Modern discussion of what identifiable astronomical phenomenon lies behind the biblical story of the so-called Star of Bethlehem was effectively begun by the astronomer Johann(es) Kep(p)ler (1571–1630), mathematician to Rudolph II, Holy Roman Emperor 1576–1612. In the years 1604–5 a supernova appeared in the constellation Ophiuchus and excited considerable discussion in Europe. Kepler kept a detailed record of his observations of the star. In the preceding year, 17 December 1603, at Prague he had also witnessed a conjunction of the planets Jupiter and Saturn with Mars moving into the vicinity soon after, which interested him in his capacity as court astrologer. The supernova appeared in the neighbourhood of these three planets. In medieval times the conjunction of Jupiter and Saturn, known as the “great conjunction” (recurring only once every 19·86 years on average), was regarded as of great astrological significance. Kepler calculated that a similar conjunction with Mars moving into the vicinity soon after had occurred in the year 7 B.C. = Julian year 39. On that occasion the conjunction had been a triple conjunction, a very much rarer event than the normal single conjunction.
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