Date: December 20, 2010

Title: Aristarchus: The Copernicus of Antiquity

Podcaster: Adam Fuller

Description: Did you know the ancient Greek mathematician Aristarchus invented a more accurate version of Copernicus’s heliocentric model 1700 years before Copernicus was born? In today’s podcast we’ll discuss an ancient Greek math cult’s 300 year dominance over the evolution of cosmology, and how Aristarchus’s heliocentric model was ultimately forgotten until Copernicus’s groundbreaking book, “On the Revolutions of the Celestial Spheres.” The title of today’s podcast is taken from the 1920 monography by Sir Thomas Heath.

Bio: Adam Fuller is a graduate student in the Planetary Sciences department at Johns Hopkins University in Baltimore, Maryland. He graduated from Columbia University in the City of New York with a B.S. in Astrophysics in 2009. He also has a B.A. in Journalism from The University of North Carolina at Chapel Hill. His research interests include planetary atmospheres, planetary formation, exoplanets and astrobiology. Outside of school, he has a couple of friends from Samos and is a dedicated uncle to three proto-astrophysicists.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously.

Additional sponsorship for this episode of “365 Days of Astronomy” has been provided by Mike Smithwick, author of the award winning astronomy software, Distant Suns, now for the iPad and iPhone.

Transcript:

We often take for granted the idea that the first person to discover something gets credit for that discovery. A quick glance through the history of science, however, shows that’s not always the case. In fact, there’s a joke in mathematics that says equations and theorems are often named after the last person to discover them. A discovery sometimes happens in parallel by different people in different places, but, for whatever reason, the world only hears about one of the discoverers. Or a discovery is made and then forgotten until someone else independently rediscovers it. In the case of today’s 365 Days of Astronomy podcast, the world just wasn’t ready for the discovery because either it was such a radical departure from the prevailing cultural worldview and the necessary tools and observations to prove that discovery required technology and more advanced than what was currently available. The discovery had to wait 1700 years before it finally stuck.

Hi, my name is Adam Fuller, and I’m a grad student in the Department of Planetary Sciences at Johns Hopkins University in Baltimore, Maryland. Today I’d like to share with you my interest in the history of ancient Greece and show you that even 2,000 years ago science progressed with the same faltering starts and accidental discoveries that we experience today. And when I’m finished, hopefully from now on the name Aristarchus will pop into your head every time you think of Copernicus.

Many ancient Greek scholars traveled to Egypt and Babylon or read Babylonian texts as part of their mathematical training. The Babylonians were fascinated by numbers and produced huge amounts of data using sophisticated arithmetical techniques in an attempt to predict the motions of heavenly bodies. But while they viewed the cosmos in purely mathematical terms, the Greeks brought their mastery of geometry to bear on Babylonian methods to not just ask “when?” but also “why?” They were the first to pursue reasons for the motion of the sun, moon, and stars, making them the first to engage in what we would today recognize as science.

It all started with Thales of Miletus. Alive between 625 and 547 BC, Aristotle and Plato both considered him to be the founder of Greek natural philosophy. He’s the guy who supposedly spent so much time staring up at the sky that he fell down a well. He believed that the Earth a flat disc floating in the middle of a giant ocean, and that the air was water evaporating from this ocean. Keeping with the aquatic theme, he also thought the sun and stars were made of water and floated about the Earth. Thales is also famous for being the first Greek to predict a solar eclipse.

One of Thales’s pupils, Anaximander, was the first person who proposed that the universe was actually something akin to a machine. Parting from his mentor’s ideas, Anaximander believed the universe was a void, and the Earth was more like a hockey puck freely suspended in the middle of this void. Around this puck were two hollow wheels, filled with fire, with holes that revealed the fire inside. The outer wheel was about 27 times the diameter of the Earth with its hole representing the sun. The inner wheel was about 18 times the Earth’s diameter, and its hole was the moon. Between the Earth and the moon wheel was a sphere that enveloped the Earth. It was also filled with fire, and the innumerable holes in it were the stars.

These wheels and sphere were Anaximander’s attempt to explain the motion of heavenly bodies, but their distances and sizes weren’t derived from any geometric arguments. They merely reflect an ancient worship of the numbers 3 and 9. This sort of thinking—that nature was a representation of the purity of numbers—was characteristic of almost all Greek thought and heavily influenced Western science until Kepler and Galileo. We’ll see this—quite literal—“cult of mathematics” again.

