THE DISCOVERY OF JUPITER'S SATELLITE MADE BY GAN DE 2000 YEARS BEFORE GALILEO
XI ZE-ZONG(Institute for the History of Natural Science, Academia Sinica)
As one of the earliest astronomers in China, Gan De (Kan Te) in the 4th century B.C. had made a lot of observations about heavenly bodies, particularly about Jupiter, which was then called "Sui Xing" (The Year-star). He had written two books: The "Sui Xing Jing" (Treatise on Jupiter) as well as the "Tian Wen Xing Zhan" (Astronomical Star Prognostication). Unfortunately these books were lost at some early time, only some portions of it were left in the "Kai Yuan Zhan Jing" (The Kai Yuan Treatise on Astrology) compiled between 718 and 726 A.D. From the 23rd chapter of "Kai Yuan Zhan Jing" we find out an astonishing passage:'' Gan De Said, in the year of chan yan... Jupiter was very large and bright, it looked like having a small reddish ('Chi') star attached to it, and this was called league".League was a usual term in the Spring and Autumn (770-476 B.C.) and Warring States (475-221 B.C.). At that time two or several countries united together for a common purpose was called a league. Here it means a system consisting of Jupiter and the small star. The latter is a subsidary star of Jupiter and its colour as was mentioned is in accordance with that of Jupiter's four principal satellites: reddish or yellow. Obviously, such a record is an evidence of the earlier discovery of Jovian satellite by Gan De. But up to now no one has mentioned it.According to "Astrophysical Quantities" codified by C.A. Allen, at mean opposition the distance of the four main satellites' distance from Jupiter and their magnitude is 2'18", 3'40", 5'51", 10'18"and 4.m9, 5.m3, 4.m6, 5.m6 respectively. These data show that they are visible under good conditions by the naked eye on rare occasions. We have entrasted an imitative observation to the comrades working at Beijing Planetarium. As a result, when a satellite with 5.m5 is 5' away from a planet with-2m (brightness of Jupiter varying between -2.m5 and-1.m4), it can be seen by good eyes, so we believe that Gan De had discoveried J Ⅲ or J Ⅳ. Of the two, J Ⅲ is more possible than J Ⅳ, for the former is brighter than the latter and is the largest satellite in the solar system."The year of chan-yan" is a second year of chronology according to the Jupiter's cycle (adopting 12 years, actually 11.86 years). Jupiter's positions during 12 years were systematically described in this section of "Kai Yuan Zhan Jing". Comparing this section with the "planetary, Lunar, and Solar position from 601 B.C. to A.D. 1" made by B. Tuckerman, we confirm that this discovery was in the summer of 364 B.C. when Jupiter was moving in lunar mansion Wei (Aquarius)-almost 2,000 years before Galileo's discovery of Jovian satellites.