Until the early 1960s the resources for studying the ancient sky were minimal. Astronomers such as Johannes Kepler could calculate the positions of the planets two thousand years ago, but laboriously by hand. Astronomers could make reasonable estimates of when Comet Halley would return and could look back to see if there was any bright comet observed by the Chinese at about the right time: many of the associations with past observations turned out to be wrong.
Two things changed archeoastronomy into a precise science: (1) the advent of the electronic computer and (2) the systematic translation and cataloguing of first Chinese and, later, Babylonian astronomical archives. Strangely, despite the huge number of Roman records that exist, few astronomical phenomena are recorded and many of the ones that are, are doubtful: despite Roman interest in auguries of all kinds, there was not much in the way of a written record to the point that there are only two Roman observations that appear to have been of Halley’s Comet.
With electronic computers we can calculate with great precision what the sky would have looked like at any time of day or night for any position on Earth, going back several thousand years. In fact, one of my favourite tricks when doing a Star of Bethlehem talk in a planetarium is to set the sky back 2000 years and move the observer’s perspective from wherever the talk is being held to Jerusalem. There is something very special about seeing the sky wind back and feeling the sensation that you are in a time machine, going back in history to the time of the Nativity. We know that if someone was looking at the dawn sky from Jerusalem on Christmas Day of 5 BC, Scorpius would be rising and the East and Sagittarius in the South-East, with Venus a morning star, approaching Superior Conjunction, low in Sagittarius. Much of the East and South-East would have been dominated by the relatively dim constellations of Ophiuchus and Libra, with Hercules and Aquarius above them.
Early attempts to catalogue ancient Chinese observations were plagued with errors and inaccuracies but, with experience, the catalogues became more and more reliable and revealed themselves to be a treasure-trove of observations of every kind of observation. There are such detailed records that we can re-construct that light curve and even the changing colour of the supernovae of 1054 and 1572, we can identify a possible period of very low solar activity between 1400 and 1510 because Chinese observations of sunspots are lacking in that period. And, of course, the Chinese observed Halley’s Comet in 240 BC, 87 BC, 12 BC, etc. The movement of the comet is described as is, frequently, its tail: in 141 AD we even have a detailed description of the tail’s colour and length[1]. Chinese chronicles even record aurorae and allow us to identify the appearance and activity of many meteor showers and meteor storms in the past.
Babylonian records at the time are much more fragmentary, but even they have had their uses. Until recently the 163 BC apparition of Halley’s Comet had resisted identification – it would have been at its brightest around the time of the Chinese monsoon – but was finally identified in a Babylonian tablet. Unfortunately, the context of the surviving Babylonian observations is uncertain: we know that they are copies of copies, but we do not know if the few tablets that we have were valuable records, or just thrown out as junk.
There has been intense scrutiny of the Chinese chronicles between 20 BC and 1 BC. Unfortunately, these appear to be seriously incomplete, as just four events are recorded. The objects in 12 BC and 10 BC are clearly Comet Halley: the 10 BC event is what is called a ghost event – a transcription of the 12 BC object with a wrong date.
We can compare these numbers with the statistics for discoveries in recent times, since western astronomy was born. Over recent centuries there has typically been a bright comet observed about every 10 years and a naked-eye comet about every 3 years, but there have been wide variations in this numbers. During the 20th Century there was a bright nova[2] approximately every 10 years, but just six novae, none of them especially bright, were detected between 1600 and 1900 (as a nova is usually only bright for a few days, it is easy to miss them unless a lot of people are watching the sky carefully). And, since the year 1000, just five Galactic supernovae have been observed: in 1006, 1054, 1181, 1572 and 1604[3].
So, we can conclude that the Chinese should, in twenty years, have observed one or two bright comets, one or two bright novae and four or five fainter comets. Clearly, their records are not complete but, we have what we have.
Interest focuses around the Chinese 5 BC and 4 BC objects that we will focus on in the second half of this post, but also on two, earlier events that were not recorded by the Chinese:
- The triple conjunction of Jupiter and Saturn in 7 BC.
- An occultation of Jupiter by the Moon in 6 BC.
The Babylonian Conjunction
Conjunctions of Jupiter and Saturn are relatively common, occurring almost exactly every twenty years: between January 1st 1 AD and December 31st 2000 there were 122. Why the extra events? Occasionally – and it happened exactly eleven times in those two thousand years – the position of the Earth, Jupiter and Saturn line up in such a way that we see three conjunctions in the space of about six months as first the Earth overtakes Jupiter in its orbit and then Jupiter overtakes Saturn, such that the changing perspective lines up the two outer planets three times in quick succession. These events are called Triple Conjunctions. The last one happened in 1980/81, the previous one in 1940/41 but, before that, there had not been one since 1682/83. They are rarely spectacular as Saturn is at least two magnitudes fainter than Jupiter and can be more than three magnitudes fainter if its rings are closed and, in a triple conjunction, rarely is the closest separation of the planets less than a degree in the sky. Astrologically though, they are significant, as Jupiter is the king of planets and Saturn, the bringer of change.
On May 29th, September 30th and December 5th 7 BC Jupiter and Saturn approached to 
almost exactly one degree and then separated again. This was in no way a spectacular event because the difference in brightness between the two planets was nearly three and a half magnitudes. All three events occurred a few degrees South-East of the star ζ Psc (Zeta Piscium).
