Jupiter's Satellites Before Galileo

While it is assumed that Galileo was the first to see Jupiter's satellites, it is quite possible that they were sighted much earlier, though not telescopically, of course. Of 5th. magnitude and several arc minutes from the planet, a satellite would be visible with the naked eye.

Ancient Chinese chronicles mention Jupiter's satellites. The observations were made by Gan De, one of China's earliest astronomers, in the 4th. B.C. Although his works are lost, portions are preserved in the Kaiyuan Treatise on Astrology, compiled between A.D. 718 & 726. Here, Gan De is quoted as saying: "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 Xunu, Xu and Wei. It was very large and bright. Apparently, there was a small reddish (chi) star appended (fu) to its side. This is called an alliance (tong meng)." An interpretation of the Chinese meaning of the words clearly means that Gan De had seen a Jovian satellite.

The Bejing Planetarium set up simulations to test the validity of Gan De's sighting. The results showed that people with good eyesight can see a satellite of magnitude 5.5 when it is five arc minutes away from a planet of magnitude -2.0. (Jupiter's varies from -1.4 to -2.5.) Thus Gan De could have seen either Callisto or Ganymede, as both would fulfill the conditions above. Ganymede is the more likely, though, as it is the brighter of the two.

The Kaiyuan Treatise gives the observed positions for a twelve year period. A comparison with modern tables of Jupiter's motion leads to the belief that Gan De's sighting was most likely made in the summer of 364 B.C., when Jupiter was in our constellation of Aquarius. Thus it appears that Galileo's announcement in 1610 was scooped by a Chinese astronomer by almost 2000 years.

Adapted from "News Notes", 'Sky and Telescope', February, 1982.

Rob Roy

Hamilton Amateur Astronomers
Maintained by Rob Roy