What is time?

“It’s a creation of man, time! Nature provides us with the oscillation and man counts these oscillations. The only condition is that we must not make mistakes when counting and we must always do it, so that we keep time!”
Giovanni Busca, Director of the Neuchâtel Observatory from 1988 to 2001

Three modes of oscillation: rotation of the earth, pendulum movement and movement of an atom.


Since antiquity, time measurement systems have been based on the rotation that the Earth makes on itself and around the sun. Astronomers call this objective time.

Time was first measured with instruments that determine the hours (clepsydra, sundial, etc.). They were perfected in the 18th century with the introduction of the minute hand and later, the seconds hand. At the same time, the observatories became the guarantors of measurement standards. Astronomers used spectacles to determine the time and timekeepers to measure the time interval between two moments.

Until the 19th century every city had its local time, but the acceleration of transport and communications required precise synchronisation of the different local and national times. In Switzerland, the Federal Council adopted the Berne mean time for all postal and telegraphic traffic in 1853; it was defined by the Neuchâtel Observatory (Messerli, 2015). Universal time was introduced in 1884 at the Washington Conference, which set the zero meridian at Greenwich.

In 1967, the atomic clock was invented, a revolution that led to a new definition of the second. It is based on 9’192’631’770 periods of oscillation of the frequency of caesium 133. The accuracy of current models is such that they lose 1 second every 13.8 billion years, the age of the Universe! The observation of the passage of the stars has been abandoned.

While time is determined with increasing precision, Einstein’s theory of relativity links space and time: time thus becomes relative, as it varies and slows down as we approach the speed of light. The philosopher Henri Bergson proposes to adopt the subjective time, lived and felt by each human being. He associates it with the notion of duration. Is time a line or a circle? The circle seduces the great civilizations because it represents perfection and the cycle of the stars. Linear time, with the concept of the “arrow of time”, has imposed itself, since it illustrates the principle of causality.

What is the nature of time? (diagram, pictures,…)

Time measurement (diagram, pictures,…)