Gan De (Chinese: 甘德; Wade-Giles: Kan Te, 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". Gan De is reported to have seen one of the moons of Jupiter (either Ganymede or Callisto) 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 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, he 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
Shi Shen and Gan De divided the celestial sphere into 365¼°, as a tropical year has 365¼ 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-volumes Treatise on Astronomical Astrology , both of which have been lost. Gan De also wrote the Astronomic star observation (天文星占, Tianwen xingzhan).
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.
In 1973, a similar catalogue by him and Shi Shen was uncovered in Mawangdui. It 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.
* Chinese star maps
1. ^ 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.
* 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.
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