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Atomic clocks showed that there were also random variations in the movement of the Earth around the Sun. Although they were very small, the need for increasingly accurate time measurement made them significant. To avoid these problems, in 1967, the definition of the second was changed to 9192631770 "ticks" of a caesium atomic clock.
About 230 atomic clocks in laboratories around the world are compared to give a timescale called International Atomic Time (TAI). The time given by astronomical observations is called Universal Time (UT1).
TAI and UT1 are slowly drifting apart because, as well as having random irregularities in its speed, the Earth is very gradually slowing down. To allow for this, "leap seconds" are added to TAI to produce a timescale called Coordinated Universal time (UTC), which is always kept within 0.9 seconds of UT1. the last leap second of the 20th century was added on 31 July 1999, increasing the difference between UTC and TAI to 31 seconds.
Time signals in Britain give UTC (or UTC plus one hour in the summer). This is slightly different from what used to be known as Greenwich Mean Time (GMT), which corresponds most closely to UT1, but the differences are not significant in everyday use. The time in other parts of the world is UTC plus or minus various numbers of hours.
Errors in the timekeeping of atomic clocks are mainly a result of the fact that the atoms are moving. If the atoms are made to move more slowly, accuracy increases.
A caesium fountain is a new kind of atomic clock. Atoms of caesium naturally are first slowed down and almost stopped using lasers. About 10 million of these atoms are collected in a magnetic field. Then the field is turned off and the lasers are used to push the atoms upwards. They rise and fall slowly under gravity, and pass through a region of radio waves as in an ordinary atomic clock. The number which have had their energy changed is measured by another laser, as in NIST-7.
Caesium fountains are still in their experimental phase but have already achieved accuracies of one second in 15 million years. It is thought that by taking one into space, where the atoms will not fall out of the radio wave region, they may be made ten times better.
An even more advanced type of clock is the trapped ion machine. Ions are like atoms but with an electric charge. This means that they can be held in one place and exposed to two pulses of radiation, separated by a delay of several minutes. This greatly reduces the possible frequency error. Unfortunately, these machines tend to use much higher frequencies than caesium clocks and these frequencies cannot be measured by electronic counters. Current trapped ion machines are less accurate than caesium fountains but it is hoped that they might eventually reach an accuracy of one second in 10 billion years.
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