Ancient Greek astronomy

Ancient Greek astronomy is the study of stars, planets, and the universe done by people who spoke the Greek language a long time ago, during classical antiquity. This includes the times of the Ancient Greek, Hellenistic, Greco-Roman, and late antique eras. It has three main parts: Classical Greek astronomy from the 5th and 4th centuries BC, Hellenistic astronomy from the 3rd century BC until the Roman Empire began in the late 1st century BC, and Greco-Roman astronomy that continued in the Roman world.

Greek astronomers were influenced by ideas from Babylonian astronomy and, to a smaller extent, from Egyptian astronomy. Their work spread far and wide, especially after it was translated into Arabic, helping scientists and math experts in the Muslim world during the Middle Ages.

The most famous Greek astronomer was Ptolemy, and his book called the Almagest influenced how people thought about astronomy for many years. Many of the star patterns, or constellations, we know today come from ancient Greek astronomy, even though the names we use now are often in Latin.

Key texts

Many important books about ancient Greek astronomy are only known by their names or a few descriptions. Some simpler books have survived because they were easy to understand and good for schools. These include works like the Phaenomena by Euclid and books by Autolycus of Pitane.

Three big textbooks were written just before the time of Ptolemy by Cleomedes, Geminus, and Theon of Smyrna. Roman writers like Pliny the Elder and Vitruvius also share some ideas about Greek astronomy. The most important source is the Almagest, where Ptolemy talks about many earlier astronomers.

Beginnings of Greek astronomy

Early Greek cosmology

Main article: Early Greek cosmology

Early Greek thinkers shared ideas with other ancient cultures. They believed in a flat Earth, a solid sky where the Sun, Moon, and stars lived, and an ocean around the world. These ideas were linked to gods like Ouranos, Gaia, and Oceanus.

Ionian school

Main article: Ionian school (philosophy)

Thales of Miletus was an important early thinker. He thought the universe was made of water and that the Earth was flat, resting on an endless ocean. His ideas led to more thinking about the world without magic. Later, Plato and Aristotle built on his work. They used math, especially geometry, to understand the world and believed we could learn by watching nature.

Anaximander, a student of Thales, noticed the sky moves around the North star. He thought of a big round sky around Earth and guessed Earth might be curved, though he thought it was shaped like a cylinder. The idea that Earth is a sphere became popular with the Pythagoreans because they loved perfect shapes.

Framework

Axioms

Ancient Greek astronomers believed several important ideas about the universe. According to Ptolemy in his book the Almagest, these ideas included that the sky looked like a ball, the Earth was a ball and stayed still in the middle of everything, and the stars were very far away compared to the Earth.

Planets

The word "planet" comes from a Greek word meaning "wanderer" because these points of light moved in the sky compared to the stars that stayed in the same place. People could see five planets without telescopes: Mercury, Venus, Mars, Jupiter, and Saturn. At first, people thought the bright light they saw in the evening and the morning were two different things, but later they learned it was the same planet, Venus.

Constellations

Eudoxus is thought to have given names to the groups of stars we call constellations. The book Phaenomena by Aratus from 270 BC describes these star patterns. Later, Ptolemy organized over a thousand stars into 48 constellations, including the twelve that make up the zodiac.

Sizes of astral bodies

Aristarchus tried to measure how big the Sun and Moon are and how far away they are from Earth. Not long after, Eratosthenes measured the size of the Earth quite close to its real size. Both Aristarchus and another astronomer named Hipparchus thought the Sun was much closer to Earth than it really is.

Geocentrism and heliocentrism

Most ancient Greeks thought the Earth was at the center of the universe, with the Sun, Moon, and planets moving around it. But some had different ideas. One idea was that a hidden fire, not the Sun, was in the middle. Another idea was that the Sun moved around the Earth but everything else moved around the Sun. Aristarchus of Samos was the first to suggest that the Sun, not the Earth, was at the center of everything.

Classical Greek astronomy

Plato and Eudoxus of Cnidus helped change Greek astronomy in the fourth century BC. They moved from studying stars to studying planets. They created a new idea: two big spheres, one inside the other, with the Earth in the center and the stars, Sun, Moon, and planets on the outer sphere. This helped explain how planets move.

Eudoxus worked in Athens and Egypt, then started a school in Cyzicus. He tried to understand how planets moved using geometric shapes. But some people found problems with his ideas, like why the Moon sometimes looks bigger.

Hellenistic astronomy

Apollonius of Perga

Main article: Apollonius of Perga

Apollonius of Perga lived around 240–190 BCE. He worked on improving old ideas about how stars and planets move. He created new ways to explain why the distance between Earth and stars seems to change. Later, other scientists like Hipparchus and Ptolemy used and built on his ideas.

Hipparchus

Main article: Hipparchus

Hipparchus was an important astronomer who lived in the 2nd century BCE. He made a list of stars and noticed something new in the sky, called a nova. He also discovered that the points where the sun appears farthest and closest in the sky shift over time, called the precession of the equinoxes. Hipparchus learned a lot from older Babylonian astronomers, which helped him figure out that a year to follow the seasons is a little less than 365.25 days, while a year to follow the stars is a little more.

Ptolemaic astronomy

Main article: Ptolemaic astronomy

Claudius Ptolemy was a mathematician who lived in Alexandria in Roman Egypt during the 2nd century AD. He studied the Earth and the stars and wrote many important books about astronomy. His most famous book, called the Almagest, explained everything known about astronomy at the time. It had 13 parts that talked about the Sun, the Moon, the planets, and the stars.

Ptolemy used special ideas called "eccentrics" and "epicycles" to explain how the planets moved. These ideas helped him make better predictions about where the planets would appear in the sky. In his view, the Earth was at the center of everything, with the Moon, the Sun, and the planets moving around it. His work helped people understand the stars and planets for many years.

Influence

Reception of Ptolemy's Almagest

Ptolemy's ideas about stars and planets became very important in Europe and Islamic astronomy until new ideas appeared in the 1500s.

Many smart people wrote about Ptolemy's book over the years. Some just tried to understand what he wrote, while others thought astronomy was very important for understanding the world.

Indian astronomy

Further information: Indian astronomy

Some ideas from ancient Greece were brought into India a long time ago. One important book called the Yavanajataka was changed from Greek into a language called Sanskrit. This happened in the 100s, thanks to a king who wanted to share these ideas.

Later, two more books about stars and planets were very important in India. These books were part of a group of five main books written by a person named Varahamihira.

Notable Greek astronomers

Many people in ancient Greece studied the stars, planets, and the movements of the sky. Some of the most famous include Anaxagoras, Archimedes, Aristarchus, Eudoxus, Hipparchus, and Thales. These thinkers tried to explain how the Earth fits into the universe and how the stars and planets move. Their ideas helped shape how people understood science for many years.