Some historians advocate the view that the story of the star of Bethlehem and the Magi was written long after the event, in order to enhance the miraculous context of the birth of Christ. From an astronomical standpoint, it is interesting to examine all of the possible celestial events that might account for the description given in the New Testament.
An astronomer might attempt to retrofit the appearance of the sky at the time of the birth of Christ. Therein lies the crux of the matter: when, exactly, did the birth take place?
The Gospels narrate how the Magi, on arrival in Jerusalem, told King Herod that, having seen the star of Christ in the east, they had come to worship the new king. Viewing the new king as a usurper, Herod issued the decree to have killed all children under the age of two in the area of Bethlehem.
The infant and his parents escaped to Egypt, however, returning two years later, by which time Herod had died. His death was followed by an eclipse of the moon, according to historical record. The date of Herod's death is a matter of dispute, but it could be sometime between 4 BC and 1 BC.
Dionysius Exiguus, who lived circa AD 500-550, was a Scythian monk who lived in Rome and had access to state and church archives long since vanished. He selected the year that we call 1 BC for the birth of Christ, nominating December 25th as the precise date. This date has astronomical connotations. In the Julian calendar it is the date of the winter solstice; January 6th is the date of the solstice in the Egyptian calendar.
Many feel this date is wrong, as Herod probably died in 4 BC, and Christ was born before Herod died. If, as some commentators suggest, however, Herod died later, closer to 1 BC, then the chronology for the birth of Christ might also be later in time.
In searching for celestial events that might be potential candidates for the star of Bethlehem, it is prudent to search over the range 7 BC to 1 BC. In China, particularly detailed records of celestial manifestations go back to about 200 BC. Professional astronomers and astrologers were maintained as part of the civil service, to monitor the skies and report unusual happenings. Anything new or different was considered an augury.
Exceedingly accurate observations of planets, eclipses, comets and meteor showers were recorded, together with the appearance of new stars, often of extreme brightness. Today we may identify the latter category as either supernovae or novae.
Supernovae are stars that, at the end of their evolutionary cycle, spontaneously explode with a spectacular increase in brightness, in some cases becoming visible in daylight but thereafter gradually decaying, over periods of up to a year. A nova resembles a supernova in many respects, but on a much smaller scale.
Supernovae are rare; there have been only four in our galaxy during the past 1,000 years. Novae are much more common and may shine as spectacularly as the brightest stars in the sky for a short time.
What was the sign or portent that initiated the long trip, possibly months in duration, of the Magi from Babylon to Bethlehem, then guided them to a specific location? Chinese records indicate that no supernova was recorded in the interval 8 BC to 1 BC, and only one bright comet was noted, which lasted for 70 days.
There is also evidence for a nova or guest star, close to the bright star Altair, recorded in 5 BC. Meteors and meteorites would be too transient to explain the phenomenon.
The appearance of the planets in the sky might offer a better explanation. Given computers and our understanding of Kepler's laws of planetary motion, it is a straightforward task to chart the motion of the planets in the heavens over the time span of interest. Single conjunctions between the bright planets Jupiter and Saturn occur, on average, every 20 years.
In their orbital procession against the stars, Jupiter overtakes Saturn in the sky and, viewed from earth, they appear to cross. Very occasionally, due to earth's position and the eccentricities of the planetary orbits, there is a triple conjunction. Here the earth appears to pass Jupiter and Saturn within about 40 hours of each other. Subsequently, the planetary motion of Jupiter carries it back past Saturn a second time in retrograde manner and, later, forward past Saturn a third time. Such a triple conjunction takes about seven months.
Such are the astronomical possibilities as to what the star of Bethlehem might have been - but the event may have been supernatural, after all.
David Fegan is an astrophysicist and a professor in the department of experimental physics at University College Dublin