Download A Triple Conjunction

Possible Explanations of
the Star of Bethlehem
The are many possible explanations of the Star of Bethlehem which have received
wide support over the years. The most widely accepted are variations on comets,
novae and conjunctions, although a new candidate, which has to be taken
seriously, is the idea of a planetary occultation. What are these candidates and
what are their strengths and weaknesses in each case?
Planetary conjunctions
The idea that a planetary conjunction might have been the Star of Bethlehem is usually
credited, erroneously, to Johannes Kepler. In fact, the planetary conjunction theory only
dates back to the middle of the 19th Century. Kepler only pointed out that a conjunction
of Jupiter and Saturn had occurred near the time of the Nativity whilst himself favouring
the nova/supernova hypothesis. A planetary conjunction is when two (or more) planets
approach each other in the sky, one passing due south of the other. Conjunctions can be
quite spectacular and last for several nights although a really compact conjunction lasts
for just a few hours.
It was in 1968 when Roger Sinnott wrote a highly influential article in Sky and
Telescope pointing to the June 17th 2 BC conjunction of Venus and Jupiter as having
been particularly spectacular from Babylon that this theory took off. Sinnott’s work is
still one of the finest ever carried out in this field and all the more laudable for having
been done from planetary tables, without the assistance of a computer. Sinnott
investigated conjunctions over a wide range of dates from 12 BC to 7 AD finding more
than 200 conjunctions of the major planets. He also found no less than 20 compact
groupings of three or four planets, of which only 4 would have been observable. After
carefully filtering the events, Sinnott concluded that the 2 BC conjunction, in Leo,
would have fitted the bill.
On June 17th 2 BC, as seen from Babylon, Venus and Jupiter would have set 3 hours
after sunset, with the two planets too close together to separate by eye, having closed
considerably in the time since sunset. In fact, we now know that the disk of Venus
actually passed in front of Jupiter, occulting it partially.
The problem with conjunction though is that they are too common. When the Magi have
been waiting several hundred years for the birth of the Messiah, they would have seen
all kinds of occultations and it is hard to believe that a single occultation, however
spectacular, could have been the Star of Bethlehem, quite apart from the fact that this
one happened several years too late.
A Triple Conjunction
Any pair of superior planets (that is, planets outside the Earth’s orbit) can give rise to a
triple conjunction whereby, instead of a single pass, the planets meet and separate three
times over a period of a few months. The more exterior a pair of planets are, the more
frequent are triple conjunctions relative to normal conjunctions, although the more
infrequently a conjunction of any kind will occur.
Jupiter and Saturn will enter conjunction about every 20 years. During the last
millennium BC, however, no less than 7 triple conjunctions also took place – one every
140 years, on average – although the interval varied from 40 years (as between 861 and
821 BC and again between 563 and 523 BC) to 377 years (as between 523 BC and 146
BC). Over the millennium there were 43 “normal” conjunctions between the two planets
and 7 “triple” conjunctions.
In December 1603 Johannes Kepler observed a conjunction of Jupiter and Saturn (a
normal one), followed by a massing of Jupiter, Saturn and Mars, when all three planets
were close together in the sky. He calculated that a similar set of circumstances would
have occurred in 7 BC. In fact, the 7 BC conjunction was a triple conjunction, although
Jupiter and Saturn were never much less than a degree apart.
In 1976 David Hughes popularised this triple conjunction and suggested that it might
explain the Star of Bethlehem, particularly as it happened in the constellation of Pisces,
a constellation associated with the Jews. Sceptics point out that a far more spectacular
triple conjunction (although in the constellation of Cancer) happened in 146/145 BC.
Similarly, triple conjunctions were seen in Pisces in 861/860 BC and in 981/980 BC,
during both of which the separation of Jupiter and Saturn was less than in 7 BC.
Another important point is that the 7 BC triple conjunction was observed from Babylon,
as was the massing of Jupiter, Saturn and Mars which followed, but the Babylonian
records give no sign that they found the phenomenon of any special interest.
An occultation
A recent and interesting suggestion is that the Star of Bethlehem might have been an
occultation. At first sight this seems unpromising. Between 20 BC and 1 AD the Moon
passed in front of (occulted) the four main planets (Venus, Mars, Jupiter and Saturn)
170 times. In other words, it is hardly a rare event. However, when we calculate which
of them would have been visible from Babylon in a dark sky, the number reduces in to
just 5 over the 20 years – somewhat better, although still hardly a rare event.
Recently, however, Michael Molnar of Rutgars University has drawn attention to an
occultation of Jupiter by the Moon in 6 BC. This took place in the constellation of Aries
and was similar to a later occultation of Venus which may be referred to in a coin
known as the Antioch coin. This coin shows a Ram (Aries?), the Moon and a bright star,
and what appears to be the track of a planet.
Molnar suggests that the 6 BC occultation was the Star of Bethlehem because of its
astrological associations, Aries being the ruling star sign of Judea and Jupiter
symbolising a king, with the occultation – the reappearance of the planet from behind
the Moon – symbolising a royal birth.
I am sceptical because the March 17th 6 BC occultation took place very close to the Sun
and just after sunset. It is hard to believe that it would have been observable with the
Sun just 3 degrees below the horizon and Jupiter 5 degrees above it. This theory though
has been very well received by many astronomers and popular writers and may be more
than just an interesting anecdote in the story of the Star of Bethlehem.
Comet(s)
A theory which has been popular for many years is that the Star of Bethlehem was a
comet. There is no doubt that a bright comet is a very spectacular event and would be an
impressive “star”, but scrutiny of the Chinese and Babylonian chronicles reveals no
evidence of a bright comet. There is an event observed in 5 BC which may be an
account of a comet, but there is no description of classical elements in Chinese reports
such as the tail and the comet’s movement which make it doubtful that this was a comet.
Similarly, the Chinese reports imply that the object was stationary – most uncometary in
an object seen for two and a half months.
Such doubts do not stop many “stars” from being depicted as comets – this practice is
particularly widespread in Spain where stylised comets which show a large star with a
flowing curved tail (thus getting the best of both worlds), adorn Christmas trees and
buildings everywhere.
Nova
If the object seen by the Chinese in 5 BC was not a comet, then it can only have been a
nova (we know, from the lack of a radio source and a visible remnant that it was not a
supernova, despite its long duration of visibility). The position (southern Aquila) is
consistent with having been a nova, although a little further south of the plane of the
Milky Way than is normal.
The date of its apparition (March 5 BC), coinciding with the best guess as to the date of
the birth of Jesus, its position in the sky (in the east at dawn) and long duration of
visibility (more than 70 days), make this a very plausible Star of Bethlehem. Again
though, we would have to ask the question “why this nova?” given that the Magi must
surely have observed dozens of novae over the centuries that the spent watching the sky.
It is a little hard to see what would have made this event particularly significant to the
Magi – apart from the fact that its date coincides, as far as we can guess, with the date
of the Nativity.