Gan De


Gan De was a Chinese astronomer and astrologer born in the State of Qi also known as the Lord Gan. 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.

Observations

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 364 BC,
long before Galileo Galilei's celebrated discovery of the same in 1610.
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

Celestial comparisons

Shi Shen and Gan De divided the celestial sphere into 365°, as a tropical year has 365 days. At the time, most ancient astronomers adopted the Babylon division where the celestial sphere is divided by 360°.

Books

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.
It can be seen on the quotations under Shiji and Hanshu, but was preserved mostly in the Treatise on Astrology of the Kaiyuan Era.
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.