A surprising number of article have been written on the Star of Bethlehem that suggest a comet as a possible candidate – see for example the list at http://www.staff.science.uu.nl/~gent0113/stellamagorum/stellamagorum_com… … and www.staff.science.uu.nl/~gent0113/
Sent in by Brian Sherood Jones this is one of those tricky questions as you first have to look for a date for the nativity at Bethlehem prior to homing in on a comet. Various dates have been suggested, usually somewhere between 8BC and 4BC (largely revolving around the reign of King Herod and the date assigned to Pontius Pilate from historical records). A comet can't be read as a starting point as we don't know that the star of Bethlehem was a comet. The best fit date, shall we say 5BC for argument's sake, would require looking at the whole sky – and not just at comets. There was a good article I linked to last year.
It's worth while noting that as early as the 3rd century AD Origen of Alexandria proposed a connection with a comet – as comets could augur good tidings as well as bad vibes. Halley's comet is dated around 12 BC and another prominent comet around 9AD – but these are too early and too late (for various reasons) and therefore comets recorded in 5bc and 4bc have been seen as more likely. However, were they unusual, enough to cause a stir. We have had a lot of comets in the night skies during 2015 but none of them have really been anything out of the ordinary, and we may assume comets come and go every year.
Another possibility is that the Star of Bethlehem was symbolical, a play on the comet of 44BC that was used by Augustus Caesar to establish a dynasty – and at the same time to claim divine status. The Star of Bethlehem could be seen as a way to bestow divine status on Jesus (but this is of course a controversial suggestion) and there is a historical precedence for the Three Wise Men (as magi). On top of that we might also take into account some of Steve Mitchell's exploration of the AD dating system (in SIS articles) in which he raised the possibility there was a 7 or 9 year difference between the dating system in operation at Rome and those in the eastern empire (Constantinople and Alexandria) which was eventually brought together by Dionysius Exegius in his Easter Tables calculations. No doubt there are other factors not mentioned here and one of these might be that the star is only briefly mentioned in the Gospel of Matthew and a few early Christian texts. It is a fascinating window on the ancient world where astrology (as far as predictions and omens are concerned) was still accepted. Was it real, that is the question? Whatever, it has spawned copious writings over the centuries and no doubt into the future too.