For many years the Chinese observations from 5 and 4 BC have created confusion. We have two, similar records in similar dates of consecutive years.
First, the Chi’en-han-shu.
Chinese records generally use three words to describe new objects in the sky: k’o-hsing, or Guest Star, for novae; hui-hsing, or Broom Star, for a comet with tail and a po-hsing, or Bushy Star, for a tail-less comet. So, many people take this as meaning that the 5 BC object was a tailed comet. However, the Chinese chronicles were far from consistent in this: bright novae, for example, Tycho’s supernova, were also called hui-hsing too.
Similarly, the chronicle gives a fixed position over two and a half months, not reasonable if the object really was a comet. For such a long-duration object the chronicles would usually indicate the motion, which would have been considerable and, as left, often included sketches and even details of the length of the tail.
The description “more than 70 days” is also highly ambiguous. May is monsoon season in China so bad weather would almost certainly have curtailed further observations – the Star would almost certainly have been visible for longer, had the weather so permitted and the astronomer was pointing out the fact that it had not vanished, except behind the omnipresent cloud and rain, hence the phrase “more than 70 days”.
Of course, we do not know details of the behaviour of the Chinese monsoon in the last century BC, but we do know its patterns of rainfall today and can guess that they are similar, at least in general terms to the conditions prevalent today in monsoon season. As the monsoon moves north it loses some of its power but, even so, at peak times, anything from half to two-thirds of the days in the month are wet, according to location.
Clouds, rain and the fact that the Full Moon would have passed very close to the location of the object in the sky each month would have been severe impediments to observation as the object faded.
Then, we have the second object, in the Korean chronicle, Samguk Sagi, which is even more conflictive, mainly because of the impossible date that is given:
Here, the problem is that the day “Chi-yu” did not exist in the second month of the year. It is an impossible date. Back in 1975 I remember vividly watching on television in a late evening current affairs programme on the BBC as Professor Richard Stephenson of Durham University talked of this observation and how he had resolved the mystery. I can honestly say that few TV interviews that I have watched have so impacted on my life.
Richard Stephenson pointed out that this chronicle was a copy of a copy of a copy, laboriously transcribed by scribes who, on occasion, made mistakes, especially if the original from which they were working was unclear. Look at the tiny difference between the Chinese pictograms Chi-yu (what was written) and I-yu (what must have been intended).
Make the change and, suddenly, the date becomes March 31st. What is more, the Chinese constellations of Chi’en-niu and Ho-Ku are neighbours in a region of the sky that is surprisingly poor in bright stars, despite being close to the Milky Way.
Richard Stephenson also pointed out that you could either accept the amazing coincidence that two, similar objects appeared in the same region of the sky in consecutive years, or you could suggest that the scribe who confused Chi-yu and I-yu also wrote “54” instead of “55” and, BINGO! It is actually a ghost record of the same object, like the 10 BC Halley observation.
Some authors have made much of the fact that the constellation cited for the object was different, without looking at the date given: March 31st.
On March 31st 5 BC, the Full Moon was close by. The whole area would have been flooded with so much light that the faint stars of Capricorn and southern Aquila would have been completely invisible (if you do not believe me, try looking at this area of the sky from a light-polluted location – I will bet a lot of money that the only star that you will see is Altair and, maybe, its two companions in Ho-Ku.
In other words, the Korean astronomers gave the reference of Altair, the main star of Ho-Ku, because it was the ONLY star that they could see near the object.
To me, the simplest interpretation of these observations is that a bright nova appeared in Southern Aquila, close to the date of the Full Moon of March 31st 5 BC.
What would the Chinese astronomers have seen? It would have looked something like this (although, admittedly, I prepared this plot some years ago for the pre-dawn sky of March 1st, before understanding the significance of the date of the Korean observation).
This view from Tehran at 5am local time on March 1st, would have been very close to the view obtained at dawn, late in the month.
So, now, we are almost at the end of our journey. The next post, the tenth, will also be the last. In it, I will try to bring together the different threads of our story.