History of time measurement

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 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. 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.

French revolution and decimal time

The French Revolution didn’t just cause a political transformation; the new republic wanted its system of measurements free of sovereign influence too. Therefore, in 1791, in a spirit of enlightened new beginnings, replacing arbitrary units such as the length of a king’s arm, the metre was standardised as the unit of length. The length of the metre was – by French law – defined to equal the 10 millionth part of the distance from the equator to the North Pole. Likewise, the litre was defined to be the volume of a cube with the edge length of 1/10 metre x 1/10 metre x 1/10 metre, and the kilogram as the mass of 1 litre of water. From 22 September 1792, time also became part of the decimal system in France – in which a day lasted 10 hours, each hour had 100 minutes and 100 seconds equalled one minute. But revolutions can only go so far, and the idea wasn’t accepted. In April 1795, France returned to the 24 hours, 60 minutes, 60 seconds system.