Greeks

Greeks

Babylonians | Greeks | Egyptians | Others | Calendars

The Greeks inherited the legacy of the Babylonians and developed the science of astronomy. By the second century A.D., they had covered most of the main branches of astronomy: They knew what caused and could predict eclipses, they had charted planets, cataloged stars, observed novae, and discovered precession. They had discovered the Earth was spherical (though that knowledge was lost later), and that it moved around the sun (though that model grew out of favor).

The Greeks were very meticulous in the forecasting and recording of eclipses. While this is important for astronomy for its own sake, modern historians have been able to determine ancient Greek dates with very high accuracy.

The "Father of History," Herodotus, credits Thales of Miletus - the "Father of Philosophy" - with being the first Greek to forecast the year of a total eclipse of the sun. We now believe this was the eclipse of May 28, 585 B.C. Thales was different from the earlier Greek astronomers, for while they were content to simply catalogue occurrences, Thales searched for general rules.

The Babylonians kept records of eclipses and also calculated their periods of recurrence. The most important series was the "Saros" - it said that 18 years and 11 days after an eclipse, another very similar eclipse would occur. It was probably through the Saros calculation that Thales was able to make his prediction of the solar eclipse.

Earlier Greeks did not realize what the planets were, but were quite disdainful towards them. They were referred to as "tramp stars," which is our word for "planets." Homer wrote of the morning and evening star (Venus) by two separate names, "Phospheros" and "Hesperos" -- he never knew that they were the same planet.

It was Pythagoras in 550 B.C. who discovered that Phospheros and Hesperos were the same.

One of the most important - and infamous - contributions of the Greeks was the geocentric model of the universe. The geocentric model means that the Earth, "geo," is at the center, "centric," of the universe, and that all other bodies move around it with the Earth at a fixed location.

This view held so much sway because of many of the philosophies of the ancient Greeks. They believed that the circle is the perfect form, and that the simplest model that made sense must be the correct one. Since they "knew" the heavens were perfect, everything must move upon a circle, and since the simplest model was that the Earth stood still and everything moved around it, then that must also be true. After all, we can't feel the Earth moving, so why should be believe that it does without any extraordinary evidence?

However, evidence against this system was obvious even to the Greeks 2500 years ago. It had to do with the motion of the planets. For periods of time, the planets seem to orbit in an eastward direction across the stars. However, for brief periods of time, they switch and go in a westward direction. This is called retrograde. The explanation for this now is simple, and is discussed in the Scientific Revolution section.

But, in the Greek's defense, they did try to measure Earth's motion to see if it did move. This was accomplished - well, it failed, actually - by looking at the stars of the course of a year to see if they moved at all. Ones that were nearby should move farther than ones that were farther away, so there should be detectable motion. We now call this motion parallax, but the closest star has such a small parallax that the Greeks of 2.5 millennia ago had no hope of measuring it. Since they saw no motion, they fell back on the assumption that the Earth stood still, and there must be some other explanation for the planetary wandering.

So, Pythagoras has an ad hoc explanation for planetary motion, which he put forth as "left-behind-ness." The body left the farthest behind was Earth's moon, which lost a whole revolution in 29.5 days. The body that left the least behind was Saturn (Uranus through Pluto were not discovered yet), which lost a whole revolution in 29.5 days.

Plato taught that the movements, occultations, conjunctions, etc. of the sky were all calculable, and they only frightened those who could not "work a sum." However, he did complain that the heavenly bodies did not always use good sense. He was sure that their movements could be understood, and if they did not make sense then it was the theory that was at fault, not the heavenly bodies.

This lead him to eventually accept the theory that the Earth might not be at the center of the universe, and he wrote "the Earth, our nurse, goes to and fro on its axis, which stretches right through the universe." In Plato's school, the theory enjoyed a long life, and it was one of his followers that hit upon the heliocentric - sun-centered - model. Unfortunately, Plato's greatest pupil, Aristotle (384-322 B.C.), disagreed.

