Since the end of the 20th century, the exact measurement of time has been one of the Observatory’s main specialities. The Earth was the first timekeeper, but its motion is irregular, so time was redefined in 1967 using the frequency of a caesium atom. The practical determination of time and improvements in the measurement of time and of standard frequencies led to the creation of a specialized laboratory, similar to a few others abroad. These laboratories collaborate intensively, and also with a number of physics laboratories. The Parisian laboratory has created the extraordinarily stable atomic clocks, which are vital for the Global Positioning System (GPS) and Galileo, and for the testing of General Relativity.

caption : Laboratory of the LPTF - credits : Observatoire de Paris