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As a consequence of the mismatch between calendar length and actual year length, a given date in a mobile calendar rotates slowly backward through the year, losing a day every four years, on average. A mobile calendar thus will match a given date in a solar calendar for four years, on average, before slipping to the previous date for another four-year visit. |
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Such a 365-day calendar will also return to a given date after a period of 1,461 of its years or 1,460 Julian years (365.254 = 1,461), a period of time known as a Sothic cycle. See the discussion of the ancient Egyptian calendar for details. |
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Mobile calendars have one singular advantage they are computationally simple. Record keeping and date calculation with such a calendar is very easy. Computation of elapsed periods of time is simple, as well. It was this fact that prompted Greek astronomers to use the Nabonassar Era calendar for their records. |
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9.7.2.1
Nabonassar Calendar |
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Refer to test program DTLT_080.C for tests of the basic functions. Program DTLT_124.C is a round-trip test of the conversion functions for all dates. DTLT_155.C checks the results of the conversions against a set of published equivalent dates. |
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This section describes the structural details of the calendar of the Nabonassar Era and discusses some historical background. |
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9.7.2.1.1.1
Structural Details |
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The Nabonassar Era calendar is one of the first mobile calendars known. It contains 12 months, each of 30 days, and an epagomenal period of five days, for a total of 365 days. No intercalation occurs. |
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The epoch of the calendar is 26 February 747 B.C., the day of the ascension of Nabonassar to the throne of Babylon. |
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Ptolemy, in his Almagest (III, 6), reckons the day as starting at noon at the meridian of Alexandria. |
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9.7.2.1.1.2
Historical Background |
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The Nabonassar Era calendar is, historically, one of the most important in the history of science. Modern astronomy has its base in the system of Newton, which was erected upon the foundation provided by Copernicus and Kepler. These men explicitly derived their work from that of Ptolemy, who represents a culmination of the Greek developments in astronomy. And the Greek work, as is now known, was based upon the Babylonian astronomical system. In particular, the lengthy series of Babylonian astronomical observations, extending over centuries, was crucial to the early studies. |
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