Perhaps no other figure in the history of astronomy is shrouded in as much mystery as Zhang Zixin 張子信 (d. 577), who fled to an unnamed island to devote himself to observation and experimentation for some thirty years, circa 526–556, before returning home to revolutionise how solar and planetary motion had hitherto been modelled. This shadowy figure represents such a radical break with the past that Jiang Xiaoyuan, for one, has invited us to consider the possibility that this Chinese astronomer’s work hints at outside influence from India. In this paper, I aim to reveal a far more complex geography of sixth-century astronomy, in which we can place Zhang Zixin’s work in one of four distinct regional mathematical cultures, equally ‘Chinese’, that split off, were forced together, butted heads and, eventually, made peace. This paper will proceed in two parts. The first will set the stage, introducing my recent work mapping the regional distribution and connections of the 224 known practitioners of the astral and mathematical sciences active in the period of political chaos and disunion from 317 to 618 CE. The second will outline how regional traditions that had grown apart were forced together by military conquest and how, once brought together, they continued to work towards opposite ends. Zhang Zixin’s clique, I offer, is probably the least likely to have been propelled to success by Indian influence, and, more than anything, it was the mathematical cultures of Chang’an and Jiangling that stood in its way for the sake of self-preservation.