Episode 39: The Guest Stars

February 29, 2024

We turn to the ways that the Chinese Emperor's astronomers predicted and interpreted eclipses, as well as the so-called "guest stars" that they occasionally reported observing in the skies. Then we discuss the role of the planets, particularly Jupiter. Towards the end we hear a few examples of astronomy in Chinese folklore.


Good evening, and welcome to the Song of Urania, a podcast about the history of astronomy from antiquity to the present with new episodes every full moon. My name is Joe Antognini.

Last month we looked at the three major cosmological theories in China that emerged during the early Imperial period. This month I wanted to focus on some of the other pillars of Chinese astronomy: eclipses, records of the guest stars, and the planets. Then towards the end I wanted to give a flavor of astronomy in Chinese folklore. But, before we can get to the folktales, I have to start with the official stuff.

Now, as I have been at pains to convey in the last two episodes, Chinese astronomy was intimately associated with the Emperor. Chinese astronomers, at least the most significant of them, were employed by the Emperor and held high positions in the Imperial Court. And more fundamentally, it was conceived as flowing from the Emperor just like all the other fruits of civilization. As one of the great early scholars of Chinese astronomy, Léopold de Saussure wrote,

While among the Greeks, the astronomer was a private person, a philosopher, a lover of the truth (as Ptolemy said of Hipparchus), as often as not on uncertain terms with the priests of his city; in China, on the contrary, he was intimately connected with the sovereign pontificate of the Son of Heaven, part of an official government service, and ritually accommodated within the very walls of the imperial palace.

Nowhere was the importance of the astronomers to the Chinese state more apparent than in the study of eclipses. As in every society, eclipses were seen as perhaps the most astrologically significant phenomenon. And they did not portend anything good. After all, when the very sun or moon is blotted out from the sky, a kind of apocalyptic feeling is only natural. And, if you have ever seen a total solar eclipse in person, you’ll know that feeling, since the Sun doesn’t just go away the same way it does during a sunset. As the sun sets in the evening it’s always low on the sky, and as such, its light is passing through a lot of atmosphere. As I talked about back in Episode 33 on the history of blue moons, the shorter wavelengths of light scatter in the atmosphere much more strongly than the longer wavelengths do, so when the sun’s light passes through that much atmosphere, the color of the light changes quite dramatically, with the reds, oranges, and yellows becoming much more prominent. This is why the hour before sunset is known as the Golden Hour in photography. Well, during a solar eclipse, the Sun’s light also dims, but because the Sun is still high in the sky, there is no corresponding change to the color. So it gets dark, but the light is still quite blue, which produces an exceedingly eerie effect.

Well, at any rate, in China, as with everywhere else, a lunar or solar eclipse was perceived as a harbinger of doom. But, there was a way out. If the eclipse was predicted in advance it was not seen as being a necessarily bad omen — it could be interpreted through a variety astrological techniques to mean a lot of different things. But if there was an unexpected eclipse that was quite different. This was an indication of great discord between heaven and Earth — and because the Emperor was at the center of the Earth, that meant great trouble for the emperor. The connection between the state of the heavens and the emperor was described in the I Ching like this:

The character of the great man is identical to that of Heaven and Earth; his brilliance is identical with that of the sun and the moon; his order is identical with that of the four seasons, and his good and evil fortunes are identical with those of spiritual beings.

So one of the principal jobs of Chinese astronomers up and down the ages was to predict lunar and solar eclipses. Now, fortunately for them, the standards of a successful prediction were somewhat lax, at least relative to what you might expect, especially in the early days. In general it was okay if the astronomers predicted an eclipse and it didn’t happen. That might simply have signified that the emperor had heard the warning from heaven, corrected his course, and averted the impending disaster. And as prediction techniques became more sophisticated it was also acceptable for eclipse predictions to be within a particular cycle. In practice this meant that the date could be off by 5 or 6 months or 11 or 12 months, though dates that were in separate cycles, like a date that was off by, say 2 or 4 months, would be counted as an incorrect prediction.

