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Records show that Babylonian astronomical observations were conducted before 2000 B.C., but it was not until the eighth century B.C. that a series of systematic observations was started and recorded. These observations were extended over several centuries and provided the Babylonians with the data base required to found their system of astronomy. |
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The name Nabonassar is the one applied by writers in classical times to the monarch Nabu-nazir, who ascended the throne of Babylon in 747 B.C. and reigned until 733 B.C. Nabonassar himself had a relatively undistinguished reign during the late Assyrian period. |
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Evidence indicates that the era was used by Babylonian astronomers and was employed for historical purposes in Babylonia until the time of the introduction of the Seleucid Era (3 April 311 B.C.) following the death of Alexander the Great. The era is the basis of the famous Canon of Kings in the writings of Ptolemy, which was the only authentic known record of Babylonian and Assyrian history for many years. The era is also used by Hipparchus for recording celestial phenomena. |
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The epoch of the era can be determined precisely by recourse to the records of astronomical events in the Almagest of Ptolemy. The eclipses in particular are useful. |
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The exact circumstances of the introduction of the era are not known. Given the fact that Nabonassar did not himself inaugurate a new dynasty or empire, one might ask why Nabonassar's reign was chosen as the era for a calendar. The answer may lie in paleoastronomy. |
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It is possible that a series of very striking astronomical events that occurred in the first year of Nabonassar's reign caused Babylonian astronomers to begin keeping records of observations and introduced the era of Nabonassar as a means of recording them, in accordance with the prevailing custom of regnal dating. The epoch is 26 February 747 B.C, and the Babylonian New Year day (1 Nisan) began at sunset on the date 23 March 747 B.C. (Julian proleptic). |
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On 23 March 747 B.C., the evening sky seen in Babylon would have contained a thin crescent moon (aged 1 day and 18 hours), visible as the new moon for the first time and therefore heralding the beginning of the new month. Both Mars and Mercury appeared close to the moon in the Pleiades, while Venus appeared very brightly between the Pleiades and the Hyades. In the early evening, Jupiter also appeared, and Saturn was a morning star still later. |
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Venus and Jupiter appeared in a close conjunction in Scorpius on 12 December 747 B.C., and Mars and Jupiter appeared in another close conjunction in Scorpius on the first anniversary date of Nabonassar's reign, 26 February 746 B.C. |
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It is not certain that Babylonian astronomers were prompted to begin their observations by these phenomena, but if they were, the accession of Nabonassar, just a month before, would have been an ideal choice consistent with the customs of the area and time. It may be one of the many ironies of history that the most famous king of Babylonia is remembered not for any deeds, but because he ascended his throne in astronomically interesting times. |
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