The influence of these forces on the orientation of the Earth in space is part of recent research, which goal it is to create precise models on the changes in the rotation and the spatial orientation of the Earth.

The exact hour we use today in the world is no longer connected to the Sun or celestial bodies, neither to traditional clocks (picture 3), It is determined by atomic clocks (picture 4). The legal time in the world is the Universally Coordinated Time (UTC). This time scale is determined by the “Bureau International des Poids et Mesures” (BIPM, Paris), based on an average of about 250 atomic clocks installed all over the world in about 40 laboratories.

One of these labs is the Royal Observatory of Belgium, which uses four atomic clocks (3 caesium and 1 hydrogen maser). The principle of an atomic clock is to generate the second of time starting from the frequency of radiation emitted or absorbed by atoms when the energy level changes. The accuracy of present atomic clocks lies in the order of a few billionths of a second per day. This means that when all errors are counted, one will have a difference of only one second after about thirty thousand years. The comparison of atomic clocks belonging to the different laboratories is done by the GPS satellite system. (picture 5)

The GPS (Global Positioning System) is a satellite navigation system used to provide time, position and speed to any receiver close to the Earth’s surface. The accuracy of positioning via GPS goes from 100 meters to a few meters in real time, dependent on the signals registered by the receiver. For geodetic applications in delayed time, GPS reaches even millimetre accuracy.