Now, until contact with Western astronomy in the 17th century, Chinese methods to predict eclipses were fundamentally phenomenological. They had extensive records of observed eclipses and identified patterns in those records. Over the centuries the patterns they identified became more sophisticated, but it was never connected quantitatively to an underlying theory of eclipses. One of the rather surprising features of Chinese eclipse theory is that although they became fairly successful at predicting eclipses, they never made use of the Saros cycle of around 19 years, as did many other civilizations. Instead, by the 1st century BC they had hit upon a 135 month cycle during which 23 eclipses would occur. By the 3rd century AD they had an improved understanding of the Moon’s orbit called the Nine Roads of the Moon, which I’ll discuss in more detail a little later. From this Chinese astronomers estimated that the Moon’s orbit was inclined to the ecliptic at an angle of about 6° and by the late 4th century an astronomer named Chiang Chi was making predictions about the extent of totality of an eclipse. By the early 7th century the techniques had become sophisticated enough that Chinese astronomers were making predictions about the time of first and last contact of an eclipse as well as the position in the heavens. Chinese eclipse prediction methods appear to have reached their zenith perhaps in the early Song dynasty around the 10th or 11th centuries. As I mentioned in the last episode, a kind of bureaucratic rot set in in the Astronomical Bureau in the later Song dynasty, despite the efforts of some quite capable astronomers to fix it. We have a reference to an eclipse in the 13th century AD where apparently a private individual predicted a different time for some solar eclipse than the royal astronomers. The royal astronomers predicted that the eclipse was to happen at night, and thus would not be seen, but the private individual predicted it would happen during the day. And indeed the eclipse did in fact happen during the day. The citizen scientist was correct and the royal astronomers were wrong. As a consequence we read in the reference that the head of the Astronomical Bureau, along with several of his officials were put on trial for negligence, found guilty, and severely punished. Throughout the later Ming dynasty from the 14th to 17th centuries the quality of the state astronomers fell even further and this was reflected in a decline in the accuracy of the eclipse predictions. And as we’ll see in the next episode, the newly arrived Jesuit astronomers were able to exploit this decline to integrate themselves into the Imperial Court.

But I am getting ahead of myself. Perhaps one contributing factor for this decline was that by the time of the Song dynasty observation and prediction had become organizationally separated from each other. The astronomers who made predictions of future eclipses were housed in an entirely separate department from the astronomers who made observations of them. Now, throughout the history of science there has always been some division of labor between theorists and experimentalists. But it has often been the case that anytime theorists and observers are brought to work more closely together there have been more discoveries, and when they have been more decoupled there has been more stagnation, at least as far as such a general rule can go. And this may have also been the case during the Song dynasty.

As for the records themselves of the eclipses, it’s perhaps no surprise that they go back quite a long way for a civilization as ancient as China’s. The earliest records are rather sporadic, but there are some oracle bones from the Shang dynasty in the 2nd millennium BC that indicate the observation of an eclipse. More systematic records begin during the Spring and Autumn Period starting in the 8th century BC. One of the commentaries on the canonical history of the era, the Spring and Autumn Annals, called the Zuo Zhuan, or Commentary of Zuo, preserves the records of solar eclipses from this period. The text describes the period from 721–468 BC and during this period 37 solar eclipses were recorded. Interestingly, the first solar eclipse in this work was recorded as taking place in 720 BC, which is just one year after the first eclipse recorded in the Almagest in the West in 721 BC. During the Han Dynasty, records of solar eclipses were preserved in the Han Shu, or Book of Han, which has 55 solar eclipses.

Now, one of the neat things about these records is that with modern astronomical techniques we can go back and check their accuracy. And generally the records are indeed quite accurate, at least in the sense that nearly all of the recorded eclipses did in fact occur. But the records were by no means complete. The Zuo Zhuan recorded 21 so-called “striking” solar eclipses, which were either total, annular, or very nearly total, and so were unlikely to be missed. But there were 14 other striking solar eclipses that occurred during this period that were not recorded. The completeness of the Book of Han is even worse, 12 striking eclipses were recorded whereas 28 were missed. Now, one possible explanation for these omissions is that they were simply missed due to the weather. It was cloudy, say, and they couldn’t be seen. The problem with this explanation is that if you look at the distribution of when the missed eclipses occurred during the year, they’re all evenly distributed throughout. If the eclipses had just been missed due to the weather you’d expect more of them to have been missed during the winter when it’s cloudier. But that doesn’t seem to have been the case.

What seems to have been going on instead is that these eclipses were probably in fact observed by the imperial astronomers, or most of them anyway, maybe a few really were missed because of bad weather. But most of the missing eclipses appear to have been omitted from the historical record for political reasons. In general, the sparsity of the eclipse record seems to correlate with the popularity of the emperor at the time. During the reigns of popular emperors very few eclipses are recorded, whereas the astronomers seem to have been more diligent at recording for posterity the occurrence of eclipses during the reigns of unpopular emperors. Now, it should be said here, that when I say the popularity of an emperor, I really mean the popularity among the elite class of scholar-officials in Chinese society, since it’s their records that we are going off of. Some of these unpopular emperors may have been quite popular with the masses, but due to who was writing the texts it’s hard to say with great confidence what the real feelings of the masses were. That said, probably many of the unpopular emperors were really just all around bad guys, unpopular with everyone and for good reason.

