Gan De
Gan De (, fl.
4th century BC) was a
Chinese astronomer born in the
State of Qi [Shiji 27 stated that he was from the State of Qi, however according to a commentary added by Xu Guang in the 4th century state that he was from the State of Lu, further citation from another work dated to the 5th century by Ruan Xiaoxu given an account that he was from the State of Chu.] also known as the
Lord Gan (Gan Gong).
We know of two books which he wrote, namely, the
Treatise on Jupiter and the 8-volumes
Treatise on Astronomical Astrology [a.k.a. the Gan's Treatise on Stars.], both of which have been lost. It can be seen on the quotations under
Shiji (volume 27) and
Hanshu (volume 26), but was preserved mostly in the
Treatise on Astrology of the Kaiyuan Era [Another 2 volumes preserved texts were attributed to him and Shi Shen and were incorporated to the Daoist Canon during the Song Dynasty, more commonly known as the Treatise on Stars of Gan and Shi. However, the book is generally not considered to be the more reliable than the Treatise on Astrology of the Kaiyuan Era, due to the anachronistic of name of places, etc. in the texts.]. In 1973, a simliar catalogue by him and Shi Shen was uncovered in
Mawangdui and was arranged under the name of Divination of Five Planets, it records the motion of Jupiter, Saturn, Venus and other planets in their orbits between
246 BC and
177 BC.
He is reported to have seen one of the
moons of Jupiter (possibly
Ganymede) with his naked eye in
364 BC, long before
Galileo Galilei's celebrated discovery of the same in
1610 (all four of the brightest moons are technically visible to the unaided eye, but in practice are normally hidden by the glare of Jupiter). By hiding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane, one or more of the Galileans can be spotted in favorable conditions.
*X. Zezong, The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo,
Chinese Physics 2 (3) (1982): 664-667.
Sky and Telescope, February , 1981.
