Science and Calendars in China and the West from Clavius to Xu Guangqi and Schall
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¥)ZÜ0{)¦¦» ,.°Wc.n¿té¦é¥ Science and Calendars in China and the West From Clavius to Xu Guangqi and Schall Peter H. Richter Bremen, April 25, 2008 Abstract The design of calendars is fundamentally different between China and the West. Chinese calendars stress the correctness of prediction and therefore rely on obser- vational precision. Western calendars, on the other hand, emphasize the ease of computation and are not worried about discrepancies with astronomical reality. The article discusses how the Jesuits who worked out the Chinese calendar reform of 1634/45 may have perceived this cultural difference. It points to the role of Xu Guangqi as sensitive supporter of mostly young Jesuits who left Europe because at the time they did not see a future there as scientists. 1 1 Introduction This article is an attempt to come to an understanding of a fascinating pe- riod of early scientific interaction between East and West, 400 years ago in China. It must be pointed out at the very start that I am neither historian nor sinologist; as theoretical physicist with interest in philosophy I have some knowledge of astronomy and the history of science, but I cannot point to any research of mine in these fields. Therefore, to experts in the field it may seem rather jaunty on my part to contribute an article on a subject which has been discussed zillions of times in the literature: the Jesuits' role as mediators of cultural exchange in the 17th century. I came to this topic by way of planetarium presentations that I used to give in Bremen, when I decided to discuss the various calendars of the world. I was thrilled to learn that the two most widespread calendars, the Gregorian and the traditional Chinese, had been shaped by German Jesuits, Christo- pher Clavius of Bamberg, and Adam Schall von Bell of Cologne, respectively. Dwelling deeper into the matter I found that the two must even have met each other when Clavius, age 74, organized that memorable reception for Galilei at the Collegio Romano, 1611, and Schall, age 19, was a student there. I learnt that Matteo Ricci, the first Jesuit in China, had been a student of Clavius, and that the publication of his diaries in Europe by Nicolas Trigault in 1615 attracted an impressive crew of jesuitic scientists to China as of 1618. My sources were mostly jesuitic literature, some of them rather lyric. Little did they talk of Xu Guangqi, except for the fact that he could be claimed as the first Chinese Christian, Dr. Paul. I learnt of Xu Guangqi from Andreas Dress during my first visit to Shanghai in March 2007. And suddenly the story appeared in different colors. There was this high ranking Chinese scholar, expert in many fields (philosophy, mathematics, artillery, agriculture) who recognized the potential benefit for China that might be gained from Ricci's knowledge of European mathemat- ics, mechanics, and astronomy. There was their joint project of translating Euclid's Elements into Chinese which Xu published in 1607, and their com- mon interest in matters of calendars. Perhaps most importantly, there was Xu's role as political protector of the Jesuits when hostility broke out against them in Nanjing around 1617. It became more and more obvious that the calendar reform cannot have been the major reason for the Jesuits to go to China even though the Emperor seems to have had a vested interest in it; after all, the matter was not that 2 complicated . But then what? Looking back at the Europe of that time, it becomes obvious that young men would not perceive it as fertile ground for science: Giordano Bruno's auto-da-f´eof 1600, Galilei's admonition in 1616, the fierce antagonism between catholics and protestants leading to the Thirty Year's War { all this taken together was rather abhorrent. China, in contrast, promised to provide asylum: a country where knowledge was the key to recognition and success. If this was the case, then their assignment to do missionary work would not have been their major interest. Of course one cannot expect to support such a claim by studying their correspondence with Rome which had to be obedient. But perhaps it is possible to find hints in the work that they produced and left in China, the calendar work as an example. This article is a superficial attempt to argue in just that direction. It goes to some depth in comparing the design principles of Western and Chinese calendars, and it stresses that the Jesuits did not transfer the Gregorian reform to China. Rather they fully respected the Chinese tradition and most probably appreciated it as being more rational than the various Western traditions (Babylonian, Egyptian, Hebrew, Roman, Julian, Gregorian, Orthodox) which are so heavily loaded with religious