At http://io9.com/5637815/ there is another version of the Comet Halley appearance in 466BC – with a bit more history. It could have been visiting the inner solar system for anything between 16,000 and 200,000 years (see also the Journal of Cosmology). The Chinese record the comet in 240BC and it is thought the Babylonians (people of the Middle East) record it in 164 and 87BC (as well as Tigranes, an Armenian king) and in 12BC it may have sparked messianic expectations – but who can tell. It does seem to possess distinct chronological possibilities if its track can be traced back via historical records into the Late Bronze Age, for example. Alternatively, it may throw up anomalies that conflict with conventional chronology. At the moment it appears to suggest BC/ AD chronology does not have any gaping holes – but the retrocalculations do of course assume that this or that comet was Halley rather than any other comet that might have visited our region of space, or flared to the extent it was visibly awe inspiring. Like all retrocalculations, or any kind of simulation, it is not necessary the model itself that is in error – but the assumptions such models are built upon. In this instance, we seem to have a comet seen in Greece in 466BC – but that does not come with a name tag. It is a comet which could be Comet Halley – or not. However, in the AD period the situation is more transparent as in AD837 it came within 3.2 million miles of the earth so that it’s tail would have been an enormous feature in the sky (and is mentioned in Japan, China, the Middle East and northern Europe) – but why not southern Europe? This is of course another retrocalculation, but what if the southern Europeans recorded a comet in another year that resembled Comet Halley – what would that imply? Comet Halley made an appearance in 1066 and it is said it inspired William the Conqueror (as he made his way across the Channel with his fleet). It is also reputed to have inspired Genghis Khan in 1222. Apparently, its appearance in 1986 was a damp squib affair – one of the dullest of apparitions. Finally, there is one other chronological conumdrum – if there is any kind of discrepancy in AD/BC dating it may actually involve a full 76 year Halley cycle which would be impossible to determine. However, the beauty of the model developed above is that each Halley passage into the solar system differed – in the view of that comet by people on the surface of the earth. For example, the reason why the 466BC comet was associated with Comet Halley was because its behaviour fitted the description. The only bugbear was the meteorite – its connection with the comet is not part of the simulation.