Gan De (Chinese: 甘德; fl. 4th century BC) was a Chinese astronomer/astrologer born in the State of Qi also known as the Lord Gan (Gan Gong). Along with Shi Shen, he is believed to be the first in history known by name to compile a star catalogue, preceded by the anonymous authors of the early Babylonian star catalogues and followed by the Greek Hipparchus who is the first known in the Western tradition to have compiled a star catalogue.
Gan De made some of the first detailed observations of Jupiter in recorded history. He described the planet as "very large and bright". In one of his observations on Jupiter, he reported a "small reddish star" next to Jupiter. The historian Xi Zezong has claimed that this was a naked-eye observation of Ganymede in the summer of 365 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 occluding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane to prevent the planet's glare from obscuring them, one or more of the Galilean moons might be spotted in favorable conditions. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Shi and Gan together made fairly accurate observations of the five major planets.
Planetary periodic comparisons
|Planet||Period||Predictions by Gan and Shi||Modern day calculation|
|Jupiter||sidereal period||12 years||11.862615 years|
|Venus||synodic period||587.25 days||583.92 days|
|Mercury||synodic period||136 days||115.88 days|
Shi Shen and Gan De divided the celestial sphere into 365 1⁄4°, as a tropical year has 365 1⁄4 days. At the time, most ancient astronomers adopted the Babylon division where the celestial sphere is divided by 360°.
As the earliest attempt to document the sky during the Warring States period, Gan De's work possesses high scientific value. He wrote two books, the Treatise on Jupiter and the 8-volume Treatise on Astronomical Astrology, both of which have been lost. Gan De also wrote the Astronomic Star Observation (天文星占, Tianwen xingzhan).
In 1973, a similar catalogue by Gan De and Shi Shen was uncovered within the Mawangdui Silk Texts. 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.
- Shiji 27 stated that he was from the State of Qi; however, according to a 4th-century BCE testimony by Xu Guang, he was actually from the State of Lu. Further citation from another work dated to the 5th century by Ruan Xiaoxu gives an account indicating that he was from the State of Chu.
- Hockey, Thomas A.  (1999). Galileo's Planet: Observing Jupiter Before Photography. CRC Press. ISBN 0-7503-0448-0
- Yi-Long, Huang (1997). "Gan De". In Helaine Selin. Encyclopaedia of the history of science, technology, and medicine in non-western cultures. Springer. p. 342. ISBN 0-7923-4066-3.
- Deng, Yinke.  (2005). Chinese Ancient Inventions. ISBN 7-5085-0837-8
- Xi Zezong, "The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo," Chinese Physics 2 (3) (1982): 664-67.
- K. P. Seidelmann, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. Retrieved 2007-08-08., p.706 (Table 15.8) and p.316 (Table 5.8.1)
- NSSDC "Mercury Fact Sheet"
- also known as Gan's Treatise on Stars.
- Peng, Yoke Ho (2000). Li, Qi and Shu: An Introduction to Science and Civilization in China. Courier Dover Publications. ISBN 0-486-41445-0
- Another 2 volumes preserved texts attributed to Gan De 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.
- Du Shiran; et al. (1992). Biographies of Ancient Chinese Scientists Series One" Gan De. Beijing: Kexue Chubanshe. pp. 25–27. ISBN 7-03-002926-7.
- Ma Linghong (2002). Discoveries and Studies on the Bamboo and Silk Texts. Shanghai: Shanghai Shudian Chubanshe. pp. 56–58. ISBN 7-80622-944-2.
- Gu Jianqing; et al. (1991). Great Lexicon on Chinese Arts of Necromancy. Guangzhou: Zhongshan University Press. p. 648. ISBN 7-306-00313-5.
- 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.