《淮南子·天文训》
假定测得出的日出时入前表数为一寸，这一寸就相当于一里，一里合一万八千寸，这样便可知道从观测点到大地东极的距离是一万八千里。
假使视日出，入前表中一寸，是寸得一里也。一里积万八千寸，得从此东万八千里。
Assuming that the value of the pre-table of sunrise measured is one inch, this inch is equivalent to one mile, and one mile is equivalent to 18,000 inches, so you can know that the distance from the observation point to the eastern pole of the earth is 18,000 miles.