National Physical Laboratory

SI units > Second (s)

Second (s)

SecondThe second is the SI base unit for time. It is defined by 9,192,631,770 oscillations of the radiation corresponding to the transition between two levels of the caesium atom.

The second is used to measure time. As well as enabling us to tell the time of the day, accurate timekeeping underpins the functioning of satellite technology like GPS or the internet and facilitates timestamping for transactions in financial trading.

Ancient civilisations used sundials and obelisks to tell the time, which was not very precise, and was restricted in cloudy weather or at night. Time was commonly measured in terms of days, relying on the rotation of the Earth. This rotation is not regular enough to give this definition sufficient accuracy.

Atomic clocks, which keep time using energy transitions in atoms, revolutionised timekeeping. NPL developed the first operational caesium-beam atomic clock in 1955. This clock was so accurate that it would only gain or lose one second in three hundred years. Modern atomic clocks can be as much as 300,000 times more accurate than this, underpinning satellite technology like GPS or the internet.


The second is not expected to be redefined in 2019.

Did you know?

  • The second was first defined by the Sumerians, over 3,500 years ago
  • The first accurate atomic clock was in operation at NPL in 1955

The science behind the unit

For thousands of years the Earth's rotation was our most stable timekeeper. However, the quartz and atomic clocks invented during the 1930s and 1950s are even better timekeepers, and show that the Earth does not rotate steadily but wobbles. Since 1967 the definition of the second has been related to the movement of electrons in a caesium atom:

The second 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 caesium 133 atom.

The atom can be pictured as a mini solar system, with the heavy nucleus at the centre surrounded by electrons in a variety of different orbits. The orbits correspond to energy levels, and electrons can only move between levels when they absorb or release just the right amount of energy.

This energy is absorbed or released in the form of electromagnetic radiation, the frequency of which depends on the difference in energy between the two levels. By measuring the frequency of the electromagnetic radiation, like counting the number of pendulum swings, we can measure the passage of time.

Time measurement has become a basic part of everyday life and accuracies of the nearest minute or a few seconds are usually good enough for most human activities, but highly accurate timing plays a vital role in many other aspects of the modern world. The Global Positioning System (GPS) satellites broadcast timing signals from onboard atomic clocks, which enable land vehicles, shipping and aircraft to know their location within a few metres.

Clocks for the 21st century based on single cold trapped ions or collections of atoms are being developed. Ions are charged atoms which can be trapped almost indefinitely by electromagnetic fields, and cooled by laser beam close to absolute zero. In this way, certain optical absorptions in the ion can exhibit a very pure frequency. This can also be achieved for atoms trapped by intersecting light beams.

At the National Physical Laboratory, optical clocks are being developed which may have accuracies of around 100 times higher than the best current microwave atomic clocks. That is equivalent to losing no more than one second in the age of the universe.

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