A contemporary of Anaximander was Anaximenes of Miletus. He changed Anaximander’s theory by filling the void with air that supported the Earth puck, sun, moon, and stars the same way a leaf floats on the wind. Anaximenes also thought the stars were bolts attached to one of two crystal spheres. The inner sphere had only a few bolts and rotated around the Earth the way a bottle cap spins on a table. The majority of the stars attached to the other crystalline sphere were located past the sun and were the most distant part of the universe. Anaximene’s model improved upon Anaximander’s in several ways. It was the first that distinguished between planets and stars, and the first to locate the stars beyond the sun. Though the ancients knew the planets followed different courses than the stars, before Anaximenes, no one had attempted to explain why.

These three natural philosophers—Thales, Anaximander, and Anaximenes—all knew each other and lived in Miletus, one of the great ancient Greek cities in what is now present day Turkey. Their models for the universe contain some ideas that were carried all the way forward to Kepler’s time. They were also a stepping stone for what was about to come out of the island of Samos only 30 miles to the west.

Pythagoras of Samos—the Pythagoras—was born in 572 BC. One of history’s greatest names, Pythagoras invented the science of acoustics, helped flesh out many of geometry’s most fundamental principles, and is perhaps best known for his discovery of the famous theorem bearing his name. What many people don’t know is that Pythagoras was also a politician and religious leader. Around 530 BC, Pythagoras left Samos for a Greek colony in southern Italy and founded the Pythagorean Brotherhood. Part religious cult and part school of thought, the Pythagorean Brotherhood sought to unify mathematics with nature. Members, including Pythagoras himself, developed ideas that pushed the world forward in huge leaps. For instance, the Pythagoreans developed a heliocentric model of the solar system more accurate than Copernicus’s roughly 1800 years before Copernicus was born.

Pythagoras was also the first to suggest that the Earth, moon, sun, and stars were spherical in shape. He discarded the wheels and spherical surfaces filled with fire and water for a series of concentric crystalline spheres. These spheres each supported a different heavenly body and each independently revolved around the earth at different rates. As they moved past each other, the spheres produced musical notes that were determined by the distances between each sphere. This was known as the ‘Music of the Spheres.’ Outside of all of this was a living and intelligent sphere that represented the universe, rotating on its own axis, the resting Earth at its center.

The first revolutionary step away from an Earth-centric model of the universe was made roughly a century after the founding of the Pythagorean Brotherhood by one of their own, Philolaus, who lived sometime between 470 and 385 BC. He rocked the ancient world by being the first to completely remove the Earth from the center of the universe and set it in orbital motion around a central fire called Hestia. The Earth completed a circle around Hestia once ever day but always kept its inhabited part permanently facing away from it. This was an idea influenced by the fact that the moon always showed the same face to the Earth despite its obvious motion through the sky. The other bodies also revolved around Hestia but at different rates: the moon took twenty-nine and a half days to complete one orbit, and it took one year for the starry sphere, the planets, and the sun. The sun, by the way, was thought to be a crystal that reflected Hestia’s light.

But Philolaus didn’t stop there. He also suggested there was a counter-earth immediately between our Earth and Hestia, protecting us from Hestia’s heat. This counter-earth meant the universe was now composed of ten bodies in circular orbits around the central fire: the sun, moon, Earth, counter-earth, the five planets, and the sphere of stars. This appealed greatly to the Pythagoreans because they considered ten to be a perfect number and circles the perfect shape.

Living between 388 and 315 BC, Heracleides of Pontus made the next big leap forward when he adopted a geocentric model with the Earth in the middle of the universe. His was different, however, from past geocentric models because he set the Earth spinning on its axis. Heracleides was also the first person to discover that Mercury and Venus actually orbit the sun in the same way the sun orbited the Earth in the geocentric model. This is another ground-shaking revelation from the Pythagoreans at a time when most people weren’t even convinced the moon orbited the Earth. How ground-shaking? If Heracleides had claimed that Mars, Jupiter, and Saturn also orbited the sun instead of the Earth, he would have anticipated Tycho Brahe’s model of the universe by 1800 years!

The final step to achieving a true heliocentric model was taken by Aristarchus of Samos. Born around 310 BC, Aristarchus lived for roughly 80 years, eventually passing away around 230 BC in Alexandria. He was the last of the Pythagoreans, and the fact that he and Pythagoras were both from the same island is somewhat akin to Dr. James Naismith, the inventor of basketball, and Michael Jordan coming from the same moderately-sized coastal town in North Carolina.

