How to misunderstand a diagram: The eccentric/equant model in a 17th century Jesuit Chinese text.

Résumé En

In February 1631 a set of solar tables by Giacomo Rho (1593-1638) and his colleagues were presented to the Chongzhen emperor of the Ming dynasty (1368-1644); these came to form part of the astronomical compendium known as the Chong zhen li shu 崇禎曆書 ’Texts on mathematical astronomy [compiled] in the Chongzhen reign’. The texts presented included a table of the solar equation of center, calculated on the basis of applying to the sun the eccentric/equant model used by Ptolemy for the outer planets, and using Kepler’s bisection of Tycho’s solar eccentricity. At some time before the end of the dynasty, an appendix by an unknown author was added to the equation table, with a diagram and an explanatory text. The diagram portrays the model used used by Rho in calculating the solar equation of centre, but as Shi Yunli and Chu Longfei noticed in their 2012 publication, the calculation procedure set out in the following text text does not correspond to the eccentric/equant model as portrayed in the diagram. In this talk I shall examine the implications of this discrepancy for the level of understanding of the underlying model by the group of Jesuit astronomers and their Chinese associates, as well as for what appears to be a hidden aspect of the history of the relevant texts.