Astronomical Time

A definition of time-scales and criteria of uniformity of a time-scale are introduced in the first part of the paper. These considerations are applied to time-scales currently used in Astronomy: Sidereal and Universal Times. Each of them is defined, their equivalence is proved, and the precision with which each can be determined is discussed. The irregularities of the rotation of the Earth are the causes of the non-uniformity of these time-scales. A uniform gravitational time, Ephemeris Time, is then defined and the precision of its determination is discussed. All the three astronomical time-scales have a necessity of their own and cannot be interchanged. In order to reach a better understanding of the concept of time, it is necessary that all time-scales — including atomic time — should coexist and be compared with the highest possible precision.