An astronomical or navigational almanac can best be thought of as a device for connecting an observer with celestial objects. For an observer with a known position and time the almanac allows the observer to identify the celestial objects. Conversely, if the observer knows what objects he is observing and when, the almanac allows him to determine his position. In either case, knowledge of the time is crucial in providing this link between the celestial objects and the observer through the almanac. This paper summarizes the various time scales used in the astronomical and nautical almanacs produced jointly by the US Naval Observatory and HM Nautical Almanac Office.
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