Gan De

Gan De (

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.^[2] He wrote two books, the Treatise on Jupiter and the 8-volume Treatise on Astronomical Astrology.^[3] Gan De also wrote the Astronomic Star Observation (天文星占, Tianwen xingzhan).^[4].

Shen and Gan together made fairly detailed observations of the five major planets during the 4th century.^[2][5] Gan De made some of the first detailed observations of Jupiter in recorded history.^[6]

Works

All Gan's writings are lost, but some fragments are preserved in the

Shi Shen and Gan De divided the celestial sphere into 3651⁄4°, as a tropical year has 3651⁄4 days. At the time, most ancient astronomers adopted the Babylon division where the celestial sphere is divided by 360°.^[2]

Planetary periodic comparisons

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

"In the year of chan yan . . . , Jupiter was in Zi, it rose in the morning and went under in the evening together with the lunar mansions Nǚ, Xū and Wēi. It was very large and bright. Apparently, there was a small reddish star appended to its side. This is called 'an alliance'."

— Treatise on Astrology of the Kaiyuan Era, xxiii

The historian and astronomer Xi Zezong published a paper in 1981 in Acta Astrophysica Sinica identifying the "small reddish star" with one of the Galilean moons of Jupiter, an interpretation hitherto unrecognized.^[10][5] Xi used the Beijing Planetarium to simulate the brightnesses of Jupiter and its moons in their relative positions from the earth as reported in the astronomical publications of Bryant Tuckerman and Clabon Allen's Astrophysical Quantities.^[10][5] He concluded that the Galiean moons of Jupiter are visible to the human eye under good conditions, and that Gan's report was an accurate account of a naked-eye observation of either Callisto or Ganymede – the two brightest and most visibly distinct moons – in summer 364 BC^[10][5]= 9637 HE.

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" (赤 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.^[10][11]

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.

See also

• Chinese star maps
• Galileo Galilei
• Hipparchus
• Nicolaus Copernicus

Notes