50th anniversary of the atomic second

50th anniversary of the atomic second

On 13 October 2017, atom-based time measurement will have replaced astronomic-based time measurement for exactly 50 years.

The second, as time unit, was originally defined as the fraction 1/86 400 of the mean solar day. This “mean solar day” was determined by astronomers from observations of the passage of stars through the sky. Due to irregularities of the Earth rotation speed, this second was variable in duration. In 1960, the 11th General Conference on Weights and Measures approved a new definition of the second, based on a fraction of the tropic year 1900 duration.

However, experimental research had already demonstrated that a much more exact and stable time unit can be generated by using transitions between two energy levels of an atom or a molecule. Considering that a high precision definition of the International System time unit is essential for science and technology, the 13th General Conference on Weights and Measures replaced on 13 October 1967 the definition of the second by the following:

“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.”

The atomic time has led to many scientific and technological advances. The GPS and the synchronization in communication networks are examples of them.

Presently, the Royal Observatory of Belgium realizes and distributes the accurate second and time with its set of atomic clocks. These latter are also used for the working of the Galileo system.

Contacts :

Dr. Pascale Defraigne
Email: p.defraigne@oma.be
Tel (before 12 October 2017): 02/3730260

Dr Bruno Bertrand
Email: b.bertrand@oma.be
Tel: 02/373 02 83