The late Carl Sagan once said of Kepler that, " he was the first astrophysicist and the last scientific astrologer." His work with planets is something I have never gotten tired of reading. Kepler was deeply religious and tried to make his scientific work pure truth, even if it conflicted with the belief of the church, his findings were factual and true.
Being part astrologer, Saturn and Jupiter attracted his attention. Jupiter takes 12 years to orbit the Sun,( actually 11.9 ). Saturn takes 30 years ( really 29.5) to do the same. Jupiter spends about a year in each member of the Zodiac, Saturn needs about 30 months to do the same. Every 20 years these two giants meet, ( would you believe 19.85?) . Each conjunction occurs approximately 117 degrees apart. Thus, Jupiter and Saturn every 60 years always meet in the same sign of the Zodiac. If you multiply 117 X 3 the answer is 351 degrees, with 9 remaining to complete a circle, or 360 degrees. Those 9 degrees slowly pushes the meeting point eastward through the constellations that make up the Zodiac, until after 40 years of what Kepler called Trigons, Jupiter and Saturn have gone full circle.
As Kepler began to work these conjunctions backwards, he was startled to find that in 7 BC both Jupiter and Saturn had conjoined not once, which was common, but three times. The first on May 29, 7 BC, when they were about two full Moon's width apart. The second time on Sept. 29th. Again they moved apart, but on Dec. 6th they moved back together for the third and final time. This triple conjunction happens every 973 years. The first was the same thing that happens every 20 years when Jupiter catches up with Saturn. The second conjunction was caused by the Earth's movement in its orbit, causing an apparent reversing motion. Since that reversing motion is only temporary on the third conjunction once again Jupiter appeared to overtake Saturn.
Was this the heavenly sign that had convinced the Magi to go to Bethlehem? They probably would have known that the 20 year conjunction was going to occur by looking at their records. But the second and third conjunction they could not have predicted. To do that they would have had to know the Sun was in the middle of the Solar System, not the Earth, and also know a lot about the orbits of planets which they didn't, and since triple conjunctions only occur every 973 years they had no knowledge of it.
There are those who insist that the sign was a special star, but there are no records of any special star recorded by any of the other peoples of the region to verify that. But there is a lot of evidence that astrology was firmly rooted in all cultures of that time.
The next triple conjunction will occur in 2230 AD.
Dec 2 First quarter Moon.
6 Jupiter near crescent Moon.
10 Full Moon.
13-14 Geminid meteor shower, Not great because of nearly full Moon.
14-28 Mercury at its highest in southeastern sky at dawn.
17 Last Quarter Moon.
21-22 Longest night of the year.
23 Crescent Moon close to Mercury in early morning sky.
24 New Moon.
26 Bright Venus and the Moon in evening sky.