Now, despite the fact that Chinese astronomers were somewhat selective in their recordings of eclipses, this selection effect largely went only one way. If there had been an eclipse, it may or may not have been recorded. But it was very rare for them to fabricate an eclipse into the record that never occurred in reality. That said, very rare is not never. After the death of the Emperor Gaozu in the early 2nd century BC, his widow, the Empress Lu effectively took control of the state by ruling in the name of her teenaged son once he nominally took the title of Emperor. In no time at all she poisoned another son that the late emperor had had with a concubine. Then to really drive the point home she apparently chopped off the limbs of the concubine, gouged out her eyes, cut off her tongue, nose, and ears, and threw her in a pigsty. So, for these deeds among others, Empress Lu evidently merited a made-up eclipse during her reign.

Well, as for a theory of eclipses, as I mentioned earlier, when it came to prediction the techniques were largely empirical. But when it came to an explanation of what caused the eclipse, there were a few competing theories. One of these theories is already familiar to us from our tour of Indian astronomy. During the Han dynasty, contact with India increased, and brought with it, most importantly, Buddhism to China. But other Indian ideas came into China during this period as well. One of these was the Rahu and Ketu theory of eclipses. The basic idea was that there were two invisible planets, named Rahu and Ketu, at the nodes of the Moon’s orbit, where it intersects with the ecliptic. When the Sun and Moon passed through the nodes, these invisible planets would swallow up the Sun or Moon, causing the eclipse. More mythologically Rahu and Ketu were conceived of as snakes, literally devouring the Sun or Moon and subsequently disgorging them. As we’ll see a little bit later, this wasn’t the only instance of invisible planets in Chinese astronomy, there was also a theory that there was an invisible counter-Jupiter.

The earliest record of eclipses being due to the shadow of the Earth or the Moon is in 20 BC by the astronomer Liu Hsiang. By a century later this theory seems to have become more popular. One reference gave a fairly complete description. It said that the Sun is like fire and the Moon and planets like water — the fire produces the light and the water reflects it. If the Earth came between the Sun and the Moon the shadow from Earth onto the Moon produced a lunar eclipse. If the Moon came between the Earth and the Sun it produced a solar eclipse. And the same could happen with planets as well. If a planet passed between the Earth and a star, it would produce an occultation, or likewise if the Moon passed between the Earth and a star.

However, these two theories were not the only theories of eclipses at the time. Another, more astrological, theory held that the Moon was influenced by other heavenly bodies. The planets and even stars like Arcturus and Antares could, through their astrological influence, cause an eclipse and temporarily extinguish the Moon. Another astronomer named Wang Chhung made a lengthy argument against the theory that eclipses are caused by shadows. And his conception was quite different. He held that celestial bodies behaved in a similar way to creatures on Earth, which all follow their own internal rhythms. Just as there is a rhythm of sleep and wakefulness, or of tides ebbing and flowing, so it is with the Sun and the Moon. Periodically their light ebbs away and then returns, and this is entirely under their own, internal rhythms.

Well, although eclipses were the most important events that the royal astronomers had to be on the lookout for, there were other celestial phenomena that they were concerned with as well. Over the ages as the astronomers observed the skies, at least the ones in the observational subdivision, they would sporadically notice and record new stars that appeared in the sky. As I mentioned in the last episode these records are today quite valuable because they are the most complete record in history of the appearance of comets, novae, and supernovae prior to the advent of the scientific revolution. The oldest of these records was written on an oracle bone around 1300 BC and reads,

On the 7th day of the month, a chi-ssu day, a great new star appeared in company with Antares.

A second oracle bone from the same site recorded, “On the hsin-wei day the new star dwindled.” Now, the hsin-wei day is two days after chi-ssu mentioned in the first oracle bone, so it is quite possible that these two oracle bones recorded the same event. This description would be consistent with a nova, which would suddenly appear in the sky and then fade after a couple of days. Now, in these early days, Chinese astronomers used the term “hsin hsing” to describe these new stars, and the term literally translates to “new star.” But around the middle of the Han dynasty, say the early 1st century AD, the term “hsin hsing” falls out of use and was replaced by the term “kho hsing,” which literally translates to “guest star.” Now, today we know that the guest stars that Chinese astronomers recorded were typically comets, novae, and very occasionally, supernovae. And it is our good fortune that the descriptions were fairly detailed. The astronomers would include the time of the appearance of the new star, the duration of its appearance, its brightness, its color and a description of its shape. There were three kinds of shapes: huixing, which literally translates to “broom star,” and refers to comets with a prominent tail; beixing, which literally translates to “fuzzy stars,” and usually referred to comets without visible tails; and poxing, which literally translates to something like “sparkling star,” and referred to a bright, point-like object, which often was a nova or supernova. An example description of a guest star from 185 AD goes like this:

In the 2nd year of the Chung-Phing reign-period, in the tenth month, on a kuei-hai day, a guest-star appeared in the midst of the constellation Nan Mén; it was as big as the half of a bamboo mat and showed the five colours in turn, now beaming now lowering. It diminished in brightness little by little and finally disappeared about July of the following year.

