Classical Greek Astronomy

History is a building process, each successive event building on the last. In the greater picture of civilization as a whole, the one culture that probably did more to shape the modern Western world was that of Classical Greece, defined as the period starting with the first Olympic Games in 776 B.C. And ending (this is open to debate) with the death of Alexander the Great in 323 B.C. During this fruitful period of 450 years, great advances were made in many fields, including political theory, philosophy, the arts, and science.

The birth of permanent settlements gave rise to civilization as we know it, which in turn spurred the growth of agriculture. With agriculture supplanting hunting and gathering as the primary means for feeding oneself, it became matter of life and death, literally, for early farmers to get their planting done at the correct time. Having no abundance of entertainment as we do today, early people often looked to the sky. Looking up, these early people started to notice patterns to the sky, such that, following a regular cycle, the same stars would be visible in the same season year after year. In time, it became apparent that the appearances of various stars could be used to predict the weather on Earth and thus help in the odds for a successful crop. Such tidbits of astronomical observations would eventually find their way into the earliest of Greek literature, with the poet Hesiod (c. 700 B.C.) stating that “when Orion and Sirius come to the middle of the sky and the rosy fingered dawn confronts Arcturus, cut off all your grapes and bring them home with you.”

As with all peoples interested in the sky, astronomy would start out as practical knowledge essential to matters of everyday life, however, the sky would soon be hijacked by people who exploited the heavens to bring themselves power on Earth.

Up until the Greeks, people had explained the way the world works through mythology and/or all-out religion, which served as the fuel for superstition and irrationality. Obviously, not everyone was intimately familiar with the annual motions of the stars. As a result, when a priest said that, for example, that the gods were angry and may not provide a favorable spring (whose coming the priest knew was evidenced by a star), the masses could be fooled into offering sacrifices and/or imploring the priest to intercede on their behalf with the gods and, as if by some human intercession, spring would come, the priest would be validated (he knew spring was coming by looking at the stars) and the people impressed. By 600 B.C., every culture had a deeply-rooted mythology describing why the world came to be the way it was, the Greeks included. At this point in history, it seemed as though the world would never wake up to reason. However, in Asia Minor (modern Turkey) science as we know it was born.

In a city called Miletus, there arose three great thinkers in the 6th century B.C. Now, looking back at the ideas of these three men and comparing them to what we know as fact today, the theories proposed were often wrong in both main idea and reasoning. However, science is a self-correcting process that builds on itself so, while the ideas of these first scientists were incorrect, that's not important, the rational approach they used was the innovation.

Thales of Miletus
The first of the great scientists from Miletus was a man called Thales, one of the legendary Seven Sages of Ancient Greece and, so far as we know, the first man to try and explain the world through reason, not myth. In his profession, Thales was a merchant and, having to travel all over the known world to sell his goods, came into contact with many different peoples and ideas. Of all the places he went, the one where Thales learned the most was Egypt. In Egypt, Thales learned to calculate distances and heights by using geometry (which would later factor into astronomy) and was even said to have predicted a solar eclipse. However, of all the ideas proposed by Thales, perhaps the most awe-inspiring was one of evolution. Thales believed that the world was all water at one point and this is where life got its start, gradually evolving from simple to complex organisms. At the same time, the water started to evaporate, exposing dry land (no need for a God/gods to intercede here), and eventually life moved out of the water and colonized the land. This was over 2,000 years before Charles Darwin and Alfred Russel-Wallace.

Anaximander
A young contemporary of Thales was a man called Anaximander, who would eventually become the second of the three great scientists hailing from Miletus. While Thales was not a career astronomer, Anaxaminder spent a lot of time thinking about the heavens and what the objects in them were to the point where he is now considered the father of cosmology. As far as history records, Anaximander was the first man to come up with a mechanical, not mythological, model of the solar system. As for the universe proposed by Anaximander, it was this: the Earth was a cylinder floating freely (without support) in space and the Sun, Moon, and stars were all fire-filled wheels with holes that allowed the light we see to escape.

