This paper provides a biographical overview of Aristotle (384–322 BCE) before examining his major contributions to astronomy. It surveys his theories on the spherical and finite universe, the stationary Earth at the center of the cosmos, the four classical elements plus a fifth celestial element called aether, and his complex theory of motion. The paper also discusses Aristotle's concept of the "prime mover" and how it was absorbed into medieval Christian and Islamic theology. While acknowledging that later scientists such as Galileo and Copernicus overturned many of his claims, the paper situates Aristotle's astronomical work within the limits of the scientific knowledge available in his time.
Aristotle (384–322 BCE) is one of the three most famous ancient philosophers — alongside Socrates and Plato — whose work has left an indelible mark on Western civilization. Aristotle was born in Macedonia, where his father served as a physician in the royal court. He went to study at Plato's Academy at the age of 17, where he remained for 20 years and became Plato's most distinguished pupil. After Plato died in 347 BCE, Aristotle moved to Assos, a city in Asia Minor, and married the adopted daughter of its ruler, Hermias. When Hermias was captured and killed by the Persians a few years later, Aristotle was invited to the Macedonian royal court, where he became tutor to the king's son Alexander — who would later earn fame as Alexander the Great.
In 335 BCE, when Alexander became king, Aristotle returned to Athens and established his own school, the Lyceum. It was during his 13 years at the Lyceum that most of his works were written. When Alexander the Great died in 323 BCE, anti-Macedonian sentiment in Athens forced Aristotle to retreat to a family estate in Euboea, where he died the following year.
The majority of Aristotle's works have been lost. Those that survived, consisting mainly of his lecture notes, reveal that he had studied and written in depth on almost every subject of his time, including anatomy, astronomy, physics, geology, zoology, aesthetics, rhetoric, logic, ethics, metaphysics, psychology, and literature. His accomplishments across such a wide range of subjects are remarkable. After the fall of Rome, his works were nearly lost to the Western world entirely. They were preserved and studied by Islamic scholars from the 9th to the 12th centuries, when Aristotelian texts were translated into Arabic. Aristotle was rediscovered by the Western world in the 13th century, and his ideas were held in such high esteem during the Middle Ages that his philosophy attained the status of dogma. His work has relevance to theology, education, literary criticism, the analysis of human action, and political thought, and it has helped to shape modern language and common sense.
Although Aristotle was among the first to introduce elements of empiricism into philosophical inquiry, he did not consistently apply the scientific principle of observation and experiment across all of his work. Some of his theories on astronomy, found in works such as Metaphysics, Physics, and On the Heavens, are based on philosophical speculation rather than empirical observation. This may help explain why his basic astronomical theories were later proved wrong by the more rigorously scientific work of Galileo and Isaac Newton. Some writers have criticized Aristotle's astronomical theories harshly, holding them responsible for retarding progress in the field, given that his writings were treated as authoritative for many centuries. A. Berry, in his book A Short History of Astronomy (1961), writes:
"There are also in Aristotle's writings a number of astronomical speculations, founded on no solid evidence and of little value… his original contributions are not comparable with his contributions to the mental and moral sciences, but are inferior in value to his work in other natural sciences…" (quoted in O'Connor and Robertson)
It must be remembered, however, that Aristotle did not have access to modern instruments such as the powerful telescopes that later enabled more accurate observation of the heavens. His reliance on philosophical speculation when theorizing about the universe was therefore a natural consequence of his era, and his contributions to astronomy should be evaluated in the context of the scientific knowledge available at the time.
Aristotle proposed that the universe was spherical and finite, and that the Earth stood at its center. He argued for a spherical universe on the grounds that the sphere is the most "perfect" shape, and he held that the universe must be finite because anything possessing a center cannot be infinite. Aristotle also argued that the Earth itself was spherical, citing the fact that all earthly substances tend to move toward the center, and that as one travels northward or southward, the visible stars change position in the sky.
He supported this theory with empirical observation and is credited as the first thinker to offer actual proof that the Earth is round. By observing a lunar eclipse — during which the Earth casts its shadow on the Moon — he noticed that the shadow was curved. Since only a round object can cast a curved shadow, he inferred that the Earth must be spherical. (Others before him, including Plato, had theorized about the roundness of the Earth but had not been able to offer proof.) Aristotle also used a simple device called a camera obscura — a screen with a small hole placed between the sun and the ground, which projected an image of the sun — to observe the sun indirectly. This device is still used today to observe solar eclipses.
Aristotle further theorized that the Earth was not only at the center of the universe but was also stationary, in contrast to the other planets. He supported this claim by arguing that if the Earth were moving, an observer on it would see the fixed stars in motion, just as the planets appear to move. Since the fixed stars do not appear to move in this way, he concluded that the Earth must be at rest. This geocentric model remained accepted for centuries before Galileo and Copernicus disproved it through more systematic observation.
In his astronomy, Aristotle accepted the theory prevalent in his time that there were four basic elements — earth, air, fire, and water — and proposed that the central region of the universe was composed of these four elements. He added, however, the important idea that the celestial bodies were made of a fifth element, which he called aether. Aether was eternal and unalterable, found in its purest form in the outermost celestial regions, but increasingly "contaminated" in the region below the Moon.
Aristotle's theory of motion is complex and has been interpreted in various ways. He used it to explain the movement of objects both on Earth and in the heavens. According to Aristotle, the universe consists of two distinct realms: the sub-lunar world, which is always changing and in which objects move in rectilinear (straight-line) motion; and the super-lunar world, the realm of the unchanging, eternal celestial bodies (made of aether) that move in circular motion. He further maintained that the four elements of the sub-lunar world tend to move in straight lines, but that the direction of their movement depends on their specific gravity — earth, being heavy, moves downward; fire moves upward; and water and air occupy the middle.
"Four elements, aether, and two-world motion theory"
"Cosmic first cause absorbed into religious thought"
O'Connor, J. J., and E. F. Robertson. "Greek Astronomy." University of St. Andrews Website. April 1996.
You’re 79% through this paper. Sign up to read the remaining 2 sections.
Sign Up Now — Instant Access Already a member? Log inAlways verify citation format against your institution’s current style guide requirements.