Gan De

Gan De (; fl. 4th century BC), also known as the Lord Gan (Gan Gong), was an ancient Chinese astronomer and astrologer born in the State of Qi. 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 of Hellenistic astronomy to have compiled a star catalogue. He also made observations of the planets, particularly Jupiter. His writings are lost, but some of his works' titles and fragments quoted from them are known from later texts.

Gan De may have been the first to describe one of the Galilean moons of Jupiter, usually invisible without the aid of telescopes. In the 20th century, a fragment of Gan's work, in a later compilation of astronomical texts, was identified by Xi Zezong as describing a naked-eye observation of either of the two largest and brightest moons, Ganymede or Callisto in summer 365 BC.

Life Gan was one of the earliest practitioners of Chinese astronomy. As the earliest attempt to document the sky during the Warring States period, Gan De's work possesses high scientific value. Gan De also wrote the Astronomic Star Observation (天文星占, Tianwen xingzhan).

Shen and Gan together made fairly detailed observations of the five major planets during the 4th century.

Works All Gan's writings are lost, but some fragments are preserved in the Great Tang Treatise on Astrology of the Kaiyuan Era (Kai Yuan Zhan Jing), an 8th-century CE text whose compilation dates to the period between 718 and 726, and the titles of two treatises – On Jupiter (Sui Xing Jing) and Astronomical Star Prognostication (Tian Wen Xing Zhan) – are known.

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°.

Satellite of Jupiter Chapter 23 of the Treatise on Astrology of the Kaiyuan Era, which details the apparent positions of Jupiter during the course of a conjectural twelve year sidereal period (in reality 11.9 years), quotes Gan's notes on the "stars" observed in 365 BC, when Jupiter appeared in the lunar station of the constellation Aquarius, the Chinese lunar mansion of Wēi: = 9637HE.

Since Ganymede is larger and brighter than is Callisto, Xi reasoned that it was likely Ganymede to which Gan's "small reddish star" refers.

However, the description of the "small reddish star" is not explained; it is not known why Gan might have referred to either moon as "reddish" ( chi, a light red colour), since neither's colour is distinguishable by the human eye alone, and even using a telescope their colour appears uniform with Jupiter's.

Related texts 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.