Given this description, particularly that it lasted for more than half a year, this event was presumably a Type Ia supernova, and this is in fact the earliest plausible record of a supernova in human history. A candidate for the remnant from this supernova is still visible today with the rather impersonal name RCW 86.

Well guest stars were not nearly as astrologically significant as eclipses were, and there wasn’t as consistent an astrological interpretation of these objects, but generally speaking, they were seen as being especially relevant for military campaigns.

Now when we were looking at eclipses one of the things we could do is go back and check the accuracy of the Chinese records. Because our modern models of orbital motion are so good, we more or less know exactly when every lunar and solar eclipse occurred millennia ago, so we now know which ones the royal astronomers saw and which ones they missed, or more probably, saw, but omitted from the record. And we also know which ones they simply made up. Unfortunately we can’t do the same thing for the guest stars, at least not to the same completeness. Some comets, notably Halley’s comet, are periodic and so we know when they would have been in the skies in Ancient China, but other than that it would be hard to know which events they missed or whether they made up any guest stars as they did on very rare occasions with the eclipses. But one of the interesting ways that we can validate the accuracy of the records of guest stars is to look at their distribution across the sky. For the “poxing,” the “sparkling stars,” we now know that these were almost all novae. Novae are outbursts that can occur when a white dwarf is accreting matter from a red giant and the ones that we can see with the naked eye are all within our galaxy. And if we look at the distribution of observed novae in the Chinese records we find that they are concentrated along the galactic plane, which is just what we would expect. So this indicates that in general these guest stars were not fabricated to criticize the government or the like because astronomers of the Han Dynasty would have had no reason to put them in the right place on the sky.

Now, novae are quite common, on average there is a nova that is visible to the naked eye every year or two. Supernovae, on the other hand, can be quite a bit more spectacular, but are also much rarer. On average they happen in our galaxy only every century or so. As it happens the last galactic supernova was Kepler’s Supernova in 1604, so we are now several centuries overdue for one. I am hopeful that one will occur in my lifetime, it will no doubt cause quite a commotion among astronomers. But at any rate, one of the most spectacular supernovae ever observed was in the summer of 1054 which produced the object that is today called the Crab Nebula. Now, this object was evidently seen all over the globe as it could hardly be missed. At its brightest it was brighter than Venus and visible in daylight. But the Chinese were the only civilization to record its appearance contemporaneously. In fact we are rather spoiled for choice in the Chinese records because there are actually five independent observations of the event across the empire. One record from the city of Khaifeng said,

In the fifth month of the first year of the Chih-Ho reign-period, Yang Wei-Te, the Chief Calendrical Computer said, ‘Prostrating myself, I have observed the appearance of a guest-star; on the star there was a slightly iridescent yellow colour. Respectfully, according to the disposition for emperors, I have prognosticated, and the result said, “The guest-star does not infringe upon Aldebaran; this shows that a Plentiful One is Lord, and that the country has a Great Worthy.” I request that this prognostication be given to the Bureau of Historiography to be preserved.‘

A few weeks later the record notes its disappearance by saying

Originally this star had become visible in June (+ 1054) in the eastern heavens in Thien-kuanz (Taurus). It was visible by day, like Venus; pointed rays shone out from it on all sides. The colour was reddish-white. Altogether it was visible for twenty-three days.

Now we do have records from other civilizations of this particular supernova, but they all come quite a bit later and they are in some cases rather oblique. There have been, for instance, petroglyphs in New Mexico that have been interpreted as depicting this supernova. There is a Japanese chronicle which also mentions it, but it was written a century and a half later. Likewise there is an Arabic reference, but it was written around a century later. And if you squint there are maybe some European texts which may have some oblique references to this supernova. But none of these records are as clear as the Chinese records or contemporaneous as contemporaneous as them.

Well most of the guest stars that were observed were less spectacular than the supernova of 1054. They were mostly comets. But one of the discoveries that Chinese astronomers can claim is that they were the first to notice that the tails of comets point away from the Sun rather than along their direction of motion.

In addition to guest stars, another transient phenomenon that the imperial astronomers made note of was meteors, and in particular, meteor showers. However, in one case there is a wonderful description of a meteor impact from Shen Kuo in the 11th century AD:

In the 1st year of the Chih-Phing reign period (1064 AD), there was a tremendous noise like thunder at Chhang-chou about noon. A fiery star as big as the moon appeared in the south-east. In a moment there was a further thunderclap while the star moved to the south-west, and then with more thunder it fell in the garden of the Hsii family in the I-hsing district. Fire was seen reflected in the sky far and near, and fences in the garden round about were all burnt. When they had been extinguished, a bowl-shaped hole was seen in the ground, with the meteorite glowing within it for a long time. Even when the glow ceased it was too hot to be approached. Finally the earth was dug up, and a round stone as big as a fist, still hot, was found, with one side elongated (i.e. pear-shaped). Its colour and weight were like iron. The governor, Chéng Shen, sent it to the Chin Shari temple at Jun-Chou, where it is still kept in a box and shown to visitors.

