Star of Bethlehem: The astronomical explanations
It might seem churlish to dissect such an enduring image of Christmas as the star of Bethlehem, but a quiet astronomical debate has been bubbling away for decades. Could some real cosmic event have drawn "three wise men" on a journey to find a newborn king?
This debate requires one very big assumption - that the story of the star and the journey is true.
Prof David Hughes, an astronomer from the University of Sheffield, first published a review of the theories on the famous star in the 1970s.
Who were the three wise men
- The Three Wise Men, or Three Kings are believed to have been based on the Magi - Persian sages able to read the stars
- In Spain and Spanish speaking countries they are a much-celebrated part of Christmas. It is actually the three wise men (Los Reyes Magos) who have the task to bring gifts to all children on the Epiphany (6 January). Traditionally, children send their wish lists to the wise men. Children are also told that they will receive coal instead of presents if they behave badly. Edible sweet coal is widely available in shops
- People leave their shoes out in the living room, under which they will find their presents the next morning. It's also traditional to leave some food and water for the kings and their camels
- A parade of the three wise men takes place on the evening of the 5 January in all major Spanish cities and towns. The kings are seen riding their camels and throwing sweets into the crowd.
Having spent many years studying the astronomical explanations and reviewing the associated biblical stories, he is now an expert on the subject.
But there are some intriguing historical parallels.
The three kings were religious scholars known as the Magi - revered Babylonian astronomers and astrologists. They studied the stars and planets, interpreting the meaning behind cosmic events.
Anything very unusual was considered an omen, so the star must have been both rare and visually spectacular. And, says Hughes, it would have had a very clear message for the Magi.
This leads the astronomer to conclude that the star of Bethlehem was probably not a star at all, and that it was more than one single event.
"If you read the Bible carefully," says Hughes, "the Magi saw something when they were in their own country - [probably Babylon] - so they travelled to Jerusalem and had a word with King Herod."
According to the story, the Magi told Herod of the sign they had seen and, says Hughes, "when they left Jerusalem [for] Bethlehem, they saw something again".
Reading the stars
The tradition of celestial events signifying events on earth originated in Mesopotamia more than 3,000 years ago.
Astronomer-astrologers reported omens to the king; these were anything unusual - perhaps the moon moving in front of a planet, or a lunar eclipse. Their job was to interpret the meaning of these phenomena.
There was also an elite class of Babylonian diviners who created nativity charts. They recorded the positions of the planets, the Sun, the Moon and other astronomical data at the time of a child's birth, in order to make predictions about that person's life.
About 2,000 years ago the Greeks turned this into the sort of horoscope we would recognise today, charting the zodiac signs in which the Sun and Moon rose, as well as the planets that rose in the east for each moment in time.
Some believe that the wise men from the East, or the "Magi" of the nativity, were astrologers from Mesopotamia, and that the star rising in the east was the horoscope that predicted the birth of a king.
If so, they were reading a nativity chart in reverse; they had the prediction and sought to find the child who had been born at that precise moment.
Hughes's best explanation for this series of events is something known as a triple conjunction between Jupiter and Saturn - with the two planets coming close together in the sky three times over a short period.
"[This happens when] you get an alignment between the Sun, the Earth, Jupiter and Saturn," says Hughes.
Tim O'Brien, associate director of Jodrell Bank Observatory in Cheshire, suggests this would have looked striking. "It's remarkable how much your attention is drawn when two very bright objects come together in the sky," he explains.
And once the planets lined up in their orbits, Earth would "overtake" the others, meaning that Jupiter and Saturn would appear to change direction in the night sky.
"At that time, people would have set great store by the motions of the planets," says O'Brien.
Even more significantly, the event is believed to have been in the constellation Pisces, which represents one of the signs of the zodiac.
"You would [only] get a triple conjunction like this about every 900 years," he says, so for astronomers in Babylon 2,000 years ago, it would have been a signal of something very significant.
"A triple conjunction of this kind ticks all the boxes."
The second favoured explanation is a very bright comet.
While certainly spectacular and ethereal in appearance, comets are essentially "big, dirty snowballs" flying through space.
"When they come close to the Sun, this ice melts - solar wind blows this material out into space, so you get a tail of matter coming off the comet," explains O'Brien.
This tail, which points away from the Sun, is one of the things that has made the comet idea popular, explains Hughes.
"Quite a few people have said that comets seem to 'stand over' the Earth, because of their coma and tail sometimes looking like an arrow," says Hughes.
The most timely record was of a bright comet appearing in the constellation of Capricorn in 5BC, which was recorded by astronomers in China.
A less likely, but more famous candidate was Halley's comet, which was visible around 12BC.
Those who favour this theory point out that the 5BC comet would have been in the southern sky as seen from Jerusalem, with the head of the comet close to the horizon and the tail is pointing vertically upward.
"Quite a lot of people liked the comet idea, so it crops up in quite a lot of Christmas cards," says Hughes.
"The snag is that they're not that rare. They were also commonly associated with the 'four Ds' - doom, death, disease and disaster," he suggests. "So if it did contain a message, it would have been a bad omen."
Another theory is that the star was light from the birth of a new star, or nova.
There are records - again from astronomers in the Far East - of a new star in the small, northern constellation of Aquila in 4BC.
Hughes says: "People who like this theory say this new star would have been [positioned] directly over Jerusalem."
Dr Robert Cockcroft, manager of the McCallion Planetarium at McMaster University in Ontario says a nova is "a good candidate" for the star of Bethlehem.
"It can 'appear' as a new star in a constellation, and fade again over the following months," he explains.
"It is also not too bright, explaining why we don't have any records of it in the west." Cockcroft suggests that this might also have given the three wise men something to follow.
While other "omens" would have been needed to cause the Magi to set out on their journey west to Jerusalem, he says , it would take them months to get there, "by which time Aquila [and the new star could have] risen in the sky to appear in the south.
"Bethlehem lies due south of Jerusalem, so that Magi could 'follow' the star to Bethlehem."
Other more improbable but entertaining theories have been proposed over the years, says Hughes.
One he describes as particularly far-fetched was suggested in a 1979 academic paper by the Greek astronomer George Banos. He proposed that the Christmas star was actually the planet Uranus.
Banos suggested that the Magi discovered the planet 1,800 years before the astronomer William Herschel formally recorded the discovery in 1781.
"His idea was that the Magi discovered Uranus, that this was the star of Bethlehem and they then tried to hush up the discovery," Hughes explains.