It’s time for a Christmas blog post, and there’s really only one thing to write about. On 21 December, there will be a great conjunction, in which Jupiter and Saturn will appear very close together in the early evening sky. Look for them near the western horizon, as they get closer and closer over the next two weeks. The diagram below (from fourmilab.ch) shows what the solar system will look like at conjunction. A line from Earth to Jupiter continues on to Saturn, but skims by the Sun (which is why the “kissing planets” are only visible in the early evening). These two giant planets have not appeared so close for several centuries.
The Christmas connection relates to the Magi mentioned in the Bible: “After Jesus was born in Bethlehem in Judea, during the time of King Herod, Magi from the east came to Jerusalem and asked, ‘Where is the one who has been born king of the Jews? We saw his star when it rose and have come to worship him.’” (Matthew 2:1–2, NIV)
One theory as to what the Magi might have seen was a set of similar conjunctions of Jupiter and Saturn in 7 BC (the great astronomer Johannes Kepler was the first to suggest this – and yes, thanks to a calendrical error 500 years later, Jesus was born around 7–4 BC). There were three such conjunctions in 7 BC: in May, in late September, and again in December. Here is a view of the September one, as seen from Ctesiphon in Parthia (from fourmilab.ch again). The planets Jupiter (♃) and Saturn (♄) would have risen just before sunset, and been visible in the evening twilight (and then throughout the night):
The Babylonians and Persians had an elaborate system of seeing omens in the sky. For example: “If Jupiter becomes steady in the morning: enemy kings will be reconciled. … If Jupiter passes Regulus and gets ahead of it, and afterwards Regulus, which it passed and got ahead of, stays with it in its setting, someone will rise, kill the king, and seize the throne.” So a conjunction is the sort of thing that would have gotten the attention of star-gazers in the Parthian Empire (the planet Jupiter was associated with the god Marduk and with kingship). Others have suggested a nova recorded in China in March of 5 BC. Yet others have suggested that a sequence of astronomical events led the star-gazers to search for a newborn king specifically in the frontier province of Judaea:
Countless paintings show the journey of the Magi across the desert (the one above is from James Tissot). If they were sensible, they would likely have travelled along the trade routes via Palmyra and Damascus. “A cold coming we had of it” wrote the poet T.S. Eliot (adapting lines from a 1622 homily by Lancelot Andrewes), “Just the worst time of the year / For a journey, and such a long journey.”
The real danger was Herod, of course. He had murdered his own sons Alexander and Aristobulus in 7 BC (and was to murder a third son, Antipater, in 4 BC). According to Macrobius (Saturnalia Book 2, IV, 11), Caesar Augustus had quipped “Better to be Herod’s swine [Greek hus] than his son [huios],” making a Greek pun referencing Herod’s Jewish religion and its prohibition on pork. Naturally, a man like that would be less than thrilled at the suggestion that another heir to the throne might exist:
“When King Herod heard this he was disturbed, and all Jerusalem with him. When he had called together all the people’s chief priests and teachers of the law, he asked them where the Messiah was to be born. ‘In Bethlehem in Judea,’ they replied, ‘for this is what the prophet has written:
“But you, Bethlehem, in the land of Judah,
are by no means least among the rulers of Judah;
for out of you will come a ruler
who will shepherd my people Israel.”’ [a summary of Micah 5:2–5]
Then Herod called the Magi secretly and found out from them the exact time the star had appeared. He sent them to Bethlehem and said, ‘Go and search carefully for the child. As soon as you find him, report to me, so that I too may go and worship him.’” (Matthew 2:3–8, NIV)
This is the kind of trouble you get when you mix star-gazing boffins and international diplomacy (I have been to enough international scientific events to know how that works). The gospel account makes a further confusing reference to the “star” and mentions the famous gifts of “gold, as to a king; myrrh, as to one who was mortal; and incense, as to a God”:
“After they had heard the king, they went on their way, and the star they had seen when it rose went ahead of them until it stopped over the place where the child was. When they saw the star, they were overjoyed. On coming to the house, they saw the child with his mother Mary, and they bowed down and worshiped him. Then they opened their treasures and presented him with gifts of gold, frankincense and myrrh. And having been warned in a dream not to go back to Herod, they returned to their country by another route.
When they had gone, an angel of the Lord appeared to Joseph in a dream. ‘Get up,’ he said, ‘take the child and his mother and escape to Egypt. Stay there until I tell you, for Herod is going to search for the child to kill him.’” (Matthew 2:9–13, NIV)
This “Flight into Egypt” has been a common subject of Christian art. The painting above, by Adam Elsheimer (1609), includes a beautifully painted Milky Way.
Egypt, of course, was a logical destination. We know first-century Alexandria primarily as the scientific and mathematical centre of the world of that time, but it also had a thriving Jewish community, with hundreds of thousands of Jews living in the city, and hundreds of thousands more in the rest of Egypt. The Hellenistic Jewish philosopher Philo was still a boy at this time, as was the Greek scientist Heron, but the Musaeum was fully active. Eventually, Alexandria was also to become one of the most important Christian cities, and the statement “gold, as to a king; myrrh, as to one who was mortal; and incense, as to a God” was from Alexandria. An active Coptic Church is still there (though suffering hardship).
But troubled as the world of 2020 may be, let me wish all my readers a Merry Christmas, and a better 2021!
Scientific Gems 