Now, the purpose of all these observations of transient phenomena was fundamentally astrological in nature. The royal astronomers were looking for omens that would be relevant to the Emperor. But from the records we have it appears that they applied a degree of skepticism in the preparation of their omens as well. There is an illuminating passage from the 14th century by the astronomer Yang Yu about a celestial phenomenon he saw and the debate over how to interpret it:

When I was a Co-signatory Observer in the Bureau of Astronomy, there came a special imperial edict that we were to pay particular attention to celestial presages. On the first day of the seventh month in the sixth year of the Chih-Yuan reign-period (1340 AD), there came (to my house) one of the Senior Observers, a Mr Chang, who asked me to go to the Observatory as quickly as possible. When we arrived there together, we were met by Commissioner Li, dressed up in state apparel, who said: ‘Last night there appeared the Ching Hsing phenomenon. That is a very auspicious omen. I believe that it ought to be memorialised immediately. I suppose we shall be richly rewarded.’ So I looked up the files which contained the records of earlier memorials, and came to a very different conclusion. I said, ‘Although the phenomenon has occurred on the last day of the month (i.e. at the new moon), its shape was slightly different from what it ought to be. Besides, if the Ching Hsing appears, there ought to be reports coming in of wine-sweet springs, phoenixes, purple herbs, and felicitous clouds, in order to corroborate the omen). But on the contrary there are epidemics and catastrophes in Shensi, brigands and robbers in the central provinces, and in Fukien rebels are active. I am afraid it won’t do. Why should the Tao of heaven be pro- claiming the opposite (to the Tao of earth)? ‘But Mr Li was most obstinate, and stuck to his Opinion. So I said ‘Up to now, only the six Observers here have seen the phenomenon. In the unlikely possibility of its having been generally seen by people throughout the Country, will they not have taken it as an omen of evil?’ Finally he agreed to wait and see if it appeared again (that night), before we memorialised about it. And indeed only nine days later the planet Venus ‘crossed the meridian’. All this shows how careful one has to be not to take lightly responsibilities like these.‘

The Ching Hsing phenomenon in question in this passage is what is today called Earthshine, when light reflected from the Earth illuminates the dark side of the moon, usually close to the new moon.

Well, as you might expect, over the centuries Chinese astronomers also made note of the motion of the planets. The five classical planets were typically known as wu pu, the five walkers, or wu wei, the five threads. Each planet had an astrological association with a cardinal point and an element in a system called wuxing, which literally translates to the “five phases,” or “five agents.” This system developed out of the School of Naturalists during the period of the Hundred Schools of Thought in the Warring States Era. Like most of these purported hundred schools of thought, the School of the Naturalists was not a single cohesive philosophy, but a collection of ideas from disparate philosophers who all had similar kinds of thoughts to each other. But one common feature to philosophers who fell under the School of Naturalists label is that they took an unusual interest in the nature of matter and the physical world. In this way, they were somewhat similar to the Ionian school in Greece that we discussed back in Episode 10. But unlike the Ionian school, philosophers in the School of Naturalists were not monists. They instead held that all matter was composed of five elements: earth, wood, fire, metal, and water. These elements were then related to each other in dynamic ways. There was the generating cycle, where one element would naturally generate the production of another. So wood feeds fire. Fire produces earth in the form of ash. Earth creates metal in the form of minerals underground. Metal produces water in the form of condensation. And water, in turn, allows wood to grow. But the elements could act in destructive ways with each other as well. Fire melts metal. Metal can chop wood. Wood depletes the Earth as it grows. Earth obstructs the flow of water. And water extinguishes fire.

Well each of these elements was associated with one of the planets and one of the cardinal directions. Mercury was associated with water and the direction north, and was called “the Hour-star.” Venus was associated with metal and the direction of West and was called the “Great White One.” Mars was paired with fire, unsurprisingly, and the South and was called the “Fitful glitterer.” Jupiter got Wood and East and was called the “Year Star.” And finally Saturn was associated with Earth and the center, and was called the “Exorcist.”

