The Quest for a Uniform Time: From Ephemeris Time to Atomic Time
In his 1939 paper H. Spencer Jones presented compelling evidence that de Sitter’s proportionality constant Q did not differ from unity. This meant that the fluctuations of the Moon and Mercury were proportional to their respective mean motions. Jones also verified that the secular accelerations of the Sun and Mercury were proportional to their mean motions. (The secular accelerations of the Moon and Mercury, on the other hand, were not proportional to their respective mean motions, because tidal friction not only slows the Earth’s rotation but also changes the Moon’s mean motion.) The two proportionalities – of the fluctuations of the Moon and Mercury to their mean motions, and of the secular accelerations of the Sun and Mercury to their mean motions – were straightforwardly interpretable as due to variations in the Earth’s rotation, that is, as errors in the accepted measure of time. Other hypotheses left these proportionalities as unexplained coincidences. The general conclusion in the astronomical community was that the Earth’s rotation was slowing and fluctuating.
KeywordsUniversal Time Uniform Time Atomic Time Naval Observatory International Astronomical
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