Astronomy of Ancient Greece
The following notes are excerpted from Journey to the Cosmic Frontier by John D. Fix, and Archives of the Universe by Marcia Bartusiak.
Thales of Miletus (c. 636–546 B.C.) is credited with being the first natural philosopher to attempt an explanation of heavenly motion without resorting to myths and supernatural forces. For this, we might reasonably call him the first man of science. He taught that the earth was a flat disk floating on water. He was also given credit for predicting a solar eclipse in 585 B.C. during a battle between the Persians and Lydians that astonished them so much they ceased hostilities. Modern astronomers doubt Thales had the technical skills to predict a solar eclipse so precisely, but he may have learned of the saros cycle from the Babylonians and had a bit of luck (in addition to possibly confusing a lunar eclipse—which is much easier to predict—with a solar). A teacher of Anaximander and possibly Pythagoras. None of his writings survive.
Anaximander of Miletus, a younger contemporary of Thales, believed the earth floated in nothingness (since it did not appear to move in one direction or another). He also believed the earth to be cylindrical in shape.
Pythagoras and his followers are believed to be the first to reason the earth is spherical. The sphere was believed to be the most perfect of the geometric solids because it can be defined by a single parameter—its radius. Since the earth was thought to occupy the center of the heavens, its perfect geometry followed. Also, the Pythagoreans noted that the earth's shadow is always a curved line on the surface of the eclipsed moon. Only a sphere always produces a curved shadow.
Eudoxus of Cnidus (c. 408 – 347 B.C.) expanded upon
the Pythagoras' idea of perfect circles and accepted Plato's invention that
the planets move around Earth on crystalline spheres (the spheres had to
be perfectly transparent to see the more distant objects). From this, Eudoxus
developed a system of 27 concentric spheres centered on Earth. The outer
sphere contained the fixed stars while each wandering star,
or planet, required four spheres. The Sun and Moon each required three
spheres.
His conception of the heavens was purely mathematical and Eudoxus made no attempt to derive a mechanical explanation of these motions. Within 50 years the model had to be discarded, but his observations of the planets and stars were later utilized by Hipparchus.
Aristotle (384–322 B.C.) studied under Plato, but
unlike his teacher, believed ultimate reality existed in the physical
realm and not merely in the realm of ideas. Thus, experience in the physical
world could be used to understand reality. Of 150 writings, 30 of Aristotle's
treatises survive. In De caelo (On the Heavens) he stated
that the earth is a sphere of no great consequence
about 45,000 miles in circumference. The number is rather high,
however, and Aristotle never stated the source of his assertion.
Aristotle more successfully reasoned the earth had to be spherical based
on observations that stars rise above or dip below the horizon when one
travels north or south and that falling objects always fall perpendicular
to the ground when dropped. Believing that like
elements
were attracted to each other, a rock should be drawn
to Earth's center. Thus, the earth had to be spherical for a falling rock
to always fall perpendicularly to the ground. He also argued that the heavens
were immutable and that all change occurred in the earthly realm. The heavenly
objects were also believed to be of fundamentally different composition
than objects in the earthly realm.
Aristarchus of Samos (310–230 B.C.) made the first documented attempt to ascertain the distances to the Moon and Sun using geometry. His logic and geometry was correct, but inaccurate measurements resulted in the wrong answer. Although his results were incorrect, his realization that the Sun was many times larger than Earth probably caused him to propose a heliocentric (sun-centered) model of the heavens. Unfortunately his idea was hundreds of years ahead of its time and lack of observational evidence stopped the model from gaining acceptance. The writings of Aristarchus do not survive, but his ideas are cited in The Sand Reckoner by Archimedes.
The first accurate calculation of Earth's circumference is credited to Eratosthenes. His writings do not exist, but is summarized in On the Orbits of the Heavenly Bodies (c. 75 B.C.) by Cleomedes and many other works.
Hipparchus of Rhodes (190–120 B.C.) observed a nova stellum (today we would call it a supernova) in 134 B.C. and this strange, new object in the heavens prompted him to construct the first accurate catalog of 850 star positions and magnitudes. Believing that an accurate accounting of stellar positions and magnitudes would be helpful for determining if a star's position or brightness changed, his observations actually led to one of the most serious challenges to Aristotle's view of an immutable cosmos. Hipparchus noticed that the entire celestial sphere precessed (shifted) over time. By today's measurement, we know the precession occurs at a rate of 1° every 72 years. Astronomers had to wait nearly 1,700 years, however, until a young Isaac Newton's law of universal gravitation was formulated and could explain the phenomenon of precession.
Claudius Ptolemy (A.D. 100–175) was one of the last in a line of Greek astronomers. His Mathematike Syntaxis (later translated into Arabic as Al-Majisti, then Latin as Almagestum and finally, the English The Almagest) decided the order of planets around Earth with as many planets on one side of the Sun as the other. Since the Sun was the source of light and heat, that balance resonated with his Aristotelian beliefs. He also estimated the distance to the fixed stars as 19,865 Earth radii, or 7.5 million miles, and refined Hipparchus' magnitude system.
Like Aristotle before him, Ptolemy assumed a stationary Earth since no object tossed straight up into the air was seen to move eastward against the daily westward motion of the Sun. Not until Galileo and others formulated the principle of inertia was this argument satisfactorily refuted.
In The Almagest, Ptolemy proposed explanations for the length of the year and the varying lengths of the seasons. He also invoked a system in which circular motion at a constant speed was observed from a stationary point away from Earth (the equant), however, and that detail did not sit well with other followers of Aristotle's teachings or with the later Islamic and western scholars. This detail was necessitated by Ptolemy's insistence on matching his theoretical model to observations, but philosophical doubts about the correctness of a model that had heavenly motions centered on a point other than Earth resulted in numerous refinements and debates until the time of Copernicus.