The Christmas season is definitely upon us, so I thought I’d post something seasonal. Patrick Moore was sceptical of some of the explanations offered for the Star of Bethlehem, which led the Magi to the infant Jesus. In the 1983 edition of the Yearbook of Astronomy, he dismisses the idea that it could have been an particularly bright appearance of Venus, noting that the planet was far too well known to appear new. He also noted that no new stars were recorded in the astronomical records of the time.
However, in the 1981 Yearbook of Astronomy, he speculated that an extremely rare triple conjunction of the planets Jupiter and Saturn occurred in 7 BC, and that this could be the basis of the Star of Bethelehem because of the immense significance this would have had for contemporary astrologers. He wrote
If we regard the periods [time taken for planet to complete one rotation around the Sun – Beastrabban] as 12 and 30 years approximately, we see that Jupiter covers about 30 degrees a year, while Saturn moves through 12 degrees. Thus, Jupiter gainis 18 degrees a year on Saturn, and conjunction of the two planets can only occur at an interval of 20 years. If both planets travelled in circular orbits it can be shown that only one in six of these conjunctions could possibly be triple, and we should then expect to have a triple conjunction every 120 years. However, both of these giant planets have accentric orbits, and both are subject to severe perturbations, so tyhat this average is never realized. the last three tiple conjunctions of Jupiter and Saturn occurred in 1452, 1683 and 1940, and the intervals here are more than double the 120 years. In all of these cases, the time between the two oppositions was of the order of a day or less.
One tripole conjunction of Jupiter and Saturn has received a great deal of attnetion. This is the conjunction of BC7 which, it was suggested, could be the explanation for the Star of Bethlehem. This old idea had been rejected in many quaarters because the two planets were well separated in latitude and were, in any case, familiar objects to the Magi. In recent times the subject has been revived, but now the astrological significance of the even has been emphasized. This seems a more reasonable suggestion, though it does not explain all the details of the story. Certainly the rarity of this triple conjunction (which the Magi would never have witnessed before) would give added significance to the event. (p. 83).