A brief historic perspective of the time measurement J. Mauricio López R. Centro Nacional de Metrología

State of the art evolution for time measurement during the last 400 years 1,E-02 5,E-05 1,E-08 1,E-12 1,E-01 1,E+00 7,E+00 2,E+00 5,E+00 1,E-13 1,E-09 1,E-05 1,E-01 1,E Year segundos/día Pendulum Quartz cristal First atomic clocks Stability seconds / Day 9 UTC Harrison clock

From the born of the the humanity until very recently the celestial bodies were used as “clocks”

Kukulkan pyramid (~1000 AC) The Maya culture leave us evidences about thier time measurement capabilities based on astronomical observations.

“Fire clocks” were used at the Midle Edge mainly on monasteries to mesure time in order to rule their religious activities

Clepsidras are another type of ancient clocks based on watter flow and the time needed for it to fill fixed volumes. In some cases the designs of this type of clocks were very complex.

The sun dial clocks were used for very long time as key pices to measure time. For “high accuracy” time measuremnt based on this type of clocks consideration of the coordinates where the clock is located are needed.

Sand clock

Galileo Galilei ( )  : la latitud de la posición del péndulo The Pendulum Galileo Galilei discovered that the oscillation period of a pendulum is independent of the amplitude of the oscillation (that is true only for small amplitudes of oscillation). Due to the period of the oscillation of a pendulum is much more shorter than the period of the Earth rotation, the pendulum is a very convenient method to measure time for short time periods.

After Galileo´s discovery about the period of a pendulum, pendulum clocks were the most accurated way to measure time for short term.

The invention of the mechanical clocks was a significant advance for the time measurement. Actually, the pendulum can be substituted by a mechanical bar taking advantage of properties of the inertial moment. This type of clock were very popular in Europe during several centuries.

The substitution of the gravity action on a piece of mass by springs, as energy source for mechanical clocks, was a mayor achievement on the progress of the time measurement capabilities. This “new” technology permitted the production of small “accurate” clocks.

The discovery of the oscillation properties of the Quartz crystals was a revolution on the way to measure time. With Quartz crystals were possible to built a small and accurate clocks and also to produces frequencies in a very large interval of values that can be used to built “high” accurate clocks for scientific purposes.

With a very interesting combination of Quartz crystal oscillators and quantum mechanics was possible to built the atomic clocks. Now a days the atomic clocks constitutes the most accurate way to measure time. It also interesting to notice that the best measurement (in terms of uncertainty) made by the humanity is performed with atomic clocks.

La calibración de la transición hiperfina del estado base del Cesio-133 El gráfico muestra la comparación entre la duración de segundos astronómicos y segundos atómicos. Se observa la inestabilidad de los segundos astronómicos. Mediciones similares fueron realizadas entre 1957 y 1967 en el National Physical Laboratory (NPL) de Inglaterra para calibrar la transición hiperfina del estado base del átomo de Cesio-133 cuyos resultados derivaron posteriormente (1967) en la adopción de una defición para el segundo en términos del áotmo de Cesio-133

The 13th Conférence Générale des Poids et Mesures (CGPM), Considering  That the definition of the second adopted by the Comité International des Poids et Mesures (CIPM) in 1956 (Resolution 1) and ratified by Resolution 9 of the 11th CGPM (1960), later upheld by Resolution 5 of the 12th CGPM (1964), is inadequate for the present needs of metrology,  That at its meeting of 1964 the Comité International des Poids et Mesures (CIPM), empowered by Resolution 5 of the 12th CGPM (1964), recommended, in order to fulfil these requirements, a caesium atomic frequency standard for temporary use,  That this frequency standard has now been sufficiently tested and found sufficiently accurate to provide a definition of the second fulfilling present requirements,  That the time has now come to replace the definition now in force of the unit of time of the Système International d'Unités by an atomic definition based on that standard, Decides The SI unit of time is the second defined as follows: "The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom"; Resolution 1 adopted by the CIPM at its meeting of 1956 and Resolution 9 of the 11th CGPM are now abrogated. Resolution 1 of the 13th CGPM (10-16 October 1967)

Evolution of the Cs atomic clocks