The idea that the length of the sun’s shadow increases 1 cun for every 1000 li north and decreases 1 cun for every 1000 li south has profound effect in the history of Chinese astronomy. Based on new archaeological and archaeoastronomical discoveries and studies, as well as textual investigations, this paper concludes the idea of 1 cun for 1000 li originated from observations of the sun’s shadow from two important archaeological sites, the Taosi site and the Wangchenggang site, during the critical period when the first state was being formed. The length of the sun’s shadow at summer solstice observed by the dwellers of Taosi site was 1.6 chi (16 cun), and that observed by the dwellers of Wangchenggang site was 1.5 chi (15 cun). The former handed down into Zhoubi suanjing while the latter into Zhouli. Study on the length and distance system of early China demonstrates that the distance between the two sites is very near to 1000 li at that time. It is a special period in Chinese history when measurement of the sun’s shadow and large-scale geographical survey began subsequently, and thus the idea of 1 cun for 1000 li came into existence.