These five elements first appeared in a work by a philosopher named Chou Yen, and although the School of Naturalists even as a loose grouping disappeared after the intellectual purges that took place during the establishment of the Qin Dynasty, the general wuxing system persisted in Chinese thought. These ideas also became integrated with another system, the dualistic system of the yin and yang. In this system all entities have some mixture of these two opposing but complementary natures. Yin is a passive and receptive nature and is associated with the feminine, and yang is the active and expansive nature and is associated with the masculine. In this system the Sun was yang as it produced light that emanated out away from it and permeated the universe. The Moon was yin as it received the light emitted by the Sun. As the stars shone with their own light, they were called the “Hsiao Yang,” or “lesser yang.” Now, generally the systems of wuxing and the yin and yang were most closely associated with Taoism, but the ideas were by no means restricted to Taoist schools of thought, and were generally important concepts in Chinese culture.

Well, the royal astronomers had a relatively fine set of gradations for the motions of the planets as they observed them. They kept a particular eye out for all the major stations like a heliacal rising or a stationary point. But they also marked whether the planet’s motion was prograde, or retrograde, which was literally written as a “retreat.” And they would mark the speed of the planet’s motion, along with its acceleration, if it had an unexpectedly rapid advance or if it unexpectedly slowed down. If the planet remained at a stationary point for more than 20 days it was called su or shou, which literally translates to guarding the constellation it was near.

By the end of the first century AD the synodic periods of all the planets had been measured to good accuracy, which was about 75 years ahead of records of similar quality in the West. Now, although the five planets had their various astrological associations, the most important was Jupiter. Jupiter’s sidereal period is very close to 12 years, so its period was seen as a kind of “greater year.” This is also why it’s name was, as I mentioned earlier, the “Year-Star.” Just as there were 12 months in a year, there were 12 years in this longer Jovian cycle. And this is probably the origin of the famous Chinese zodiac where each year is associated with a particular animal in a 12 year cycle. But while Jupiter’s period was important, astrologically there was actually a more important body, called Taiyin, or the “counter-Jupiter.” The Counter-Jupiter was an unseen celestial body that moved in the opposite direction as Jupiter, but with the same period. We have seen similar kinds of hypothetical celestial bodies in other cultures. Probably the most relevant analogy was the Pythagorean idea of the Counter-Earth, that there was a second Earth on the opposite side of the Central Fire which we were always facing away from. In Chinese astrology it was not so much the position of Jupiter that mattered, but the position of the Counter-Jupiter. A book called the Chi Ni Tzu recorded some of the astrological implications of the position of the Counter-Jupiter that developed in the 4th century BC:

When the Taiyin is at the position of the element Metal during the first three years, there will be abundant harvests. When it is at Water, there will be damage to crops for three years. When it stands in Wood, for another three years, there will be prosperity; when it stands in Fire, again for three years, there will be drought. So some times are suitable for storing agricultural products while at other times rice may be given away. Accumulations need not be for more than three years. Deciding wisely and with judgment, and helped by the Tao (of Nature) one may draw on surpluses to mend shortages. In the first year there will be a double harvest, in the second a normal one, in the third a poor one. During floods chariots should be built, during droughts boats should be prepared. Bumper harvests come every six years, and famines every twelve. Thus the sage, predicting the recurrences of Nature, prepares for future adversity.

Now in Chinese culture, along with other cultures of southeast Asia, the twelve year cycle of Jupiter’s sidereal period then factored into a longer cycle of 60 years, called the sexagenary cycle. This came about because it was the length of time to cycle through both the 12 years of the Chinese zodiac and also through the 5 elements. So each year was not only associated with an animal of the Chinese zodiac, but it was also associated with one of the five elements. In practice the elements were doubled up so that two years in a row get the same element, but either way, it takes 60 years to come back to where you started. As it happens, Jupiter and Saturn also come to a conjunction nearly every 60 years, which was another reason for the significance of this period of time.

Now for most purposes of astrology this sexagenary cycle was sufficient. But as in India, astronomers later constructed even longer cycles. One of the foundations of these longer cycles was called the pu, which is the Callippic cycle of 76 years that I talked about back in Episode 17. The Callippic cycle is special because it contains very nearly an integer number of years and synodic months. Incidentally, as far as we know, the Chinese never used the less accurate Metonic cycle of 19 years in their astronomy, even though this cycle is far more common across the globe.

20 pu was called a “sui,” of 1520 years. And then 3 sui was a shou, and 7 shou was a chi of 31,920 years. This corresponds to 4 Julian periods in the Western calendar, and according to the philosophers, after this time, “all things come to an end and return to their original state.”

Now, in addition to the cycles based around the “pu,” there was also a “hui,” of 513 years, which consists of 47 lunar eclipse periods. Unfortunately a hui is not an even number of days, so they put together three hui to make up a “tung” of 1539 years, which is an even number of days. Then 3 tung, or 4617 years, had an even number of days, months, years, and eclipse periods.

