Ephemeris time, relativity, and the problem of uniform time in astronomy.

This paper contains a discussion of the difficult problems of time and its uniformity, with application to astronomy. The existing status of Ephemeris Time and Atomic Time is reviewed, and a view is given of the fundamental precepts governing useful time scales. Those relativistic concepts are discussed which are required for a thorough understanding of time processes, including the theoretical and practical differences between coordinate time and proper time. Finally, the IAU resolutions dealing with Ephemeris Time are described together with the background and motivations leading to a wish to replace it by a 'dynamical' time scale.