Anaximenes
Like Anaxaminder was to Thales, Anaximenes was to Anaximander, a student. In terms of theory, Anaximenes wasn't as revolutionary as his predecessors (Thales = the world can be explained rationally, Anaxaminder = the heavenly bodies are physical places), his idea would be much longer-lasting. In the model of the Universe proposed by Anaximenes, the stars were no longer fire-filled wheels, but were bright points of light fixed to the inside of a sphere, in which were the Earth, Sun, and other planets. In time, this idea would grow into the celestial sphere, which would be central to astronomical thought for about 2,000 years.

After the three scientists from Miletus, the rational approach to explaining the world was well established and would continue to be refined as new ideas were put forth in the coming centuries.

Pythagoras
More famous for his famous triangle theorem, Pythagoras was also interested in astronomy and, despite living from 582-500 B.C., put forth an idea about the mechanics of the solar system that was surprisingly accurate. The first fundamentally correct insight by Pythagoras was this: the Earth moves. Second correct idea: all celestial bodies were spherical. Now, despite having these main ideas correct, Pythagoras also got a lot of things wrong, too. First, instead of the Earth simply revolving around the Sun, the Earth revolved around what was termed the “Central Fire,” which was invisible because it was blocked by a “Counter Earth.” Obviously, these ideas are extremely abstract, needlessly complicated, and would have been confusing to the masses. Obviously, for all of his brains, Pythagoras never heard of Occam's Razor. As for how Pythagoras came to the conclusion that all heavenly bodies were spherical, that's lost to history.

Anaxagoras
The next great figure in classical Greek astronomy, besides being a great thinker, was also somewhat of a martyr for free thought. This man was Anaxagoras, who moved to Athens in the middle of the 5th century B.C., the time at which Athens was at its peak of political and intellectual power. In Athens, Anaxagoras became a close friend of Pericles, the man who is credited with making Athens the great city it was while also sowing the seeds of its downfall at the same time. It is believed that this friendship with the city's leader is what put Anaxagoras on a collision course with the religious authorities in Athens. In his cosmology, Anaxagoras was remarkably rational and proposed natural explanations for eclipses, meteors, and rainbows, all while creating his own ideas of the solar system. In his model, Anaxagoras was, so far as we know, the first man to make guesses as regarding the sizes of the heavenly bodies when he declared that the Sun was a giant, blazing hot piece of metal in the sky bigger than all of Greece. Also, Anaxagoras thought that the planets, Moon, and stars were all part of Earth torn from the planet and ignited by rapid rotation. Also, Anaxagoras was the first man to profess that the Moon shines by reflected sunlight and that the stars are suns infinitely far away. Obviously, while Anaxagoras was correct about some things and wrong about others, what was important was the fact that, for the first time, someone dared answer the question of where the Sun, Moon, and stars came from.

Unfortunately, for some people, this was audacity gone wild.

The Greeks were an interesting people when it came to their thought processes. At no other time in history has a people ever been so logical and superstitious at the same time as was the case in Classical Greece. At the same time Thales was teaching that the world came about naturally, Anaxamenes was proposing mechanical models of the solar system, and Pythagoras already knew that Earth was spherical, other Greeks were basing important decisions on oracles and divination through, among other things, examining the entrails of sacrificial animals. It was this second group who, with Anaxagoras, decided to make an attack upon reason.