demands. A stronger argument would require a thorough in- vestigation into the Chinese writings of the Jesuits. So far I did not succeed to find out how much of this has already been done by Sinologists. A prac- tical problem seems to be that the combination of ancient astronomical and sinological knowledge is rare. The article has four parts. The first, Sec. 2, is a general review of the art of calendar making. Sec. 3 explains the Gregorian calendar with its roots and its design as a theoretical construct. Then, in Sec. 4, the Chinese counterpart is presented as an attempt to predict events as correctly as possible. The history of the reforms from Clavius to Xu and Schall, and an evaluation from today's perspective, are given in Sec. 5. 3 2 Science and the art of calendar design 2.1 The challenge of calendar making The origin of science lies buried in the distant past, but undoubtedly it was stimulated by the experience that life on earth is closely related to celestial phenomena. These phenomena range from the daily cycles of the Sun and the starry sky, the yearly cycle of the seasons, the motion of the Moon and the other \wandering stars", i. e. the planets Mercury yh (Shuixing), Venus h (Jinxing), Mars Ûh (Huoxing), Jupiter ÷h (Muxing), Saturn Hh (Tuxing), but also less regular events like thunderstorms, rainbows, floods, or lunar and solar eclipses. Except for the daily and yearly cycles, it was not at all clear how these celestial phenomena influence our lives, and how they can be predicted. Priest-scientists were assigned the task to find out. They did so by a combination of observation, generalization, calculation, and speculation. One result of their studies, with immediate practical relevance, were calendars but they were embedded in more comprehensive schemes like astronomy and astrology, science and religion. Calendars are therefore an expression of knowledge on matters concerning the interaction of heaven and earth. Calendar reforms, if worked out by ex- perts in the field, reflect progress of that knowledge. However, to the extent that political interests interfere with their design they may occasionally lose reliability or even become useless. Of the celestial phenomena mentioned above, some are more predictable than others. The length of a day (depending on the season), the solar year, and lunar months present no major difficulties. The motion of planets is already more complicated, but with sufficient effort still predictable. Whether or not their constellations have any observable effect on human life as claimed by astrology, has always been a matter of dispute. In ancient times, astrology was treasured as a serious discipline, but in the end, astronomy and astrol- ogy parted: the former developing into a modern scientific discipline, the latter into a speculative art of character and fortune telling. On the other hand, there is no question that the phenomena of weather and climate are highly important for human activities. Yet, their unpredictability beyond season-related tendencies was recognized early on; so they are not included in respectable calendars. In the end, calendar makers concentrated on the motion of Sun and Moon among the stars. The art of calendar design is to define rules which solve the 4 following problems: 1. How to assign integer numbers of days to years if the period of the Sun is not a rational, let alone integer multiple of a day? 2. How to assign integer numbers of days to months if the period of the Moon is also an irrational multiple of a day, and even worse, not always the same? 3. How to assign integer numbers of months to years if the solar period is not a rational multiple of the lunar period? In other words: how to \couple" the Moon to the Sun in spite of their unrelated paces? Many different solutions were presented in the course of history, some of them trivial in that they ignored either the Moon or the Sun: The old Egyptian calendar only referred to the Sun and defined the year to be 365 days; it did not care about the discrepancy of some 0.25 days which made the calendrical year drift through the \true" astronomical year with the Sothis period of 1460 years. In contrast, the Islamic calendar only refers to the Moon and defines the year to be 12 lunations; it does not care about the discrepancy of some 11 days to the solar year which results in 33 true solar years being counted as 34 Islamic years. Notice that the difference of those two calendars is not only the shift from purely solar to purely lunar character but also from \mathematical" to \astronomical", or \theoretical" to \observational": the Egyptian calendar defined the length of a year, the Islamic calendar insists that the beginning of a month (new Moon) be determined by observation (as a result of which the dateline is not fixed on Earth but changes location and shape from month to month).