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# UNIT OF THE MONTH MARCH 2019: SECOND

"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. So, let's welcome our March unit of the month - the second - the SI unit of time.

**A short history of time**

Humanity has sought to measure time for thousands of years. Natural markers of the passage of time, such as day passing into night and changing of the seasons, held huge significance throughout history but the first civilization to apply astronomical observations for measuring time was the ancient Egyptians, first using sundials clocks to divide the day into measurable parts. The division of the day and night into 12 parts probably came from the Sumerians that used the sexagesimal (base 60) system of counting from around the in the third millennium BC. Rather than count using all ten fingers as with the decimal system, they counted with fingers joints (minus the thumbs), of which there are 12 per hand. The number 12 is also convenient – as it is divisible by 2, 3, 4 and 6. Division of the whole day into 24 equal hours we probably owe to the ancient Babylonians, as well as the division of the hour into 60 minutes. The Babylonian’s also divided the wheel to 360 degrees. In the second century AD, Ptolemy (a Greco-Roman who lived in the city of Alexandria), introduced the concept of latitude and longitude, expressed in angular degrees in the cartographic system, that was also divided angular degrees of 60 equal parts, called ‘pars minuta minor’ (small tiny part – the arc minute). Each was further divided into 60 equal parts, called ‘pars minuta secunda’ (second tiny part - the arc second). At the beginning of the 11th century AD, the Persian astronomer Al-Biruni used the same system to determine the period of revolution of the Moon around the Earth, dividing the hour for minutes and seconds in sexagesimal system

**Horology ... from solar to optical**

The history time measurement dates from approximately 2000 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 Schortt clock, the most accurate pendulum clock commercially produced, that had the highest standard for timekeeping between the 1920s and the 1940s. This 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 1930s. Quartz crystals are piezoelectric, a word derived from Greek piézo, meaning “I squeeze”, and ēlektron, which means “amber”, an ancient source of electric charge. With quartz, the opposite is also true: 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, as it happens, by Lord Kelvin back 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 in the UK. The caesium atom remains the most popular solution, the frequency of which occurs in the microwave band, slightly above 9 GHz. Clocks which frequencies as high as is in the optical band are in development, that, in theory, promise accuracy equivalent of losing or gaining just one second in about 30 billion years.

**Definitions of the second**

For most of its history, the second was defined as a fraction of the average solar day, based on the position of the sun in the sky. One second was exactly 1/86 400 of the average solar day.

In 1956, the International Committee for Weights and Measures (CIPM) proposed new definition of the second as follows:

**The second is the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time**

This definition was formally adopted by the General Conference of Weights and Measures (CGPM) in 1960.

In 1967, after the development of atomic clocks, a revolution in the definition of the second took place which still applies today:

**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.**

However, on 20 May 2019, the latest redefinition of the second will come into force:

**The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, 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 rebuilt so that after redefinition all base SI units have consistency in construction.

**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 that 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 measured accurately - such as for household electricity, gas, and other bills.

**Time scales – what’s that?**

As communications systems bring us closer together, it’s convenient to have one-time scale apply to all the earth. 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 several clocks from around the world. The time scale based on the ‘group time standard’ was called the International Atomic Time scale (TAI) and its creation is coordinated by the International Bureau of Weights and Measures in Paris (BIPM).

Because rotation and orbital motion of the earth has no direct relationship to atomic phenomena in atoms, discrepancies occur between TAI and historical astronomical time. Universal Coordinated Time UTC became the primary time standard on 1 January 1960 – that is TAI time shifted to coordinate with the average solar time at the zero meridian. In 1972, it was decided that UTC would be the official time in all countries. 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.

**Summer daylight saving time – is it needed?**

From spring to autumn, in many regions, dawn arrives rather early in the morning, meaning when we wake up, it’s often already daylight. By shifting clocks forward in the summer, so making us wake earlier, better use can be made of the available natural light for longer in the evenings. This is intended to increase productivity and improve safety, by reducing the number of road accidents at commuting times.

However, two time-changes the year disrupts our biological clocks and can cause confusion. Last year, the European Union held a summertime consultation, finding over 80% of respondents from across Europe preferred abolition, but any change would need approval from national governments and the European Parliament to become law.

**Did you know...**

The tropical year, associated with the earth’s orbit around the sun, takes exactly 365.242... days, that is 365 full days and almost one fourth of the day. This additional fraction of the day causes, that despite that common year is 365 days long, to equate the calendar year with a tropical year, once every four years we have a leap year of 366 days... except years divisible by 100, unless they are divisible by 400.... Easy, isn’t it... For example, year 1900 was not a leap year, because it is divisible by 100 and non-divisible by 400, but year 2000 is a leap year because it is divisible by both 100 and 400.Very occasionally there are 61 seconds in a minute, called a leap second, applied to UTC to synchronise atomic time with solar time. Since UTC was introduced on 1 January 1972, initially with a 10 second lag behind International Atomic Time (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 Service (IERS), but you probably won’t need to pause your next new year celebrations, as, so far, no leap second has been announced for 2019.On March 12, 2019 we were celebrating 30th anniversary of World Wide Web (www). 30 years (from 12.03.1989 to 12.03.2019) expressed in SI base unit of time it is exactly 946 684 800 s.