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 to compile a star catalogue, 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". 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. 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 || 11.862615 years |- | Venus || synodic period || 587.25 days || 583.92 days |- | Mercury || synodic period || 136 days || 115.88 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°.

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-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 and 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

• Du Shiran et al (1992). Biographies of Ancient Chinese Scientists Series One: Gan De. Beijing: Kexue Chubanshe. ISBN 7-03-002926-7.
• Ma Linghong (2002). Discoveries and Studies on the Bamboo and Silk Texts. Shanghai: Shanghai Shudian Chubanshe. ISBN 7-80622-944-2.
• Gu Jianqing et al (1991). Great Lexicon on Chinese Arts of Necromancy. Guangzhou: Zhongshan University Press. ISBN 7-306-00313-5.
• 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.