 1989, including work on atomic clocks.

### Image: The atomic clock fountain of cesium atoms. This clock of 200 kg and 1.5 m high, is the primary standard of time and frequency in the United States, with a total uncertainty of 5.10-16 (2005). Using this clock, time measurement is made with incredible precision with this rare metal atom, the cesium. credit: NIST (National Institute of Standards and technology).

A gnomon (Greek literally "one that knows or examines") is an astronomical instrument used to determine the height of the sun. In prehistoric times, the gnomon is a simple stick planted in land that can indicate the solar time. At that time, the man lives crop and livestock, it does not feel the need to have a greater time precision. During antiquity, the gnomon and sundial enough to measure, on a clear day, hour day. Today the gnomon is a simple vertical rod placed on a table, the length of the shadow projected at a given latitude, inquires on the height of the sun. Projected on the curves of a sundial, the extremity of the shade determines the time throughout the year.

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