Aristarchus’s only surviving work is an early book entitled “On the sizes and distances of the sun and moon.” The problems he tackles in this work would have been easily solved with trigonometry, had trigonometry been invented yet. In fact, at this point in the history, his friend Archimedes—yes, the Archimedes—hadn’t even approximated the value for pi. Aristarchus’s solution instead relies on a series of geometric arguments so elegant that astronomers used his results for the next 1700 years, despite a whopping two and a half degree error in the value he used for the angle between the sun and the half moon. Even the notoriously fastidious Tycho Brahe relied upon Aristarchus’s results when he made his detailed astronomical observations.

As for his heliocentric model, the ancient natural philosophers and historians all agreed that Aristarchus was the first person to put the sun at the center of the universe and then line the planets up in their proper order along circular orbits. He had the moon orbiting the Earth, and he retained Heracleides’s notion of an Earth spinning on its axis. In Archimedes work, The Sand Reckoner, Aristarchus’s model is explicitly mentioned, providing a first-hand eyewitness report of sorts from one of history’s greatest mathematicians.
Aristarchus’s heliocentric model was a huge—and much more accurate—leap forward, but at the same time it was still a natural progression from Heracleides’s model, which itself was an evolution of prior models. And the Pythagoreans were the only ones following this line of thinking. Others were developing models that followed much different lines, like the elaborate geometric model by Eudoxus of Cnidos that used multiple concentric spheres spinning on different axes for each separate heavenly body.

Unfortunately, Aristarchus’s model was eventually discarded when, sometime between 162 and 127 BC, Hipparchus set about checking its predictions against his observations. He was the first mathematician to use trigonometry, and using his own observations, he realized that the orbits of the planets in the heliocentric model were not circles but instead ellipses. Since ellipses weren’t considered as perfect as circles, and since the Greeks believed that nature was perfect, Hipparchus threw away the heliocentric model and adopted a geocentric model that more closely matched observations. And that pretty much spelled the end of the heliocentric model for the next 1700 years, at least until Copernicus came along.
The history of ancient Greek science is fascinating because we see example after example of the sophistication of their thinking and their embrace of a proto-scientific method. In just over 400 years they built from scratch a highly accurate model of the solar system and the mathematical tools that allowed them to verify the model. Along the way we see how much they did with so little and how close they were to inventing or discovering many things we take for granted today. They show us that in the right environment, science will progress and flourish, but that it’s still a very fragile thing that can be completely shut down if the prevailing culture isn’t ready for its discoveries.

So I hope you’ve found this podcast informative. I certainly have. After learning how science progressed during other times and contexts, I’ve come to appreciate exactly how science is built up and how difficult it is to make groundbreaking discoveries and rediscoveries like the heliocentric model. Should we change its name from the “Copernican model” to the “Aristarchian model?” I’ll leave that to you to decide, but, please, remember Aristarchus the next time you think about Copernicus.

If you’d like to read more, I’ve included several references in the transcript. I’d also like to apologize to my friend, Steven Simos. He spent 30 minutes on the phone coaching me on the proper Greek pronunciation of most of these names, and I’ve still managed to butcher all of them. Sorry, man. Everyone else, have a great day and keep listening.

References:

Archimedes. The Sand Reckoner. http://www.lix.polytechnique.fr/Labo/Ilan.Vardi/sand_reckoner.ps

Heath, Sir Thomas. The Copernicus of Antiquity: Aristarchus of Samos. New York: The MacMillan Company, 1920.

Leverington, David. Babylon to Voyager and Beyond: A History of Planetary Astronomy. New York: Cambridge University Press, 2003.

Wikipedia contributors. “Aristarchus of Samos.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 17 Dec. 2010. Web. 17 Dec. 2010.

Wikipedia contributors. “Copernican principle.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 28 Nov. 2010. Web. 17 Dec. 2010.

Wikipedia contributors. “Heliocentrism.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Dec. 2010. Web. 18 Dec. 2010.

Wikipedia contributors. “Hipparchus.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 25 Nov. 2010. Web. 18 Dec. 2010.

Wikipedia contributors. “Nicolaus Copernicus.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Dec. 2010. Web. 17 Dec. 2010.

Wikipedia contributors. “Parmenides.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 11 Dec. 2010. Web. 18 Dec. 2010.

End of podcast:

365 Days of Astronomy
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