Whatever his accusers' motives were is unknown, but several leading men of Athens, who just happened to be political opponents of Pericles, accused Anaxagoras of impiety, a religious crime, because of his teachings about the universe. Result: Anaxagoras found himself imprisoned for shocking the sensibilities of everyday people and insulting the Olympian gods. It was only through the influence of Pericles that the charges against his friend Anaxagoras were finally dropped. However, in Athens, the tide had turned, the spirit of free thought that had propelled the Greeks to greatness was giving way to a drive for conformity. With the philosophical-religious climate so hostile, Anaxagoras, now free from prison, was forced to flee to Lampsacus, where he lived out the rest of his life, continuing to teach the ideas that enough leading Athenians deemed too dangerous for the good of the people. After his death, the people of Lampsacus built an altar to reason, which they dedicated to Anaxagoras. This hostility to free thought would come to a head in 399 B.C. when the Athenian assembly condemned the famed philosopher Socrates to death for, among other things, blasphemy.

After Anaxagoras, Greece would be plunged into the on and off 30 year struggle that was the Peloponnesian War, in which Athens, Sparta, and their respective allies fought an on and off war with the goal of becoming the supreme power in Greece. Obviously, with so much fighting going on, time for any science that could not help out in the field of military development was limited. Then, finally, after peace returned, the scientists of Greece once again took up the struggle to understand the universe.

Eudoxus
In the late 300s B.C., Eudoxus refined the model of the solar system to a new level of sophistication, or cumbersomeness, your call there. In the model of Anaxamenes, the Earth was at the center and was orbited by the planets with the stars lining the inside of a spherical vault. However, in its simplicity, the old model of Anaxamenes failed to address some undeniable observations about solar and planetary motion. First, while the Sun moves through the sky from East to West, it also moves along the Zodiac. Why was that? Also, planets appear to slow, stop, reverse course, move backwards, stop again, and continue in their forward motion. Why could this be? These are the two questions Eudoxus sought to answer.

In explaining the seemingly inexplicable motions of the stars and planets, Eudoxus set them on multiple celestial spheres. Starting with the stars, whose motions were easiest to explain, Eudoxus set them on a single sphere that rotated from East to West once per day, easy enough. However, this simplicity wouldn't last long. The Sun, besides moving across the sky once a day, also moves through the Zodiac once per year and both of these motions had to be rectified in the model. So, to explain why the Sun does what it does, Eudoxus set the Sun on two spheres, one moving East to West once per day and the other going Eastward once per year to account for the movement through the Zodiac. Sound confusing? It gets better. The planets need four, (yes, four) spheres. Sphere 1 rotated Westward once per day for the daily motion of the planet. Sphere 2 rotated Eastward once a year to account for the planet's motion through the Zodiac. Spheres 3 and 4 were slightly inclined to each other and were used to explain retrograde motion of planets, which occurs in an elongated figure 8 motion if observed carefully. In retrospect, this model was needlessly cumbersome and failed to account for the sometimes very obvious change in planet brightness. However, at the time, it was the best thing going.

Aristotle

After Eudoxus, the field of astronomy would not see advances for a time, but the discovery of “proofs” for some of the already postulated ideas. The man responsible for these, a philosopher whose shadow would stretch nearly 2,000 years into the future as the center of western thought: Aristotle. By the time of Aristotle, who lived from 384-322 B.C., several ideas had been proposed, but no real proofs made. Aristotle was the man who changed this. At this time, it was widely believed that Earth was spherical. Aristotle came up with two proofs of this fact. First, some stars are seen in Greece that aren't in Egypt, and vice versa. Second, the shape of the Earth's shadow on the Moon during an eclipse. If Earth was flat, the shadow would be a line. Next, if Earth was spherical, what about the other heavenly bodies? Again, looking at the Moon during an eclipse, if the Moon was a two-dimensional circle, Earth's shadow would be perfectly circular, yet, it was slightly distorted because, as Aristotle correctly inferred, the Moon was spherical, too. So, if Earth and Moon were spherical, was it out of the question to assume that the stars and planets were circular, too? According to Aristotle, no.