Aristotle's heavy scientific words contrasted with light and eloquent phrases from Plato, and they tipped the balance in the favor of geocentrism. It would take nearly 2000 years before main-stream thought returned to heliocentric ideas.

But prior to that, Greek Aristarchus of Samos (310-230 B.C.) was one of the first to actually present the heliocentric model of the solar system. However, Aristotle's views were too wide-spread and well-known, and he had too many followers for anyone to listen to an idea that didn't put our egotistical race at the center of the universe.

Ptolemy With a geocentric model, one must explain the apparent wandering of the planets in some way. Credit for the theory of concentric crystal spheres with epicycles (spheres upon spheres) and eccentricities is given to Eudoxus, but it reached the highest stage of development in the hands of Claudius Ptolemy of Alexandria.

Around A.D. 150, the Greek astronomer Ptolemy (left) (A.D. 85-165) - the last of the great Greek scientists - solidified the geocentric model, elaborating and formalizing the view in a manner that closely approximated the movements of the sun and planets. In Ptolemy's model of the universe, Earth was in a center sphere, surrounded by eight other spheres, which were, in order, the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and then the "fixed stars." The idea was absorbed by Arabs and portrayed under the name of "Ptolemaic."

In 150 B.C., Hipparchus observed a bright star (a nova) in a place that hadn't had a bright star. In excitement over the discovery, he thought he should compare all the stars with a catalogue to see if there were other new stars. There were no good recent catalogues, but there was an old record. One by one he recorded the stars and compared them with the record.

But something wasn't right. Either the old record was consistently wrong, or he was. Since he did not believe himself to be crazy, nor did he think that the old record would be so consistently wrong, there could be only one explanation to the discrepancy he found in the stars' positions: The fixed stars are not fixed.

From their movement, he concluded that the motion must be due to a "precession," which is the slow movement of Earth's axis among the stars.

Thales probably believed in a spherical Earth; Pythagoras and Plato did, as well. However, though Aristotle was grossly incorrect in his model of the universe, he must be given credit for the first study of scientific geography. He gave three reasons for his thinking:

Only a sphere could result from the tendency of matter to fall together toward a common center.
Only a sphere could through the circular shadow which we always see during a lunar eclipse.
Only on the surface of a sphere would a traveler going from North to South see new constellations rising above the horizon on (s)he moved.

After Aristotle was dead, Eratosthenes (284-192 B.C.) , the librarian of Alexandria, was able to determine the circumference of the Earth to an accuracy of 0.1-0.5%. Around 250 B.C., Eratosthenes knew that on a particular day, the sun cast no shadow in a well in the modern-day village of Assouan. At the same time, on the same day, it cast a minor shadow in Alexandria - the distance between the two was known to high accuracy, and Alexandria and Assouan are almost at the same longitude. Thus, by dividing 360° by that shadow angle and multiplying by the distance, the polar circumference was measured. Eratosthenes measured it to be 40,000 km (24,855 miles), and the current accepted figure is 40,032 km (24,875 miles)*.

Sadly, a few centuries later, Ptolemy in his infamy messed up this calculation as well. His measurement for the circumference of the Earth was short by around 30%. A nice footnote of history is that this is another reason why Columbus thought that there was a faster route to India. If he had known that there was another 1200 km to go, he probably never would have set off on his voyage.

*I have also seen the number 40,233 km (25,000 miles) for what Eratosthenes measured, and a disturbingly diverse amount of numbers for Earth's polar circumference: 39,941 km (24,818 miles) from NASA, 40,008 km (24860 miles) from "Astronomy in Ancient Greece" (a 1941 paper that is the main source for this page), the 40,032 km (24,875 miles) is from an Oceanography text book; however, the polar circumference is DEFINED as 21,600 nautical miles by international agreement, so it is the conversion factor which is what is apparently up for debate. Personally, I would trust the NASA number - they don't actually give the circumference, but rather the polar (as well as equatorial) radius (6356.8 km), from which one can then calculate the circumference in km and miles.