Measuring the length of gnomon shadow was one of the fundamental astronomical observations in ancient China. It could help determine the seasons and orientation. Thus it was crucial for calendar making, which constituted an important aspect of imperial governance. This paper focuses on origin and development of gnomon shadow measurement, discusses the usage of a discovered painted stick four thousand years ago as the oldest gnomon, and shows the astronomical and social importance of gnomon shadow measurement through analyzing its role in the transition of cosmological theories, in the establishment of imperial government, and in the calendar reforms in ancient China. Computational analysis shows that the gnomon shadow lengths for the Winter and Summer Solstices recorded in the Zhou bi suan jing might have originally been measured at the Prehistoric (ca. 4300 BP) astronomical site of Taosi in Linfen, Shanxi. In 2002, a painted stick with calibrated scales was excavated in a royal Tomb from the Taosi culture. As we restored the stick and made a series of computational analysis and simulate measurements, we found the yearly-dates corresponding to the red-belted marks on the stick fit well with the yearly-dates obtained from the “horizontal calendar” of the Taosi site. According to our analysis, the painted stick should be about 173 cm long. By opening the two halves of the stick, the length of the stick is doubled, serveing as the template of the gnomon of 8 chi (200 cm) long. Gnomon shadow measurement, as a replacement for the “horizontal calendar”, reflected the development of civilization from local to universal. Moreover, gnomon shadow lengths for the twenty-four solar terms from the BC 200 to AD 900 figures that these data featured two types, which might have signified the transition in cosmological theory from “covering sky” to “celestial sphere” as two separate academic traditions. Gnomon shadow measurement was of top priority for the rulers to establish imperial authority government during Zhou dynasty. As mentioned in the Zhou li, the capital should be constructed at the “center of land”, where the gnomon shadow length on Summer Solstice measured 1.5 Chinese Chi (approximately 37.5cm). The “center of land” symbolized where the imperial authority lied. Boundaries of territories of vassalage were also determined by the gnomon shadow length. This was based on the shadow principle “one shadow length Cun corresponds with one thousand Li on earth”.