Attention was first drawn to this event by Johannes Kepler, in 1604. He calculated that a conjunction of Jupiter and Saturn would take place in the constellation of Sagittarius between December 16th and 18th 1603. As the last important astronomer who was also an astrologer, this event excited him because it was the third in a series that had taken place successively in Leo, Aries and now, Sagittarius, the three constellations associated with fire (each constellation of the zodiac was associated with one of the elements: earth, water, air and fire). For an astrologer, this triplet of conjunctions over forty years in fire signs constituted a fiery trigon. Then, Mars joined Jupiter and Saturn, forming a triangle in Sagittarius, thus forming a “fiery trigon within a fiery trigon”. Excitement in Germany reached fever-pitch in expectation of calamities and the appearance of a comet. What followed was a brilliant new star (now known as Kepler’s Supernova), which appeared on October 10th 1604. Kepler calculated that a similar conjunction of Jupiter and Saturn would have occurred in 7 BC, before the Nativity and speculated that, like the 1603 conjunction, it was followed by a bright new star – the Star of Bethlehem. What is more, he calculated that it would have happened in Pisces, a constellation that he associated with the Jews: as is well known, early Christians identified themselves using a fish sign, standing for ιχυς – ichthus,”fish” in Greek and the abbreviation of Ιεσυς Χριστος Θεου Υιος Σοτερ, Iesous Christos Theou Yios Soter, i.e. Jesus Christ, Son of God, Saviour. Curiously, three of the seven triple conjunctions of the final millennium BC took place in Pisces: the early ones being in in 861/860 BC and in 981/980 BC.
The Triple Conjunction was popularised as an explanation of the Star of Bethlehem by British astronomer David Hughes and is still one of the most popular theories. He used the movements of the two planets in a very imaginative way to explain how the planets could lead the Magi first one way, then disappear and then stand over the baby.
After the triple conjunction ended and the planets slipped into the evening twilight, Mars joined the grouping, forming a planetary massing, also in Pisces.
Tablet BM 35429 in the British Museum is an astronomical almanac for Seleucid Era 305 
and describes the first two conjunctions this way[4]:
“Month II, the 1st of which will follow the 29th of the previous month. Jupiter and Saturn in Pisces, Venus in Gemini, Mars in Virgo.
Month VI, the 1st of which will follow the 29th of the previous month. Jupiter and Saturn in Pisces, Venus in Libra, Mars in Scorpio… On the 21st, Jupiter’s acronychal rising… On the 21st, Saturn’s acronychal rising.”
If the triple conjunction’s significance excited the Babylonians, they hid the fact well.
The Persian occultation
What about the 6 BC occultation of Jupiter by the Moon?
In 7 BC and 6 BC there were no less than twenty-five occultations of Venus, Mars, Jupiter and Saturn by the Moon, of which four were nominally visible from Persia. Two of the occultations though would have occurred in daylight. Of the remaining two, the occultation of Mars on February 16th 7 BC would have been quite easy to observe, despite coming close to Full Moon. It is the other one that has attracted the attention of Michael Molnar. March 20th 6 BC, 18:58 local time. The Sun has just set and is 3 degrees below the horizon. A 1% illuminated Moon is just 4 degrees above the horizon. It is very debatable 
whether or not the Moon would have been visible and certain that Jupiter would not have been observable as it reappeared over the course of a couple of minutes from the finest possible sliver of illuminated Moon as shown, left. This event took place in the constellation of Aries, which Molnar associates with the Jews, mainly through a Roman coin that has been christened the Antioch coin showing a ram looking over its shoulder at a bright star [alert readers will notice that this is the third zodiacal sign after Leo and Pisces that different authors have associated with the Jews]. The theory continues that the disappearance of Jupiter behind the Moon and its sudden re-appearance would have suggested the death of one King and the birth of a new one, with Aries pointing to Judea and the dying King Herod.
The argument is that even though there is no way that the Magi could have observed the occultation, especially bearing in mind the dusty desert air, they would have known that it had happened. The theory has become very popular in academic circles and treated as perhaps the most plausible Star of all by many investigators.
How many occultations of Jupiter by the Moon were there around this time? Between January 1st 20 BC and December 31st 20 AD there were SEVENTY-NINE. Yes, in numbers, 79! Of these, no less than 14 were visible from Persia. And, if you argue that the Magi would have known of the invisible event on March 20th 6 BC, they would certainly have known of the others too, many of which would have also occurred in Aries.
Particularly spectacular occultations of Jupiter by the crescent Moon would have been observed from Persia on July 13th 17 BC, January 17th 3 AD and March 14th 17 AD as well as a magnificent event with a Moon just past half on January 22nd 13 AD and another with the Moon about to set on November 18th 6 AD.
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In the next post we will look more closely at the Chinese observations of the 5 BC and 4 BC events.
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[1] The Chinese report the blue colour that we now know is characteristic of the emission of carbon monoxide gas.
[2] Maximum of magnitude 2 or brighter.
[3] It is possible that John Flamstead detected the highly reddened progenitor of the supernova remnant Cassiopeia A in 1680, but this is very uncertain.
[4] The full text of the translation is available here: http://www.jstor.org/stable/4200210.
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