MEASURES OF TIME IN ASTRONOMY

Abstract

ABSTRACT MEASURES OF TIME IN ASTRONOMY* J. DERRAL MULHOLLAND McDonald Observatory and Department of Astronomy, University of Texas at Austin Received 21 February 1972 "Time travels in divers paces... W. Shakespeare, A8 You Like It "The current definition of ephemeris time is philosophically repugnant, aesthetically , and completely inadequate." I.I. Shapiro, IAU Colloquium 9, 1970 August 'The time of an event given in the old scale, before the leap second, will be given as a date in the previous month, exceeding 24 hours ff necessary. The time of an event given in the scale after the step will be given as a date in the new month, with a negative time ff necessary. Trans. IAU, vol. 14B, p. 198 'The time is out ofjoint; 0 cursed spite, that ever I was born..." W. Shakespeare, Hamlet The prolfferation of time scales available to or known to the working astronomer has proceeded to the point that potentially harmful conflisions are widespread. The situation has been somewhat exacerbated by the major change recently effected in the time scale broadcast by WWV and similar services. A discussion is presented which attempts to the conceptual bases of time measures such as universal time and ephemeris time and ways in which the concepts are related to the actual determination of time. Areas of utility and obsolescence are indicated for the various kinds of clock time. Common measures of time not subject to clock-keeping are also described. Key words: time - year