Diyuanlun 地圓論

Diyuanlun 地圓論 "On the roundness of the Earth" is an astronomical book written during the high Qing period 清 (1644-1911) by Wang Zhenyi 王貞儀 (1768-1797), courtesy name Deqing 德卿, style Jianging Nüshi 江寧女史, from Shangyuan 上元 (today part of Nanjing 南京, Jiangsu).

Wang was the first female astronomer of China. She was also familiar with meteorology, geography, mathematics, and medicine, and was also a good poet. The scholar Qian Daxin 錢大昕 (1728-1804) called her a worthy successor of Ban Zhao 班昭 (c. 45-c. 117 CE), author of a good part of the history book Hanshu 漢書, which also includes a chapter on astronomy (see Tianwen zhi 天文志). Wang Zhenyi wrote the books Xingxiang tushi 星象圖釋, Chousuan yizhi 籌算易知, Xiangshu kuiyu 象數窺餘, Congding cesuan zhengwei 重訂策算證僞, Xiyang chousuan zengshan 西洋籌算增删 (all on astronomical mathematics), Nümeng shisong 女蒙拾誦 (on female education), and Shenke yiyu 沈疴囈語 (on medicine?), the commentary Wenxuan shifu canping 文選詩賦參評 and the brush-notes-style book Xiuzhi yujian 綉帙餘箋. Her collected writings are called Defengting ji 德風亭集. The first collection of it, Defengting chuji 德風亭初集, includes some texts on calendar and astronomy, among others, the Diyuanlun of 6 juan length. Other texts are Yingsuo gaobi bian 盈縮高卑辯, Jingxingbian 經星辯, Huang-chi erdao bian 黄赤二道辯, Suicha rizhi bianyi 歲差日至辯疑, Diqiu bi jiuchongtian lun 地球比九重天論, Sui lun ding yu dixin lun 歲輪定於地心論, Riyue wuxing suitian zuoxuan lun 日月五星隨天左旋論, Gougu sanjiao jie 勾股三角解 and Rishi jie 月食解.

The Diyuanlun presents six arguments to demonstrate that the Earth is indeed a sphere. If one follows a compass and continues traveling eastward, the author says, one will eventually return to the starting point. As one travels southward, the North Star appears lower in the sky; upon reaching the equator, it lies on the horizon. Conversely, as one travels northward, the North Star appears higher; at the North Pole, it is directly overhead. The higher one ascends, the farther one can see, with visibility increasing with altitude. When observing an approaching ship from the shore, its mast becomes visible before its hull. Conversely, as a ship sails away, its full shape is visible first, then only the mast, until it disappears completely. During a lunar eclipse, when the Earth passes between the Sun and the Moon, the Earth's shadow partially or fully obscures the Moon. The shadow cast on the Moon is always round, indicating the Earth's spherical shape. The Sun rises earlier in the east than in the west, with sunrise progressively delayed the farther west one travels. There must be a location where the time difference between east and west is precisely twelve hours, in other words, when it is noon in the east, it is midnight in the west.

The text also raises the question of why people on the opposite side of the Earth do not fall off and why those on the sides do not topple over. Wang Zhenyi provides her explanation: Because the Earth has a massive gravitational force, and this force originates from a single point at its centre, all people, regardless of their location, stand with their heads facing upward and their feet firmly planted on the ground.

The text is included in the series Jinling congshu 金陵叢書. It was reprinted in 1916 by the Shenxiu Studio 慎修書屋.

A book with a similar title, Diyuanshuo 地圓說, was compiled by Jiao Tinghu 焦廷琥 (1783-1821). It is included in the Xuxiu siku quanshu 續修四庫全書.