Science: History is written in the stars

The Star of Bethlehem is a perennial topic at Christmas. It is one of the great mysteries debated for almost 2,000 years, but shall we ever know for certain what it was? Some people argue that it never even existed. Astronomers seek to explain it with any one of a number of astronomical phenomena seen around the time of the Nativity. Many Christians feel that it was a miracle that, as a matter of faith, needs no explaining. However, modern scholarship is slowly moving towards solving the mystery. Though we probably shall never know for certain, we may be as close to solving the mystery of the Star of Bethlehem as we ever can be.

The gospel of St Matthew talks about the appearance of a "star", which alerted the Magi to the birth of the Messiah.

A writer called Origen discussed the nature of the star in his work Contra Celsum, around AD248. Hundreds of other authors have speculated, including the great astronomer Johannes Kepler who, almost 400 years ago, proposed a theory that was popular at least until the early Sixties. Arthur C Clarke worked it into "The Star", one of the finest of his short stories, published in the mid-Fifties. Around 1302 an Italian painter named Giotto di Bondone painted the Star in a fresco in the Scrovegni Chapel in Padua; it is still one of the most famous of all representations of the Christmas star. What though, was this star?

If you are out walking early on Christmas morning and the sky is clear, you cannot fail to notice a brilliant "star". Many people who see it will wonder whether it is the Star of Bethlehem; others, knowing it to be the planet Venus, will wonder whether this most beautiful of objects could have been the Star. However, it is certain that it was not. To understand why, we must think a little about who the Magi were and where they may have come from.

Almost nothing is known of the Magi. St Luke, the only other Gospel writer to mention the Nativity, mentions neither them, nor the Star. The idea that there were three of them seems to derive from the fact that they offered three gifts to the baby Jesus. It is likely that the Magi were astrologers. They would have watched the sky diligently, looking for signs and casting horoscopes. As such, they would have known the planets well and would never have been fooled by Venus.

We do not know about the origin of the Magi. Some scholars have suggested they came from Babylonia, a centre of early astrology. Some early representations of the Nativity show the Magi in Persian dress. If they were Persian, they could have travelled nearly 1,000 miles over a mountain range, two deserts and two major rivers to reach their destination. If so, the Star must have been a very important sign to them.

So what was the Star, and when might it have appeared? Modern scholarship suggests that Christ was born around 5BC. We can date the death of King Herod quite exactly at March or April 4BC and so can assume that Jesus was likely to have been born within about two years of Herod's death. We are also quite certain that the first Christmas was not on 25 December. This was a date first used perhaps two or three centuries later, as the early Christian church adapted the ancient pagan festival of Saturnalia - the shortest day - to the new Christian tradition. Many of the traditions of Saturnalia are familiar in our modern Christmas, such as feasting, and decorating houses with green branches.

In fact, the suggestion that the shepherds were watching their flocks by night is consistent with the Nativity occurring at lambing-time - around March or April. This is also consistent with other contextual clues, such as the inn's being full; this would have happened if Mary and Joseph had arrived at around the time of the Passover celebration.

There is thus good reason to think that the Nativity occurred round about March in 5BC; so did anything astronomical happen then that could be interpreted as a sign?

Many theories have been proposed. One of the most popular is that of the planetary conjunction, when two planets pass very close to one another. Sometimes the configuration can be striking. According to the planets involved and the constellation where the conjunction took place, the event may have had great astrological significance for the Magi. We know that in 7BC there was an interesting triple conjunction of Jupiter and Saturn in Pisces, when the planets passed close to each other three times within seven months. As Pisces is often stated to be associated with the Jews, such an event could have been tremendously significant to the Magi. Two months after the final conjunction, the planet Mars added itself to the configuration, an event that we know was observed in Babylon.

More recently, it has been suggested that the Star might have been an occultation of Jupiter by the Moon. On 17 April, 6BC, the thin crescent Moon passed in front of Jupiter, hiding it for approximately an hour until it reappeared from behind the Moon's disk. The reappearance of the king of planets would have suggested a royal birth. This theory is plausible, though it has several difficulties - in particular the fact that such occultations of Jupiter are common, and that the event took place so low in the twilight sky that it would have been impossible to observe directly, although it might have been predicted by the Magi.

Is there a better theory? To answer that question we must look to ancient Chinese texts. We know from Chinese records that the Star was not Halley's Comet. The Chinese observed it in 12BC, from 26 August to 20 October, as it crossed the sky. This is far too early to have been the Star. We also know from Chinese records that there was no observation of a supernova - the unbelievably powerful terminal explosion of an ancient, massive star - around that time, so the theory proposed by Kepler and echoed by Arthur C Clarke can finally be laid to rest.

However, the Chinese did observe something around 5BC. The Chinese chronicle the Ch'ien-han-shu states that an object, probably a nova, or new star, was observed in March of 5BC and that it remained visible for 70 days. We know that the nova appeared near the star Theta Aquilae, and there is no suggestion that it moved as it would have done had it been a comet. This object would have appeared in the east, in the dawn sky, and would have endured long enough to guide the Magi across the desert. However, by the time they arrived in Jerusalem it would have risen several hours earlier, appearing due south at dawn - just the right direction to have guided them south to Bethlehem.

For the Magi it would have been the culmination of various signs: the triple conjunction; the massing of Mars, Jupiter and Saturn; possibly the occultation of Jupiter. These signs would have told them to expect he birth of the Messiah, and the explosion of the nova would have told them it had finally happened.

The coincidence in date, the duration of the visibility of the Chinese nova and its position in the sky make it hard to believe that the Star of Bethlehem could have been anything else. It is even possible that we may one day be able to identify the Star. In 1925 a nova was seen very close to the position in the sky given by the Ch'ien-han-shu. This object is now known as DO Aquilae. It was not visible to the naked eye in 1925, but just possibly an earlier, greater explosion of this star might have been the Star of Bethlehem. More likely it was another faint nearby star that exploded. Whatever it was, we are probably close to being able to identify the Star of Bethlehem. If it was a nova, the tell-tale cloud of hydrogen gas that its explosion ejected, probably still just detectable, will one day give the star away.

Dr Mark Kidger is an astronomer at the Instituto de Astrofisica de Canarias, Tenerife. His book, `The Star of Bethlehem', is published by Princeton University Press, pounds 14.50