Unit of the month – the second

In preparation for the SI changes that are coming in on 20 May 2019, we are looking at a unit a month

Following the recent decision, taken by measurement scientists from around the world, to revise the International System of Units (SI), on the 20th of each month we will be looking at one of the seven SI base units. You'll be able to find out where it's used in everyday life, how it's defined now, and the changes that will come into force on 20 May 2019.

20 November 2018 the metre (m)
20 December 2018 the candela (cd)
20 January 2019 the ampere (A)
20 February 2019 the kelvin (K)
20 March 2019 the second (s)
20 April 2019 the mole (mol)
20 May 2019 the kilogram (kg)









"What’s the time?" – is one of the most frequently asked questions in all languages, and a subject of fascination ever since human societies first tried to coordinate.

A short history of time

Horology... from solar to optical

The history of time measurement dates from approximately 3500 BC. The first clocks were sundials, that noted movement of a shadow cast by a gnomon. Designs varied from the monumental Egyptian obelisks, down to smaller, portable and even pocketable versions. Solar clocks have some obvious shortcomings: not working at night and construction needed to account for latitude (that affects elevation of the sun). Another avenue of innovation was the water clock, measuring time using the flow of water. These had other drawbacks, such as needing a constant pressure of water to maintain flow at a constant rate.

Mechanical clocks were developed in Europe during the 13th and 14th centuries, making use of mechanical energy stored in a spring or weight. Hundreds of years of innovation culminated in the Shortt-Synchronome free pendulum clock, the most accurate pendulum clock commercially produced, that had the highest performance for timekeeping between the 1920s and the 1940s. This clock used two electrically coupled pendulum systems (basic and auxiliary) and electromagnets regulated an auxiliary clock.

The quartz era as the base for time standards began in the late 1920s. Quartz crystals are piezoelectric, applying a voltage to quartz causes vibrations at a precise frequency. Typical modern quartz clocks today may be accurate to about half a second per day, while the most accurate may be out by just about a second over 20–30 years.

Superseding quartz clocks for precision were atomic clocks, an idea first proposed by Lord Kelvin in 1879. Atomic clocks are based on quantum phenomena in atoms, that allow only step-changes of energies at set values, accompanied by the emission of electromagnetic radiation at fixed frequencies.

The first accurate atomic clock, based on a transition of the caesium-133 atom, was built in 1955 at the National Physical Laboratory (NPL). The caesium atom remains the most popular choice for atomic clocks, with the frequency of its clock transition in the microwave band, slightly above 9 GHz. Clocks based on transition frequencies as high as those in the optical band are in development, that, in theory, promise accuracy equivalent to losing or gaining just one second in about 30 billion years.

The changing definition of the second

For most of its history, the second was defined as a fraction of the mean solar day (based on the position of the Sun in the sky). One second was exactly 1/86400 of the mean solar day.

In 1967 came a revolution in the definition of the SI second, based on the development of atomic clocks, which still applies today and is as follows:

second – time equal to 9 192 631 770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

However, on 20 May 2019, the SI definition of the second will be changed, as follows

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s–1.

What’s the difference? From a technical point of view – none. Microwave caesium radiation will still be used with the same defined frequency. However, the wording of the definition is to be revised so that after redefinition of some of the SI units, all basic SI units will have consistency in the construction of their definitions.

Time metrology

Our civilization today is highly time-dependent. Working hours, school timetables, shop opening hours and train timetables all, more or less, keep to time. Some industries require much higher levels of timing accuracy, such as banking, stock exchanges, telecommunications, internet communications and satellite navigation.

If it were not for accurate timing, we might get lost when navigating by GPS, not connect with the person who we’re trying to call, not guarantee bank transfers via the internet, or reliable exchange information by e-mail.

Time is the most accurately measured quantity in metrology, and very accurate timing is a pre-requisite for measuring other quantities, such as determining laser frequencies for measuring length or for reproducing the standard electric voltage based on the Josephson effect.

While such precision isn’t always necessary, most of us come into contact with time measurements in our daily lives that we’d prefer to be carried out accurately – such as for household electricity, gas, and other bills.

Time scales

As communications systems bring us closer together, it is convenient to have one time scale as the reference for timekeeping in all countries. Atomic clocks are very accurate, but no single clock can be totally relied upon to set the time for the whole world. So, to establish a uniform time scale, it was decided to average many clocks from around the world. The time scale based on the ‘group time standard’ is called International Atomic Time (TAI) and its creation is coordinated by the International Bureau of Weights and Measures (BIPM) in Paris.

Because the rotation of the Earth has no direct relationship to atomic phenomena, TAI and astronomical time tend to diverge. A second time scale, Coordinated Universal Time (UTC), was therefore introduced in 1960. UTC is based on TAI, but is adjusted so that it remains close to time based on the Earth’s rotation measured at the prime meridian. UTC is subject to irregularities, such as slowing of the Earth’s rotation, so coordination is needed with atomic time to maintain noon at midday, so the benefits of agreement of the measure of time are preserved, and kept in compliance with astronomical time.

Did you know...

The tropical year, associated with the Earth’s orbit around the Sun, takes approximately 365.242 days. However, a normal year in the Gregorian calendar is 365 days long and, therefore, every four years (except for years that are a multiple of 100, but not of 400) we have a leap year of 366 days to equate the calendar year with a tropical year.

Very occasionally, there are 61 seconds in a UTC minute. The extra second, called a 'leap second', is applied to UTC to keep it in close agreement with mean solar time. Since the leap second procedure was introduced on 1 January 1972, initially with UTC having a 10-second lag behind TAI, there have been 27 leap seconds, the most recent on 31 December 2016 at 23:59:60 UTC. Decisions on when to set leap seconds are made by the International Earth Rotation and Reference Systems Service (IERS), but you may not need to pause your next New Year celebrations, as, so far, no leap second has been announced for the end of 2019.

(Thanks to our colleagues at the Central Office of Measures (GUM), Poland, for providing this information.)

Find out more about NPL’s Time and Frequency research.

20 Mar 2019