Geminus facts for kids

Geminus of Rhodes (Greek: Γεμῖνος ὁ Ῥόδιος) was an ancient Greek astronomer and mathematician. He lived around the 1st century BC.

One of his books about astronomy, called Introduction to the Phenomena, still exists today. It was written as a beginner's guide for students learning about the stars and planets. Geminus also wrote a book about mathematics. However, only small parts of that work have survived, quoted by other writers later on.

About Geminus's Life

We don't know much about Geminus's life. It's not even certain if he was born in Rhodes. But he wrote about mountains on Rhodes in his astronomy books. This suggests he probably worked there.

His exact dates are also not known for sure. Some clues in his writings suggest he lived around 70 BC. This would mean he might have been a student of a famous thinker named Posidonius. However, some experts think he might have lived as late as 50 AD.

A crater on the Moon is named Geminus in his honor.

Geminus's Work in Astronomy

The only complete book by Geminus that we still have is his Introduction to the Phenomena (Greek: Εἰσαγωγὴ εἰς τὰ Φαινόμενα). It's often simply called the Isagoge.

This book was a basic guide to astronomy. It was based on the ideas of earlier astronomers, like Hipparchus. Geminus wrote it to help new students understand the subject.

In the book, Geminus explained many things, including:

• The zodiac and how the Sun moves through it.
• Different constellations, which are groups of stars.
• The idea of the celestial sphere, an imaginary ball around Earth where stars appear.
• How days and nights happen.
• When the signs of the zodiac rise and set.
• How Moon cycles work and how they relate to calendars.
• The different phases of the Moon.
• Eclipses, when the Sun or Moon is blocked.
• Different star phases.
• Earth's climate zones and various places on the globe.
• Why it's not smart to try and predict the weather using stars.

Geminus also wrote comments on another work by Posidonius, called On Meteorology. Some parts of these comments were saved by a writer named Simplicius.

Geminus's Work in Mathematics

Geminus also wrote a lot about mathematics. He had a big work called the Doctrine of Mathematics. This book is lost, but many parts of it were saved by other ancient writers like Proclus and Eutocius.

Geminus divided mathematics into two main parts:

• Mental (Greek: νοητά): This was about ideas and theories. It included geometry and arithmetic, which is the study of numbers.
• Observable (Greek: αἰσθητά): This was about practical uses of math. It included subjects like mechanics (how things move), astronomy, optics (light), geodesy (measuring the Earth), canonics (which is about musical harmony), and logistics (practical calculations).

Long sections of his math work were also saved by a writer named Al-Nayrizi. These sections were part of Al-Nayrizi's comments on Euclid's famous book, Elements.