Astronomers are concerned with the measurement of time rather than with its philosophical definition [5, 9]. They have arrived at several different time-measures which I will briefly describe. A clock is a time-measuring device, and a "natural clock" is provided by a recurrent natural phenomenon such as the periodic passage of the Sun across the observer's meridian. A "man-made clock" is a device constructed by man which produces a repetitive phenomenon. It might be a dial with a rotating pointer that passes over marks made on the rim of the dial. A natural, and a man-made, clock proceed at the same rate if the natural phenomenon recurs at the same reading of the man-made clock.