Earth's circumference mile in the seventeenth century and the metre in the eighteenth. Earth's polar circumference is almost exactly 40,000 km because the metre was originally calibrated on this measurement (1/10-millionth of the distance between the poles and the equator), which is almost exactly 21,600 nautical miles (being defined as one minute of a degree – i.e. 360 multiplied by 60). It has been described as "the single most important measurement" of the Earth, assuming the Earth to be a sphere (the Earth is an technically an ellipsoid, albeit with just 0.3% flattening). Earth's circumference lines are the foundation of the coordinate
Chi (unit) of China, its special administrative region of Hong Kong, and Taiwan. In its ancient and modern forms, the chi is divided into 10 smaller units known as cun (the "Chinese inch"). 10 chi are equal to 1 zhàng. In the People's Republic of China "chi" has been defined since 1984 as exactly 1/3 of a metre, i.e., 33 cm (approximately 1.094 ft). However, in the Hong Kong SAR the corresponding unit, pronounced "chek" in Cantonese, is defined as exactly 0.371475 m (exactly in). The two units are sometimes referred to in English as "Chinese foot" and "Hong Kong foot". In