Zhao Youqin

Zhao Youqin (趙友欽 1271-?) was a Chinese mathematician, astronomer, alchemist, and Taoist monk. He is most well known for his book Ge xiang xin shu (革象新书), translated either as New Elucidation of the Heavenly Bodies or New Writing on the Image of Alteration, wherein he described a new method to calculate pi. He also described an instrument used to calculate the angle between a star and the North Pole, and another used to find the differences in the right ascension of two celestial objects. In the experiment, he had two light sources - tables with candles on them - which he covered with a board with a hole in the center, and observed the light as it passed through the hole onto a screen above. In doing so, he found that a larger hole let more light through; more candles led to a brighter image; the shape of the image on the screen is independent of the shape and size of the pinhole; and that moving the screen further away led to a dimmer image.

Zhao has another extant book, Xian Fo tongyuan, dedicated to becoming immortal using meditation through "Inner Alchemy." He also wrote at least two Daoist books, Xian Fo tong yuan [lun] ([A Discourse] on the Common Origins of [the Teachings of] Immortals and Buddhas) and Jin dan nan wen (Difficult Problems of Gold Cinnabar).

Works

Ge xiang xin shu (革象新书) Xian Fo tongyuan Xian Fo tong yuan [lun] ([A Discourse] on the Common Origins of [the Teachings of] Immortals and Buddhas) Jin dan nan wen (Difficult Problems of Gold Cinnabar) (no known extant copy)