Image of comet Siding Spring. Comets are often suggested as the 'Star of Bethelehm' but they are unlikely to be a candidate.
During the festive season astronomers are often asked about the story of the ’Star of Bethlehem’ in the Christian Bible: Was it real? What could it have been? We cannot know the answer to the first question. It is possible the story is a later invention, made up in order to fulfil a prophecy. But even if this is the case it is interesting to consider why the story was important, what it may have meant.
Two common suggestions are that perhaps it refers to a comet or a supernova. These are two kinds of astronomical object that can suddenly appear bright in the night sky.
Comets have been regarded in many cultures as being potent omens. However almost universally they foretold doom, whereas the Magi, or “wise men” who saw the star expected it to show something good.
Supernovae are stars that explode suddenly as they die, releasing enormous amounts of light and appearing in our skies as a new bright star. The astronomer Johannes Kepler saw a supernova in 1604 and soon after suggested that the ’Star of Bethlehem’ might have been a supernova. But Chinese astronomers of the time kept careful records of supernovae, and there is none that matches in timing.
Comets, supernovae and other astronomical objects also have another problem: they move across the sky through the course of a night. The gospel suggests that the ‘Star’ was first seen in the eastern sky, yet it led the Magi to travel westward. How can a moving object provide this kind of guidance?
This question suggests a different kind of explanation. It may have been that what was important was not the location in the sky, but the position on an astrological chart. Scientists no longer consider astrology a science, but in ancient times astronomy and astrology were inseparable. The Magi were Zoroastrian priests (not kings) who were careful observers of the sky and practitioners of astrology. In their system the sign of Aries was associated with Judea, so it may have been this that led them to Jerusalem.
Three events that have been considered are a close conjunction of Jupiter and Venus in 2 BCE, a massing of Jupiter, Saturn and Venus in 7 BCE, and a pair of occultations of Jupiter by the Moon in 6 BCE.
A conjunction is when two planets appear near each other in the sky. In 2 BCE Jupiter and Venus appeared so close that they appeared as a single bright star. This event occurs too late for the commonly accepted date range for Jesus’ birth, but if the story is a later creation it might refer to a memory of this event.
A massing of the planets is when more than two planets appear near each other in the sky. Conjunctions of two planets are relatively common but a massing of three planets is much rarer so would have been of more interest.
An occultation by the Moon happens when a planet like Jupiter appears to pass directly behind the Moon as seen from Earth. This happens quite rarely and so is an unusual event. Unfortunately the occultations would have been very difficult to observe and so it is doubtful whether the Magi actually saw these events.
So while there are certainly difficulties for the astrological interpretation of the ’Star of Bethlehem’ it is quite likely that this is the kind of event these skywatchers were interested in. Fortunately in modern times astronomers no longer have to worry about the astrological significance of celestial events!