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.



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.

The biggest problem though with the comet theory is that comets have the wrong associations. Throughout history comets have inspired fear and dread. Such attitudes still exist today in some parts of the world. Traditionally, comets have been associated with death, war, famine and disaster; such associations are known to have dated back at very least to Chinese astronomy several centuries BCE. In “Julius Caesar” William Shakespere sums up the prevailing attitude to comets:

“When beggars die there are no comets seen, but the heavens themselves blaze forth the death of princes.”

Lest we think that such attitudes are primitive, when Comet Delavan appeared in 1914 it was widely believed by even the educated public to be a harbinger of the forthcoming World War.

How reasonable is it to think that an event so intimately linked with death and disaster in so many cultures, including the Judeo-Roman culture around the time of the Nativity (the appearance of a bright comet in 77BC, which supposedly had the form of a sword was, at the time, associated with the fall of Jerusalem), could have been regarded as a sign of the birth of the Messiah? Had a bright comet appeared, surely King Herod would have known of it and would have feared it, something completely different to the impression that Matthew’s Gospel gives.



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.