Unfortunately, for all his brilliance, Aristotle also had some wrong ideas that were to be very long-lasting in their influence. Building on the earlier, and wrong, idea of Empedocles, who was the first man to find physical proof of air, Aristotle also assumed that the universe was made up of four elements: Earth, water, fire, and air. Now, by looking at the world, one notices that Earth and water are on the ground while fire and air go into the sky. So, taking this logic to the universe as a whole, Aristotle assumed that the heavy elements (Earth and water) are all on Earth while the light ones (fire and air) are in the sky. So, since the planets and stars are in the sky, they must be fire and air. So, what sense does the idea of a moving Earth make if it's so heavy? This was Aristotle's reasoning that would confine most scientists to a geo (Earth)-centered universe for almost 2,000 years.

However, there was one rebel to this orthodoxy before Copernicus in the 1500s: Aristarchus of Samos.

Aristarchus of Samos

Aristarchus lived from 310-230 B.C. On the island of Samos. Besides being an astronomer, Aristarchus was also a mathemitician and, so far as we know, the first man to try and measure the distances to the heavenly bodies, namely the Sun and Moon. Problem: no one knew how big the Earth was at the time. So, while Aristarchus used sound geometry, his incorrect assumed size for Earth resulting in his computations of distance being way off the mark. However, if Aristarchus had been lucky enough to live half a century later (more to follow), humans may have known that the Moon was about 250,000 miles and the Sun 93 million miles away before the birth of Christ. Unfortunately, it wasn't to be.

However, while Aristarchus was the first man to try and find the distances to the heavens, his major achievement was that he was the first man to reason that the Sun, not the Earth, was at the center of the solar system. Sadly, no original works of Aristarchus survive, only mentions of his ideas by later writers. How thrilling it would be to read Aristarchus and discover his reasoning process that led him to discover how the solar system really worked. A start is with geometry. Thanks to later writers, we know that Aristarchus calculated the Sun to be about 7 times bigger than the Earth. So, as a start, Aristarchus may have reasoned that it made no sense for a giant Sun to orbit a tiny Earth. To finish his model where the Sun, not the Earth, stood at the center, Aristarchus argued that the motion of the sky was only apparent, caused by the motion of the Earth turning on its axis once a day. However, save Seleucus of Seleucia, the ideas of Aristarchus met a wall of resistance.

First, if Earth moved, why wasn't there a great wind caused by it speeding through the heavens? Second, if Earth rotated on its axis once a day, why do falling objects still land directly under from where they fell and not to the West? However, the third argument, the unchanging nature of the stars, was perhaps the most compelling. If the Earth moved and the stars remained still, according to mainstream Greek thought, two things should happen: first, the stars should move relative to each other (stellar parallax) and change in brightness as the Earth moved around the Sun. Unfortunately, the critics of Aristarchus never considered the idea that the stars could be almost infinitely far away, thus negating both stellar parallax and brightness changes.

Eratosthenes
The man who could have helped Aristarchus discover that Moon was really 250,000 miles and the Sun 93 million miles away was Eratosthenes, who was the first man to measure the circumference of the Earth. The whole drive to do such an audacious thing came from curious stories coming out of Egypt. In Syene, Southern Egypt, it was said that at noon on the longest day of the year, and only on this day and at this time, the Sun would illuminate the water at the bottom of a deep well that was in shadow at every other time in the year. However, in Alexandria, where Eratosthenes served as chief librarian of the great Alexandria library, the Sun cast definite shadows at noon on the Summer Solstice. So how could this be?

Well, Eratosthenes was quick to realize that, for this to happen, the Earth had to be spherical. However, while lesser minds may have been content in this knowledge, Eratosthenes was not, he thirsted for more. Calculating the angle of the shadows, Eratosthenes determined that the shadow in Alexandria was at an angle of about 7.2 degrees, or about 1/50th of a circle. Thinking in terms of the big picture, he reasoned that the distance from Alexandria to Syene was about 1/50th the distance around the Earth. So, finding the distance between these two cities and then multiplying by 50 would give the circumference of the Earth, easy in principle but not in practice. As hard as it may be to believe, Eratosthenes hired a man to pace out the distance between the two cities! Despite no one having paced out such a great distance before, the distance was determined with remarkable accuracy, which resulted in Eratosthenes coming to a circumference within a few percent of Earth's actual circumference of just under 25,000 miles.

