Christmas Star shines in a cosmic ballet

Heather Couper and Nigel Henbest wonder what celestial sight heralded the birth of Jesus
What was the Star of Bethlehem? "... when Jesus was born in Bethlehem of Judea in the days of Herod the King, behold, there came wise men from the east to Jerusalem, saying, where is he that is born King of the Jews? For we have seen his star in the east, and are come to worship him" (Matthew 11, 1-2).

With the brilliant planet Venus now starting to put on a dazzling display in the western sky, it is an appropriate time to ask what the Christmas Star might have been.

Astronomy may be able to come up with an answer, but only if we can pinpoint the likely date of Christ's birth. Confusingly, it was not in the year zero (which didn't exist, as 1 BC led straight to AD 1), and probably took place several years earlier. The contradiction arises in the calculations of a sixth-century monk called Dionysius Exiguus, who attempted to draw up a calendar by adding together the lengths of the reigns of all the Roman Emperors. But alas, he managed to forget the four-year reign of Octavian - consigning Jesus to a birth some time BC!

We can date Christ's birth to the reign of Herod, who we know from biblical sources died shortly after an eclipse of the Moon. There were eclipses in 4, 5 and 7 BC.

What riveting sky-sights were there on show a few years BC that might have been the Christmas Star? We can eliminate one suspect immediately: Halley's Comet. Although the Italian Renaissance painter Giotto di Bondone executed a beautiful nativity scene with the comet (which he had seen in 1301) as his "star", it was never thus. Halley's Comet, which enters the inner Solar System every 76 years, made a close pass to Earth in 12 BC - but that's just too early.

Chinese astronomers observed a "broom-star" - possibly a comet with a tail - for 70 days in 5 BC. Strangely, they reported no movement of their broom-star, which is unusual for a comet. Some astronomers believe that the Chinese instead saw a nova - a faint star that dramatically flares up when a companion star dumps material on it.

Whether a comet or a nova, whatever appeared in 5 BC would have been a striking sight in the sky. So why did no one in the west mention it? In particular, why was Herod so surprised when the "wise men" came to his palace claiming to have seen Christ's star in the east?

The answer may lie in the nature of the wise men themselves. They were probably not kings, but astrologers hailing from Babylon. And, like astrologers today, they would not have been interested in sudden, unexpected happenings in the sky, but in the predictable movements of the Moon and planets and the "aspects" they make with each other. So the "star" may have been a much more subtle affair, apparent only to astrologers.

Two unusual groupings of planets took place around the time of Christ's birth. In 7 BC, Jupiter and Saturn indulged in a cosmic ballet that they put on only once every 139 years. That year, in the constellation of Pisces, they appeared to come together and draw apart again on three separate occasions. This "triple conjunction"spoke volumes to the astrologers. In their interpretation, Jupiter was the king of the gods, Saturn stood both for justice and the land of Palestine, while Pisces represented the Jewish people. The sky was saying that a Jewish Messiah would soon be born.

In 2 BC an even more spectacular conjunction took place. On 17 June, "godly" Jupiter almost appeared to merge with "female" Venus in the constellation of Leo - another star pattern associated with the Jews. Stunning though the conjunction must have been, it was probably not the Christmas Star - for Herod would have been dead by then.

What's Up

The heavens are taking on a decidedly more wintry look now with the appearance of Orion in the east. From now until March, the mighty hunter - along with his adversary Taurus and two faithful dogs - will occupy centre-stage in the night sky.

Just to the east of the Orion tableau is the constellation of Gemini. Around the night of 14 December, look to the skies to see many more bright shooting stars than usual (up to 90 an hour), apparently emanating from Gemini's direction. This is an effect of perspective: the Geminid meteors actually come from a nearby expired comet that rejoices in the name of Phaethon.

Staying in the Solar System, look west to spot the elusive planet Mercury. At the end of the month, it is setting more than an hour after the Sun. Mars, too, is visible in the evening twilight. And Saturn is still on view in the evening sky, as a steadily shining, brightish "star" below the barren Square of Pegasus.

But the bright new arrival on the scene is Venus, settling in for a six- month residency as the Evening Star. On Christmas Eve the thin crescent Moon will be about 14 moonwidths to the north - making a pretty photo- opportunity if you're not partying.

December Diary (all times GMT)

7 1.27am full moon

14 10am Maximum of Geminids meteor shower

15 5.32am Moon at last quarter

22 2.23am New moon

22 8.17am Winter Solstice

28 7.06pm Moon at first quarter

BBC Radio 4 is broadcasting a two-part series 'Modern Magi' with Heather Couper, the Bishop of Monmouth and Paul Vallely, also of the 'Independent', on a 20th century journey to Bethlehem, at 11am on Saturdays 23 and 30 December.