Gan De facts for kids

Gan De (Chinese: 甘德), also known as Lord Gan, was an amazing ancient Chinese astronomer and astrologer. He lived around the 4th century BC in the State of Qi. Gan De, along with another astronomer named Shi Shen, was one of the first people known by name to create a star catalogue. This is like a map or list of stars in the sky.

Before them, some unknown people in ancient Babylon made star lists. After them, a Greek astronomer named Hipparchus also made a famous star catalogue. Gan De was also very interested in planets, especially Jupiter. Even though most of his original writings are lost, we know about his work from parts quoted in later books.

Gan De might have been the first person to describe one of Jupiter's moons without a telescope! In the 1980s, a historian named Xi Zezong found a part of Gan De's work. It seemed to describe seeing one of Jupiter's largest moons, Ganymede or Callisto, in the summer of 365 BC. This is incredible because these moons are usually too faint to see without a telescope.

Who Was Gan De?

Gan De was one of the first people to study the sky in ancient China. His work was very important for understanding the sky during the Warring States period. He wrote two books: Treatise on Jupiter and an 8-volume book called Treatise on Astronomical Astrology. He also wrote Astronomic Star Observation.

Gan De and Shi Shen worked together. They carefully watched the five main planets during the 4th century BC. Gan De made some of the earliest detailed notes about Jupiter in history.

Gan De's Amazing Discoveries

Sadly, most of Gan De's original books are lost. But we know about his work because parts of his writings were saved in other books. One important book is the Great Tang Treatise on Astrology of the Kaiyuan Era. This book was put together in the 8th century AD. We also know the names of two of his books: On Jupiter and Astronomical Star Prognostication. Other parts of his work are found in the Records of the Grand Historian and the Book of Han.

How Gan De Mapped the Sky

Shi Shen and Gan De had a special way of dividing the sky. They divided the celestial sphere (the imaginary ball around Earth where stars appear) into 365 and a quarter degrees. This was because a year has about 365 and a quarter days. At that time, most other ancient astronomers, like those from Babylon, divided the sky into 360 degrees.

How Gan De Tracked Planets

Gan De and Shi Shen also made very good predictions about how long it takes for planets to complete their orbits. Here's a comparison:

Gan De and Jupiter's Moons

One of the most exciting parts of Gan De's work is about Jupiter's moons. In a book called Treatise on Astrology of the Kaiyuan Era, there's a quote from Gan De's notes from 365 BC. He wrote about Jupiter being in a certain part of the sky. He said:

"Jupiter was very large and bright. Apparently, there was a small reddish star attached to its side. This is called 'an alliance'."

—Treatise on Astrology of the Kaiyuan Era, xxiii

In 1981, the historian and astronomer Xi Zezong suggested that this "small reddish star" was actually one of Jupiter's Galilean moons. These are the four largest moons of Jupiter. Xi Zezong used special tools to figure out how bright Jupiter and its moons would have looked from Earth back then. He concluded that Gan De's report was a true story of seeing either Callisto or Ganymede with his own eyes. These are the two brightest moons.

Since Ganymede is bigger and brighter than Callisto, Xi Zezong thought it was probably Ganymede that Gan De saw. Other astronomers, like David Hughes, have also noted that some people with very good eyesight have reported seeing Jupiter's moons without a telescope in special conditions. This means that Galileo Galilei, who discovered these moons with his telescope in 1610, might not have been the very first person to ever see them. It's possible to block out Jupiter's bright light with something like a tree branch, which can make the moons easier to spot.

However, it's still a mystery why Gan De described the moon as "reddish." Neither of Jupiter's moons looks reddish to the human eye, even with a telescope.

Other Related Writings

In 1973, another old text by Gan De and Shi Shen was found. It was part of the Mawangdui Silk Texts. This text, called Divination of Five Planets, describes how Jupiter, Saturn, Venus, and other planets moved between 246 BC and 177 BC.

See also

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