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astronomical day system Quick reference
A Dictionary of Weights, Measures, and Units
...day system astronomy . A calendrical scheme comprising a simple day count measured from a pre-historic instant, thus providing a reference base independent of perceived eras and years of the various calendars . Unlike the familiar day, the astronomical day begins at noon, its date that of the half-preceding regular day. Applied holistically around Earth, rather than by local time zones, each day begins at 12:00 Universal Time . Termed the Julian day number and identified by a prefixed JD , the count has its zero at 01 January 4713 bce . The...
