The Star of Bethlehem: Was it Jupiter?

A conjunction is when celestial objects group closely in the night sky. In this image, the moon, Venus and Jupiter were all in conjunction on Dec. 1, 2008, an event that won't reoccur until 2013. Image credit: Ian O'Neill/Astroengine.com.

Interpreting events as depicted in the Bible is no easy task. Although it is well known that some events in its text can be pinned on a moment in history, it takes a lot of work to separate the fact from the fiction.

But there is one significant event in the Bible that scholars have debated for centuries: The Star of Bethlehem.

Throughout the years, the fabled drifting star has been pinned on an array of possible cosmic events. Was it a comet? Was it a distant supernova? Perhaps it was neither, argues BBC astronomer and Discovery News contributor Mark Thompson.

In the run-up to the Christmas season, Thompson decided to do some research into what the Star of Bethlehem could have been. It had to be a significant event for "The Three Wise Men," as depicted in the Gospel of Matthew, to follow to Jesus' birthplace.

Naturally, there's a lot of uncertainty about the accuracy and validity of the Bible's interpretations about what happened over 2,000 years ago, but Thompson discovered a significant astronomical event that might explain what happened.

After scouring through historical records and using computer simulations of the positions of the planets and stars as they would have appeared, he thinks the Star of Bethlehem could be explained by the motion of Jupiter.

Jupiter's Retrograde Motion

Between Sept. 3 B.C. and May 2 B.C. there were three conjunctions (on Sept. 14, 3 B.C., Feb. 17, 2 B.C. and May 8, 2 B.C.) where Jupiter passed close to the star Regulus (the brightest star in the constellation Leo). This rare sequence of events would have looked very strange to those familiar with the night sky.

Thompson found that the gas giant passed Regulus in an easterly motion before appearing to reverse direction, passing the star again in a westerly direction. This change in direction is known as retrograde motion. Due to the near-circular orbits of Earth and Jupiter, as Earth has a faster orbital period than Jupiter, from our point of view we will appear to "overtake" the gas giant. The motion of Jupiter will therefore appear to change direction for several weeks before changing direction again continuing its easterly drift.

The Three Wise Men, thought by many to have been zoroastrianist priests (who were also renowned astrologers) might have noticed this strange motion and considered it to be a 'sign.'

"The retrograde motion meant the planet was travelling in a westerly direction in the sky and so the [Three Wise Men] may have followed it from Persia," Thompson told the UK's Telegraph.

"By camel it would have taken about three months and interestingly this is roughly about the same time Jupiter was travelling in this westward direction."

Culminating in a Jupiter and Venus Conjunction?

Interestingly, Indiana University researchers noted in 2003 that there was a spectacular conjunction between Jupiter and Venus a month after Thompson's time line. Both planets are thought to have overlapped in the night sky making them indistinguishable to the naked eye on June 17, 2 B.C..

Could Jupiter have led the Three Wise Men in the direction of Bethlehem after its bizarre change in direction, eventually culminating in a dazzling conjunction with Venus?

This of course could just be a coincidence, and there are a huge number of other explanations for the Bible's Star of Bethlehem. But if the "Three Wise Men" following the Star of Bethlehem was a real event, these known conjunctions involving Jupiter are a strong -- and fascinating -- argument as to what it might have been recorded in ancient text.

Note : Mark Thompson, astronomy presenter for the BBC's One Show, will be co-hosting "Stargazing Live" on BBC2 with Prof. Brian Cox in January 2011. It promises to be a great series of shows, so be sure to tune in!