What is our place in the universe? Throughout the history, world civilizations were trying to answer this question. The knowledge of Babylonian and Egyptian civilizations made up the basis for Greek astronomy. The works of Greek astronomers happen to be of great value to mankind. What astronomical ideas and models became fundaments? Let’s find it out!
The Sphere of the World
By the 5th century B.C., the Earth was perceived as a sphere. This was a great discovery for ancient people who thought that the Earth was flat. Empedocles and Anaxagoras found arguments for the spherical nature of the Earth. According to the astronomers, the shadow of the Earth on the moon could be observed during a lunar eclipse. As the shadow passed the moon, its shape was clearly round. This served as an undeniable proof of the sphere shape of the Earth.
The Sphere of the Earth
While the lunar eclipse is quite a rare event, so it can’t be viewed as a firm proof. However, sailors were also talking about the roundness of the Earth. When a ship becomes visible on the horizon, the top of the ship was visible first. A lot of Greek works used this fact as the evidence of the roundness of the Earth.
Indicating the Size of the Earth
The Greeks often applied mathematics to analyze the nature of their world. In the 3rd Century B.C., Aristarchus of Samos tried to estimate the size of the Earth by using the data collected during a lunar eclipse. When the Earth was located between the sun and the moon, it caused a lunar eclipse. By measuring the Earth’s shadow on the moon, he managed to estimate its size.
A slightly different approach was used by Eratosthenes in 240 B.C. He calculated Earth’s circumference by measuring the shadows cast in Alexandria and Syene. The eligibility of this approach has not been totally confirmed. Nevertheless, these estimated were quite close to 100% accuracy.
Aristotle’s Fundamental Elements
Aristotle questioned the existence of four fundamental elements – fire, air, water, and earth. Moreover, the astronomer ignored the notion of void space. Aristotle stated that you could further reduce the elements into two pairs of qualities: hot/cold and wet/dry. The combination of these qualities affected the formation of elements. All the qualities were mutually replaceable. For instance, hot water turns into steam that looks like air.
The Elements in Aristotle’s Cosmic Model
According to Aristotle’s Cosmology, every element (earth, water, fire, and air) had a weight. The lighter substances like fire and air moved away from the center of the universe, while the heavier elements like earth and water stopped in the center. In this view, the world change was caused by the mix of elements. Aristotle considered the terrestrial as a place of birth and death, referring to these elements.
The Wandering and Fixed Stars in the Celestial Philosophy
The ancient Greeks found two types of celestial objects in the sky – fixed and wandering stars. While some objects seemed to move at exactly the same speed every night, others (nine planets) behaved chaotically. All of them belonged to the sphere that was split into an outer celestial realm and an inner terrestrial one. While the earth was changing, the sky remained the same.
The Unchanging Celestial Region
No change takes place in the outermost heaven and its proper parts. In Aristotle’s epoch, observational evidence was quite limited. But the astronomer stated that objects that seemed to be moving in the heavens could be explained as moving in the terrestrial realm. The celestial spheres were controlled by the movers responsible for the motion of the wandering stars. The Greeks perceived the mover as the god who could control any object in the heavens.
Ptolemy’s Circles on Circles
Claudius Ptolemy (90-168) is the author of multiple astronomical works. By using the observations of Hipparchus and Eudoxus, as well as knowledge of Babylonians, Ptolemy created the concept of predicting the stars’ trajectory. In his most famous work of Almagest, he synthesized and refined the astronomical ideas. Once the work was translated into Arabic and Latin, the Almagest turned into the primary astronomy work for the next thousand years.
The Almagest contains a lot of tables. This is why the book can be used as a guide for predicting the locations of the stars. Compared to the previous works in astronomy, the Almagest has clear instructions for presenting a system and describing the nature of the heavens. Ptolemy’s attempts to accurately predict the place of the stars made it possible to develop a much more complicated model later on.
The Ptolemaic Model
Ptolemy and other astronomers of his epoch were offering to add circles on the circular orbits of the wandering stars to explain their trajectory on the sky. These circles on circles received the name of epicycles. According to the Greek tradition, the heavens were described as a place of perfect circular motion. The level of perfection could be achieved by means of circles. This is how the effect of disorienting illustrations was achieved.
To avoid the complex nature of circles, Ptolemy introduced some new concepts. To accurately analyze a planetary trajectory, he decided to use eccentric circles. With the eccentric circle, the central element between planets was located in the orbit. Ptolemy put the epicycles on another set of circles known as deferents. Thus, the planets changed their positions on circles that went on circular orbits. Ptolemy also introduced equants, a mechanism that allowed the planets to move at different speeds on their way around the circles. The final model was marked with a complex nature, although its predictive power was quite high.
Ptolemy and Aristotle’s Cosmic Legacy
Ptolemy aimed to develop a mathematical concept that involves mathematical models with predictive power. Meanwhile, Aristotle introduced a physical model of the heavens. Ptolemy was inclined to integrate his model of the heavens into the analysis of its physical reality. He managed to use the mathematical models and data for forecasting the motion of heavenly objects. Although the name of Ptolemy is less known, his contribution continued the works of Aristotle.
The Greek contribution to astrology can hardly be exaggerated. Aristotle, Ptolemy, Empedocles, and Anaxagoras explored the earth and stars by means of observations. Amazingly, their findings helped the human civilization reach the current stage of development.