Gan De, a famous court astronomer of the Chu State in the Warring States Period (475-221BC), lived in the middle of the 4th century BC. He is one of the compilers of the world's earliest star chart and the discoverer of the second satellite of Jupiter.

  Compiling the World's Oldest Star Chart with Shi Shen

The achievements in astronomy and calendar during the Warring States Period are very impressive. The various feudal states all had their own court astronomers, which greatly promoted the development of the celestial observation. The most famous among them -- Gan De from the State of Qi and Shi Shen from the State of Wei -- together wrote The Gan and Shi Book of the Stars, which accurately record the positions of 120 stars, constituting the world's earliest star chart.

Through a long-time observation of the stars in the sky, Gan De and Shi Shen established their own naming system of the sky districts (called "xing guan" in China and "constellation" in the West) by first naming the constellation, which was then followed by the number of stars.

Chen Zhuo, an astronomer during the Three Kingdoms Period (220-280), through analyzing the star charts of Gan De, Shi Shen, and Wu Xian (also a court astronomer), reached a conclusion that there were 1,464 stars and 283 constellations, among which 146 were constellations named by Gan De.

In the West, however, no star chart appeared before the 3rd century BC. Therefore, Gan De and Shi Shen's star chart is the world's oldest.

  Discovering the Second Satellite of the Jupiter

Apart from observing the stars, Gan De also conducted long-term observation and quantitative research on the planets' movement. At his time, most people believed the planets moved in the same direction. Gan De and Shi Shen, however, found that Jupiter and Venus moved in the opposite direction.

In addition, Gan also established the concept of planets convergence cycle period, and calculated a relatively accurate time for the convergence of Jupiter, Venus, and Mercury. For example, he said that Jupiter's time period was 400 days, only a 1.1-day difference from the accurate 398.9 days.

Gan De was an expert on the observation of Jupiter, and wrote a monograph on it, in which he mentioned the existence of a little red star on the Jupiter.

Xi Zezong, a famous astronomic historian, once pointed out the fact that Gan De discovered the second satellite of Jupiter in the middle of the 4th century BC, while it is well known that the discovery of Jupiter's satellites happened in 1610, after the invention of the telescope. Therefore, it is amazing that Gan De discovered the second satellite of Jupiter with his naked eyes, 2000 years before Galileo.

In the 1980s, astronomic workers in China, through field observation proved that the satellite could be seen with the naked eye under certain conditions. With his unyielding perseverance and meticulous observation, Gan De wrote a brilliant chapter in the world's astronomic history.

Author: Jessie