There were likewise cycles based around the motions of the planets. In addition to the conjunctions of Jupiter and Saturn which occur every 60 years or so, simultaneous conjunctions of Mars, Jupiter, and Saturn happen every 516 years, which is very close to the length of a hui. This may have been the origin of the idea set out in a work called the Meng Tzu that a great sage would appear every 500 years. All together, the great period of the planets was 138,240 years. And if we combine that with the 3 tung, the great cycle of the universe in Chinese astronomy was 23,639,040 years long. It was generally believed that at the beginning of the world, all the planets were in conjunction with each other, and that when this grand conjunction recurred it would mark the end of the world.

Well, in addition to the motions of the planets there is of course the motion of the Moon, and Chinese astronomers naturally took a keen interest in it, too. Now it seems that most cultures independently came to the idea that the moon reflects the light of the Sun rather than producing its own light, but the Chinese were relatively early to come to this conclusion. The first claim that the Moon’s light is the reflected light of the Sun shows up in the Chou Pei around the 4th century BC where it says: “The Sun gives to the moon her appearance, so the moonlight shines brightly forth.” The astronomer Ching Fang gave more extensive description in the 1st century BC:

The moon and the planets have shape but no light. This they receive only when the sun illuminates them. The former masters regarded the sun as round like a crossbow bullet, and they thought the moon had the nature of a mirror. Some of them recognized the moon as a ball too. Those parts of the moon which the sun illumines look bright, those parts which it does not, remain dark.

It seems that from roughly the 4th century BC onwards, Chinese astronomers more or less continuously held that the Moon reflected the light of the Sun. Around the 6th century AD imperial astronomers were exposed to several alternative theories of the Moon’s light from India, but these ideas never took hold.

Now, also very early on, sometime before 300 BC, Chinese astronomers measured the motion of the Moon and found that it moves eastward by 13° every day, which is approximately correct. But also around this time, the astronomer Shi Shen, whom we met in the last episode as one of the creators of the first star catalogs in China, recognized that the motion of the moon was variable, sometimes moving faster and sometimes slower, and that it moved north and south of the ecliptic. Around 10 BC a more sophisticated description of the Moon’s motion appears, called the “9 roads of the moon.” The 9 roads of the moon effectively shows the precession of the Moon’s orbit over the course of 9 years, although the real period is closer to 8 years and 10 months.

I wanted to wrap up this episode by saying a few words about the place of the Moon and the other planets in Chinese folklore. In Western culture we speak of there being a “man in the moon.” In China and throughout southeast Asia they instead speak of a rabbit in the moon. The rabbit is seated on the left side of the moon and is grinding medicine in a mortar and pestle. The story of how he got there is told in the Jakata Tales, which is a Buddhist text which actually originates out of India but was influential throughout southeast Asia, anywhere with a large Buddhist population. The story goes like this: A monkey, otter, jackal, and rabbit decided that they would all do a good deed on each full moon. One full moon, an old man came begging for food. So, the monkey gathered fruit for the man, the otter provided fish, and the jackal caught a lizard and also gave a pot of curds. But the rabbit only know how to graze grass, so he offered his own flesh to be eaten and threw himself onto a fire. Miraculously, the rabbit was not burnt. The old man revealed himself to be none of other than Sakra, the ruler of the Trayastrimsa heaven. Being impressed at the rabbit’s act of charity, he drew the rabbit’s image onto moon, draped in smoke.

As for the mortar and pestle that the rabbit is seen to be using, the rabbit was said to be the pet of Chang’e, who was the goddess of the Moon in Chinese folklore. As with the Greek myths that we discussed in Episode 8, there is no canonical version of these stories, but one telling goes like this: A long time ago, there had risen not one, but 10 suns in the sky. The Earth became scorched, the crops died, and the people suffered. An archer named Hou Yi shot down nine of the suns, leaving just the one left in the sky. For this act he was rewarded with two vials containing the elixir of life, one for himself and one for his wife, Chang’e. However, while he was out one day, his apprentice Fengmeng broke in and demanded the elixir for himself. Rather than give it to Fengmeng, Chang’e swallowed the elixir in both vials herself. She then rose into heaven, bringing her pet rabbit with her and chose the Moon as her home. There the rabbit can be seen preparing the elixir.

Well, the Moon is not the only celestial body with a corresponding mythology. Taoism has the concept of a “xian,” which is usually translated as “immortal.” Some of these xians were the gods of the various planets, and from time to time they would spend some time on Earth in a human body. One such xian is the sage Huang Shigong, which literally translates to Yellow Rock Old Man, who was said to be the incarnation of the god of Saturn. As legend goes, Huang Shigong was originally known as Wei Che and early in his life he had been a prominent court official. But after the rise to power of the tyrannical Qin Shi Huang, the first emperor of the Qin dynasty, Wei Chi refused to serve in the court and fled civilization. He took up residence as a hermit in a cave on Mount Huang and his original identity was forgotten and he became known to the locals simply as the Yellow Rock Old Man, or Huang Shigong.

