Gan De facts for kids

Gan De (Chinese: 甘德) was an important ancient Chinese astronomer and astrologer who lived around the 4th century BC. He was born in a place called the State of Qi.

Gan De, along with another astronomer named Shi Shen, is thought to be one of the very first people in history known by name to create a star catalogue. A star catalogue is like a map or list of stars and their positions in the sky. Before them, some people in ancient Babylon also made star lists, but we don't know their names. Later, a Greek astronomer named Hipparchus also made a famous star catalogue.

Gan De also spent a lot of time watching the planets, especially Jupiter. Sadly, most of his original writings are lost today. However, we know about some of his work from parts quoted in later books.

Gan De might have been the first person to describe one of Jupiter's largest moons, which are usually too faint to see without a telescope. In the 1900s, a historian and astronomer named Xi Zezong found a part of Gan De's work that seemed to describe seeing one of Jupiter's biggest moons, Ganymede or Callisto, with just his eyes in the summer of 365 BC.

Gan De's Life and Work

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, a time when different parts of China were fighting.

He wrote two main books: the 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 known at that time during the 4th century BC. Gan De made some of the earliest detailed observations of Jupiter ever recorded.

Gan De's Lost Writings

Most of Gan De's original books are lost. But we know about some of his ideas because parts of his writings were copied into later books. One important book that saved his work is the Great Tang Treatise on Astrology of the Kaiyuan Era. This book was put together in the 700s AD.

We know the titles of two of his books: On Jupiter and Astronomical Star Prognostication. Other parts of his work can be found as quotes in famous Chinese history books like the Records of the Grand Historian and the Book of Han.

How Gan De Measured the Sky

Shi Shen and Gan De had a special way of dividing the sky. They thought the sky was divided into 365 and a quarter degrees. This was because a tropical year (the time it takes for the Earth to go around the Sun) has 365 and a quarter days. At that time, most other ancient astronomers, like those from Babylon, divided the sky into 360 degrees.

Planetary Movement Predictions

Gan De and Shi Shen also made predictions about how long it takes for planets to move. Here's how their predictions compare to what we know today:

Jupiter's Moon Observation

One of the most exciting parts of Gan De's work is about Jupiter's moons. In a chapter of the Treatise on Astrology of the Kaiyuan Era, Gan De's notes from 365 BC are quoted. He described Jupiter's position in the sky and said:

"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

In 1981, the historian and astronomer Xi Zezong suggested that the "small reddish star" Gan De saw was actually one of Jupiter's Galilean moons. These are the four largest moons of Jupiter. Before Xi Zezong, no one had thought that these moons could be seen without a telescope.

Xi Zezong used a planetarium 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 account of seeing either Callisto or Ganymede with the naked eye. These two moons are the brightest and easiest to see.

Since Ganymede is bigger and brighter than Callisto, Xi Zezong thought it was probably Ganymede that Gan De called the "small reddish star."

In 1982, astronomer David Hughes wrote in the journal Nature that other people had also reported seeing Jupiter's moons without a telescope in special conditions. For example, in places with very clear skies like the Apennine Mountains in Italy or the Caribbean.

Galileo Galilei discovered the four Galilean moons using his refracting telescope in 1610. He thought no one had ever seen them before him. However, Gan De's observation suggests that some people with very good eyesight might have seen them much earlier. It's possible to see them if you block out Jupiter's bright light, for example, by hiding the planet behind a tree branch.

One mystery remains: Gan De described the "small star" as "reddish." It's not clear why he would have called it reddish, as the moons don't appear that color to the human eye, even with a telescope.

Other Related Texts

In 1973, another text similar to Gan De and Shi Shen's work was found among 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