Les « Années » et la rotation de la Terre

Abstract

Jean-Paul Parisot and Georges Lambert. The « Year » and the Earth's Rotation. The following text, while recalling the rather confused and sinuous conditions leading to the elaboration of the Western calendar, shows, on the contrary, that contemporary astronomy has succeeded in proving that physical time, linked to various movements of the Earth and the stars, is neither fixed nor perfectly explained: the length of a day changes, and there are several different ways of defining a year in the physical sense of the word ; the movement of the Moon also evolves. All these variations only take on notable significance on the geological time scale ; however, these modifications probably do account for the difference of several hours between the observations of eclipses reported by Mesopotamian or Egyptian astronomers and the numerical values of these eclipses calculated on the basis of parameters supposed constant. Moreover, further consideration should be given to the fact, generally not taken into account, that, for the moment, not enough is known about the factors that contribute towards the slowing down of the Earth's movement to be able to predict this evolution accurately. The measurement of time is fundamental to historical science. This incursion into the astronomer's vision of time can help to better understand the evolution which created what is imagined to be THE calendar, and to better appreciate, from the example of contemporary astronomy, how relative the notion of « exact » science is. This text, in addition to its bearing on archeology and ancient history, more generally offers concrete, first-rate food for thought on the relationship between geological time and historical time.