Not bad for the 3rd century B.C.

Hipparchus
After Erathosthanes, the next great astronomer was Hipparchus, widely regarded as the classical world's greatest astronomer. At the time of Hipparchus, it was believed that all stars were constant and unchanging. So far as we know, Hipparchus was the first man to notice a variable star, which served a the impetus to create a catalog of stars' brightness, which he did, creating the magnitude system in the process. As major of an achievement as this was, Hipparchus had far more to offer the world. In creating his star catalog, Hipparchus, when comparing older charts, noticed that every star he observed was about 2 degrees away from where it should have been. What was going on? While Hipparchus didn't know the reason for such motion, he had, nevertheless, discovered precession of the equinoxes.

After Hipparchus, there was a period of stagnation in classical civilization as the Greeks/Macedonians gave way to the Romans as the dominant power in the known world. So, after all the wars, stability, and free time for learning, returned, producing the last of antiquity's great astronomers, Claudius Ptolemy.

Claudius Ptolemy
Going back a few centuries to Eudoxus (and discounting the ignored correct model of Aristarchus), the picture of the solar system hadn't really changed in several centuries despite the fact that the Eudoxus model was extremely complex to say the least. With Ptolemy, who lived from 90-168 A.D., what would be the final word on the solar system for nearly 1,500 years would be written. Taking the failure of Eudoxus to explain the brightness change of the planets and the overall cumbersomeness of his model, Ptolemy streamlined the solar system and explained planetary brightness change in a single stroke. Like Eudoxus, Ptolemy put the Earth at the center of the solar system. Outside, the Moon was put in orbit around the Earth with the planets and Sun (in correct order) outside the Moon. Result: a much simpler, easier to understand model of the solar system. Now, as for the planets' change in brightness, Ptolemy solved this by adding epicycles to the orbits, which explained both retrograde motion and brightness changes at once. The system was so well received that was accepted without challenge for nearly 1,500 years.

After Ptolemy, the Greco-Roman civilization would start into its long, slow decline thanks to political and then religious instability. In 180 A.D., Marcus Aurelius, the last of the 5 Good Emperors, died, leaving the throne to his mad son, Commodus, the first in a pretty much unbroken string of lunatics to rule Rome. At the same time that the secular government was going to pieces thanks to reckless spending, corruption, and murder, the religious fabric of the Roman Empire was being torn to shreds as the Christian Church started making inroads against the old Pagan faith. In time, Christianity would win out and, in order to consolidate their religious power, Church leaders did everything they could to stamp out Pagan culture, which included the free, inquisitive spirit that inspired the Greeks to set out to try and understand the world way back in the 6th century B.C., starting with the Ionian scientist Thales of Miletus. With the collapse of the Western Roman Empire in 476 A.D., the Catholic Church was unrivaled in power and, with eternity at stake, no one dared question Church position on anything, including the cosmos, which the Church deemed to be centered around a flat Earth that never moved and populated by a collection of perfect, unchanging heavenly bodies.

In the end, the Western world would have to wait over 1,000 years for the second flood of free inquiry that was the European Renaissance.

More Ancient Astronomy
The Planet of Bethlehem?
A History of Cosmology: Prehistory to Present
Galileo's Fingers Go on Display.
Renaissance Astronomy: Part 1
Renaissance Astronomy: Part 2
Renaissance Astronomy: Part 3
The Equinox and a Magic Show from the Maya
Ancient America: the Moundbuilders
Ancient America: the Southwest
Ancient Egypt
Classical Greece
The Summer Solstice Sun and the Size of the Earth
The 1833 Leonids: History's Greatest Meteor Storm
The 10 Brightest Comets of All Time
Ben Franklin and the Truth About Daylight Savings Time

 
 

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