There have been dozens, if not hundreds of theories to explain the Star of Bethlehem. In this sense it is similar to the death of the dinosaurs where many dozens of theories have been proposed, but just two or three are real candidates to be correct.
Here are a few of the less likely ones:
If you look at the Christmas sky in 2017 you may notice that something is missing. Most years Venus is prominently visible in either the evening sky on Christmas Eve or the morning sky on Christmas Day (for example, on Christmas Day 2018 it will be a morning star in the constellation of Libra). When it is, many people notice it and ask if it is the Star of Bethlehem. Certainly, Venus is so brilliant that it is incredibly striking yet, in our urban society, rarely do we see a star-filled night sky and few people can recognise the planets. Our ancestors though were completely different. For them, the constellations and the planets were as familiar as their own neighbourhood. Certainly, the Wise Men, the Magi, would have been familiar with the planets and their movements and would have used the Sun by day and the stars by night to navigate. One of the bonne mots of the sadly missed British populariser, Patrick Moore was: “if the Wise Men were fooled by Venus, they could not have been very wise!” This sums it up perfectly. Even so, every decade or so someone comes up with a new explanation for how a special configuration of Venus could be the explanation for the Star of Bethlehem.
- An atmospheric phenomenon
Various atmospheric phenomena have been proposed, such as aurorae and ball lightning. Aurorae are very rare at the latitude of the Middle East. Just occasionally they can be seen right down to the Equator – in 1909 a display was seen from Singapore, at latitude one and a half degrees North – and it has been suggested that an unusually bright display could have impressed the Magi enough to interpret as a sign of the birth of the Messiah. This theory has multiple problems. To start with, an aurora may last a few minutes, or exceptionally, a few hours, but certainly not long enough to follow over a period of weeks or months. Secondly, the Greek text definitely says that the Star appeared in the East: aurorae are always seen in the North save from Arctic latitudes. Third, the Magi followed the Star from Jerusalem to Bethlehem, that is, southwards – see the previous comment!
What about ball lightning? This is a rare atmospheric phenomenon. It fits the bill in the sense of being extremely unusual – ball lightning is so rare that science doubted its existence until relatively recently. It is also a phenomenon that moves in the sky, consistent with those who interpret Matthew as saying that the Star moved physically and that it appeared and disappeared at different times. However, ball lightning is another short-lived phenomenon: it seems unlikely in the extreme that one could last for even a few hours, let alone weeks or months.
- A meteor, or meteors
This theory was championed in various forms by British astronomer and populariser, Patrick Moore. In its original form he suggested that a bright meteor sent the Wise Men on their way to Jerusalem and another appeared over Bethlehem to indicate that they had arrived. Later, this was modified to suggest that it could have been a Cyrillid meteor stream. Around 9 in the evening Eastern Time, on February 9th 1913, observers from Canada to Bermuda and the Atlantic off the coast of Brazil, who were fortunate enough to have clear skies were astonished to see a procession of bright meteors crossing the sky, one after another, along the same track. Over the course of some five minutes, between 40 and 60 bright meteors were seen, each taking around half a minute to cross the sky such that multiple meteors were crossing the sky, one behind another. The meteor stream was christened the Cyrillids as it occurred on the eve of St Cyril’s Day. The characteristics suggest that the meteoroids were relatively large objects (maybe a few tens of grammes each) in a temporary, captured Earth orbit that decayed into the atmosphere.
The idea of a meteor stream with its insistent east-to-west motion would certainly have suggested to the Wise Men to go west. It does though present some difficulties. A normal meteor, even a bright one, only lasts a second or two; the Cyrillid stream lasted maybe five minutes – did the Magi have jet-propelled camels capable of crossing the desert at great speed? To travel east-to-west any hypothetical meteoroid stream would have had to have entered a retrograde orbit. Then, we needed a second stream, in a polar orbit, to appear just at the right time over the cave and indicate the correct place. It is just too implausible.
This is one of the more curious theories. On February 2nd 8 BC Uranus was in conjunction with Saturn, passing approximately a degree north, very low in the dawn sky and then, on 27th February 6 BC passing even closer to Jupiter, but even lower in the dawn sky. The theory is that the proximity of Saturn attracted the attention of the Magi to Uranus’s slow movement. They then lost it again, only to recover Uranus when it passed close to Jupiter, two years later. Even if the Magi were able to spot the tiny, star-like point that was Uranus in a brightening sky, it is hardly fits the bit as a prominent object. Yes, it can be seen with the naked-eye, if you know where to look, but was hopelessly lost in the star clouds of Sagittarius at the time.
