| tonian equations of planetary motion with relativistic corrections. The solutions give the angular positions of the sun, the moon, and the planets with their moons, all with reference to the fixed stars, in terms of time. The solutions are then inverted to give time as a function of the positions of the various objects. |
| There is a score of time services around the globe. They are interconnected by radio links and continuously transmit time signals. These signals are controlled by local clocks, found to be reliable by earlier standards. An astronomical time observation consists of the determination of simultaneity between two instants: the crossing of the center of a telescope by a planet or star is one, the indication of the local clock is the other. Each night several such events are observed, photographed, and labeled by local clock time. |
| Later, using the equations of celestial mechanics, it is determined from the photographic plates how fast or slow the clock was when the object passed the reference line. The role played in this process by the equations of astronomy is identical to the role played by the designs engraved on the astrolabe: they represent our cumulative knowledge about the motion of the stars, as seen from earth. In both cases an assumption has been made: accumulated knowledge is a more reliable guide to a determination of time than the hands of the local clocks are. Accordingly, it is usually the clocks that get readjusted and not our basic theories revised. |
| Obviously, the determination of just when any of the events whose temporal separation is of interest did, in fact, occur, cannot coincide with the epoch of their occurrence. By the time the corrections to the clock readings have been calculated the time signals have already been beamed around the globe. Bulletins are therefore compiled, giving the revised estimates of the times when the signals were "in fact" propagated. These corrections are coordinated among many observatories and in due course second-order corrections are published. Time so determined is called ephemeris or tabulated time. From the point of view of the user of an Ephemeris, it is an evenly flowing time assumed to have ideally equal units, in terms of which the positions of the sun, the moon, and the planets are listed. |
| Let us recall the way in which the dial maker measured time. Two sets of simultaneities, remembered ex post facto, allowed him to declare that the friend-with-cheese event was separated from the bark-of-dog event by 6 or 18 arbitrary units of time, provided he divided his day into 24 equal hours, and depending on which of the |
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