First off the scholar who recalculated the calendar got Anno Domini wrong by at least 4 years. Herod died in 4 BC, proven by records of the date of a lunar eclipse on March 13, 4 B.C. So they looked for an astronomical event that is recorded before 4 B.C.
There were two strong candidates out of 6 possibles. These were the triple conjunction of Saturn and Jupiter (both represent kings) in the house of Pieces which is thought to represent Palestine in the astrology of the time. This occurred in 7 BC.
The other was a comet recorded in Chinese court records as having occurred in 5 BC. The programmes astronomers plumped for the comet as it met the description of having pointed at Jerusalem where the magi went to speak to Herod, then disappeared behind the sun to appear again pointing to Bethlehem. The weakness of this argument is that only the Magi seemed to have been aware of the "star", Herod was not aware of it which argues against a comet, but the conjunction was meat and drink to astrologers and only to astrologers.
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