Gan De

Gan De (, 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.

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 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

{|class="wikitable" ! Planet || Period || Predictions by Gan and Shi || Modern day calculation |- | Jupiter || sidereal period || 12 years |- | Venus || synodic period || 587.25 days |- |}

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°. 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.

See also

Chinese star maps

Hipparchus

Nicolaus Copernicus

Galileo Galilei

Notes

References

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.

Category:4th-century BC deaths Category:Ancient Chinese astronomers Category:Chinese astrologers Category:Zhou Dynasty people