Was Galileo Really the First to Discover Jupiter Had Moons – Or Did a Chinese Astronomer Beat Him by 2000 Years?

I first heard this tale on the radio (likely the science program Quirks and Quarks on CBC) back in the early 1980s. The story as I remembered it purported that the moons of Jupiter were observed with the naked eye by Chinese astronomers long before Galileo had seen them.

I recently remembered this story and decided to google around for it. This is what I found …

Galileo Galilei is credited with discovering what are now called the Galilean Moons – Io, Europa, Ganymede, and Callisto. He did so with a telescope that he had built.

These moons are roughly the size of Earth’s moon and are easily seen through low powered binoculars.

What Gan De Saw:

In the March 18, 1982 edition of Nature, a brief article was published by astronomer David Hughes entitled Was Galileo 2,000 years too late?

In this article, Hughes quotes from Galileo’s treatise entitled Sidereus Nuncius (published in Venice in 1610), where the Galileo announced his discovery of:

four planets swiftly revolving about Jupiter at differing distances and periods, and known to noone before the Author recently perceived them …

To this Hughs retorts:

Galileo’s claim to priority has recently been shown to be unjustified; and it is not a matter of being just pipped at the post, but of some 2,000 years

Hughes article goes on to say that:

Xi Ze-zong, of the Institute for the History of Natural Sciences, Academia Sinica, has recently reported in Chinese Astronomy and Astrophysics that Gan De observed Ganymede in the summer of 365 BC.

Gan De was an ancient Chinese astronomer and astrologer who lived during the 4th century BC. Although all of his writings are lost, fragments of his books On Jupiter and Astronomical Star Prognostication can be found in the 8th-century CE text Great Tang Treatise on Astrology of the Kaiyuan Era.

The Treatise quotes Gan De 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 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’.

Gan De uses the word chi to describe the color of this star – it means light red.

Are Any Moons of Jupiter Visible to the Naked Eye?

When Jupiter is at its closest to Earth, its moons Io, Europa, Ganymede and Callisto have magnitudes of 4.9, 5.3, 4.6 and 5.6 (lower magnitudes are brighter). The maximum angular distances between Jupiter and the moons are 2.3, 3.7, 5.9 and 10.1 arc minutes. Thus Ganymede is the brightest, and its apparent distance from Jupiter is about one-sixth the width of a full Moon.

But Jupiter has an apparent magnitude of -2.6 – i.e. it is 760 times brighter than Ganymede.

Hughes describes how Xi Ze-zong used a planetarium to try to answer this question. He simulated two light sources differing in brightness by a factor of 760 and separated by 5.9 arc minutes.

He found that people with good eyesight could detect the fainter object.

Did Anyone Else See A Moon Around Jupiter Before Galileo?

Hughes Nature article quotes A Cycle of Celestial Objects, an 1844 book by Admiral William Henry Smyth:

Certain esprits fort express surprise that Galileo should have been so gratified by this discovery since they hold that the satellites of Jupiter are often seen with the naked eye and they cite the Apennines and Etna and the West Indies and various other fine-climate places as the spots where such a feat is frequently done.

Admiral Smyth’s book is available for free on google books and contains a bit more information than this quote.

Smyth says Galileo’s discovery “may be considered one of the first fruits of the telescope.”

Smyth writes:

Yet in spite of such authentic and minute details, there are some who assert that the satellites had been previously seen, both by Jansen and Lippersheim. Baron de Zach attributed the discovery to Thomas Harriot; but this has been confuted by my late friend, Professor Rigaud …

Note: It is not clear to me if “previously seen” means with a telescope or with the naked eye.

Smyth writes:

Hodierna seems quite astonished that, as they appear like stars of the sixth magnitude in the telescope, they should not have been seen earlier; and is satisfied that their not having been perceived, is entirely owing to the brightness of the planet.

Smyth however concludes that the moons are not visible, as he had tried to see them from the summit of Mount Etna.

But Smyth then writes:

Baron de Zach relates, that the late Pere Hell told him, that he had known an officer of the Hungarian Guard at Vienna, who could see Jupiter’s satellites with the naked eye; as was proved by simultaneous observation with the telescope.

Smyth continues to say that the Baron:

… also states that Dr. Muschenbrok knew some persons whose unarmed eye possessed such power …

Smyth describes another instance of someone seeing a moon:

M. Mӓdler mentions that the post-master, Nernst, saw one satellite of Jupiter with the naked eye, and drew a diagram of its position; but when the telescope was applied, it was found that three of the satellites had nearly closed, and their united brightness this drew the piercing eye of M. Nernst.

Note: I’ll confess, I found Smyth’s text to be rather confusing to read.

Questions:

Gan De’s books are lost. We only have a quote. But Wikipedia says Objects resembling lenses date back 4000 years although it is unknown if they were used for their optical properties or just as decoration. Is it possible he had invented a telescope of his own, and the record of this invention was lost?
Have you ever seen Jupiter’s moons with the naked eye?
Gan De described what he saw as “a small reddish star,” but the human eye cannot see color for such a small light source. What else could he have seen?
Smyth describes a conjunction (i.e. “united brightness”) of three satellites seen by M. Nernst – could it be that individual Galilean moons are not visible due to the brightness of Jupiter, but a conjunction of three is visible?
Do you think that Gan De spotted a moon of Jupiter, or was he just breathing CO?