Well some time later, a man named Zhang Liang was passing by. Like Huang Shigong, Zhang Liang, too, used to be a big shot. Zhang Liang was from a long line of high officials in the Han State. But Zhang Liang had the unfortunate luck to be born in interesting times and his high status in the Han State became a liability after the Qin State conquered them, along with the other independent states of China at the time and established the Qin Dynasty. Zhang Liang expended his considerable resources fighting against his new overlords and attempted to assassinate the Qin Emperor on several occasions. In one of these attempts he hired the strongest man he could find and forged him an enormous hammer. Zhang Liang heard that the Emperor was touring various parts of the country and learned where his path would take him. So Zhang Liang and his hired muscle waited in ambush hidden up on a hill. As the Emperor’s entourage passed by he directed his assassin to hurl the hammer at the largest carriage pulled by the most horses in the middle of the caravan. The assassin hit the carriage right in the center and obliterated it, killing everyone inside. But to his dismay, Zhang Liang discovered that the emperor was not, in fact, in the elaborate central carriage, but a neighboring carriage, and survived the assassination attempt.

Zhang Liang narrowly managed to evade capture in the chaos that he had wrought, but he was seen and became a wanted man. So he forced to assume a false identity and retreat to the hinterlands. During his travels he was crossing a bridge, and it was here that he happened upon Huang Shigong. Huang Shigong saw the young man, bent over, took off one of his shoes, and then threw it over the side of the bridge into the water. Then he said, “You, boy, go and fetch my shoe from the water.” Zhang Liang knew that he was being trolled in some way, but being a fugitive he couldn’t afford to make any more enemies. So he bit his tongue, crawled down the embankment, collected the old man’s shoe, and brought it back to him. Huang Shigong put his shoe back on, laughed at the young man, and then walked away. Zhang Liang stood there wondering what had just happened to him. But after walking some distance, Huang Shigong turned around and returned to the Zhang Liang and told him, “I guess the boy can be taught! Meet me again at this bridge at dawn in five days.” Five days elapsed and right at dawn Zhang Liang rushed onto the bridge. But Huang Shigong was already there and told him, “You must not be late when meeting an old man. Come back five days later.” Another five days elapsed, and this time Zhang Liang left well before dawn. But when he arrived at the bridge Huang Shigong was already standing there waiting for him. Once again the old man told him off for keeping an elder waiting and said to return five days later. After another five days Zhang Liang was determined not to be beaten, so he showed up at the bridge at midnight. When Huang Shigong arrived and saw Zhang Liang already there he gave him a book that he had written on military strategy called the Three Strategies of Huang Shigong. Huang Shigong told the young man to study the book and that within ten years the circumstances will be such that he could put his knowledge to use and become an advisor to an Emperor. And indeed, within a decade nobles in the Han State rebelled to overthrow the Qin Dynasty and established the Han Dynasty in its place, and for his service in the rebellion, Zhang Liang became a trusted advisor to the new Han Emperor.

Another figure in Chinese history, Dongfang Shuo, was also said to be a xian, usually the incarnation of the god of the planet Venus or Jupiter. Dongfang Shuo was a man who was notoriously tricky to pin down. He was technically a court scholar and official, but really seemed to behave as more of a court jester who always managed to talk his way out of the mess that his high jinks had gotten him into. But he at times seemed to be a source of inexplicable wisdom. One story goes that he was traveling with the court of Emperor Wu, when the entourage came upon a monster who was blocking a mountain pass. Try as they might, the monster could not be budged and the imperial train could not get by. Dongfang Shuo stood about looking around during these efforts, and when the Emperor’s bodyguards had exhausted themselves, Dongfang Shuo went up to the monster and showered it with wine, whereupon it promptly melted away. When asked by the Emperor how he knew to do this, Dongfang Shuo replied that there had been a dungeon here during the tyrannical Qin dynasty, and so this was a place of great sorrow. This monster is the ghost of that sorrow. But wine dispels sorrow, and so it dispelled the monster.

Dongfang Shuo’s immortal nature was revealed in another legend in which Xi Wangmu, the mother goddess of the west visited the court of the emperor. She brought seven peaches of immortality, five of which she gave to the Emperor, and two of which she ate herself. Looking around the court, she noticed Dongfang Shuo and asked the emperor if he was aware that the god of Jupiter was in his court.

Well, I still have a few things to say about Chinese astronomy, but they will have to wait until the next episode. Next month we’ll look at the place of the hsiu, or lunar mansions, along with a couple of biographies of some of the most important Chinese astronomers, and then we’ll wrap up our tour of Chinese astronomy by skipping ahead to the 17th century and looking at the role that the arrival of the Jesuits in Chinese astronomy. I hope you’ll join me then. Until the next full moon, good night and clear skies.

Additional references

  • Needham, Science and Civilisation in China, Vol. 3