- Halley’s Comet
Again, a classic theory, which has received a clever, modern twist. When Halley’s Comet first returned in 1758, according to the prediction of Edmond Halley, it created great excitement. People calculated back at approximately 76 year intervals and realised that it would have appeared close to the date of the Nativity. In fact, over the last two thousand years, Halley’s period has been almost exactly 77 years and we now know that Halley duly returned in 12 BC and was observed by Chinese astronomers for a total of 56 days from August 26th. If the Magi saw their Star in 12 BC and arrived in Bethlehem seven years later, one has to say that they were in no hurry!
The modern twist is an ingenious one by Rod Jenkins, a fellow-Bristolian. He suggested that a combination of the arrival of Tiridates in Rome, combined with the 66 AD return of Halley’s Comet that Josephus suggests predicted the fall of Jerusalem was the inspiration for Matthew’s addition of the Star. It is possible, although the Messiah was supposed to free his people, not to see that country destroyed, as Judea was when Rome put down the Jewish uprising.
- A Near-Earth Asteroid
Another modern theory. On April 13th 2029 the asteroid Apophis, which is about 370 metres across, will pass around 36000 kilometres from the centre of the Earth and 31200km above the Earth’s surface. As seen from Earth it will become visible to the naked eye over eastern Siberia and will brighten as it crosses the Atlantic, reaching a magnitude of approximately three and a half: bright enough to see easily. In total it will take about 5 hours to cross the sky. It has been suggested that a previous, even closer pass by another asteroid, might have been the star that moved across the sky for the Magi.
Again, we run into the habitual problems: an asteroid passing so close to Earth would have been bright only for an hour or two and would have moved impossibly fast for a camel to follow. And, once again, the movement is in the wrong direction: it would have led the Magi away from Jerusalem.
- A supernova
This theory was popularised by Arthur C. Clarke in his 1955 short story “The Star”. In it, a Jesuit scientist leads an expedition to the ruined planet of an ancient civilisation destroyed by a supernova and discovers that it was the Star that shone over Bethlehem. At the time it was an inviting and very plausible theory, as the Chinese observations that have allowed so many ancient phenomena to be identified were still to be exploited. In fact, we know now that the very earliest supernova that can be dated was the 185 AD supernova observed close to Alpha Centauri in the sky that was visible for eight months. Unfortunately, there is no supernova that we know that would have shone close to the date of the Nativity.
- A planetary conjunction
This is another classic theory. Every year there are about ten conjunctions – close encounters – of two naked-eye planets and typically one occasion when multiple planets meet in a small radius in the sky. They can be extraordinarily striking, like this conjunction of Venus and Jupiter very low in the down sky in mid-November 2017. A conjunction has astrological implications both through the planets involved and the constellation in which the event happens. In 1968, Roger Sinnott suggested that the evening June 17th 2 BC conjunction of Venus and Jupiter would have been especially spectacular as, seen from Babylon, with the two planets merging into a single “star” just before setting. Given that it occurred in the constellation of Leo, close to the royal star, Regulus with its Old Testament connections (e.g. “Judah is a Lion’s whelp”), he argued that of all the conjunctions that occurred in a twenty year range, it was by far the most spectacular and significant and the best candidate to be THE Star.
Of course, the obvious objection is that with very few dissenters, it is agreed that King Herod died in late March or early April 4 BC, two and a half years BEFORE this conjunction. Of course, maybe the Magi had a time machine…
- A UFO
If we take everything in Matthew’s account literally, as some people do and add some imagination, the Star of Bethlehem appeared and disappeared at will, moved hither and thither, changed direction when it wished and stopped when convenient. This is the living definition of a UFO. A few people event take things further and suggest that Jesus was actually an astronaut brought to Earth by an expedition from a distant planet.
Of course, I cannot prove that the Star of Bethlehem was not a flying saucer, but that is not the same as affirming that it was. Personally, this is not one of the theories that I find more convincing on the available evidence.
So, we get close to the end of our story. Now we must look at what phenomena were seen around the time of the Nativity and, for that, we go to ancient China.
 Technically, on Christmas Day 2017, Venus is a morning star as it rises a few minutes before sunrise but, in reality, there is no possibility of seeing it as it passes behind the Sun at Superior Conjunction.
 If you have not read it, this wonderful story is here: https://sites.uni.edu/morgans/astro/course/TheStar.pdf. It won the 1956 Hugo – the science fiction equivalent of the Oscars (or Spain’s Goyas).
 Jacob says this in Genesis 49:9-10.
 A religion based on this concept is central to the plot of the Arthur C. Clarke novel “Rendezvous with Rama”.