Aristarchus Who? of Where?

by Devon Hamilton PhD - Senior Scientist / Physics
10/31/02

Galileo Galilei, Johannes Kepler, Nicolai Copernicus. If there were a Scientists' Hall of Fame, these three would be there (who knows - maybe there is one?). All of them contributed to what is arguably the greatest scientific revolution in history – the revelation of Earth's revolution about the Sun. This theory displaced the Earth from the centre of the Universe, and we've been moving it farther away from there ever since. In modern cosmology, the Universe has no centre (despite what many Torontonians may think!).

While these three astronomers are lauded (and rightfully so) for their contribution to our realization of our “place” in the Universe, the scientific notion that the Earth was not the centre of the Universe originated (at least in Western cultures) with Aristarchus of Samos. Aristarchus of where? (http://www.samos.net/istoria-eng.htm to find out more about Samos)

Aristarchus was born in 310 BCE, about 12 years after the death of Aristotle (see http://www.ucmp.berkeley.edu/history/aristotle.html). Very little of Aristarchus' mathematical and astronomical work survives today. Most of our knowledge of his work relies on the comments of other writers, many of whom did not share his views of the Universe. Aristarchus is considered to be the first person to propose that the Earth orbits around the Sun – an idea that was anathema to the Aristotelian scholars who dominated Greek and later Roman science.

Aristarchus proposed that the Earth orbited the Sun in a perfect circle, and rotated about its own axis – giving rise to both the daily and yearly motions of the night sky. This directly contradicted the religious and philosophical views of the time – views that were formulated by Plato and Aristotle. Science as a study of the Universe was in its infancy, and the notion of disproving or supporting a hypothesis using experimentation and observations was simply not done. Observations were rejected in favour of idealized thought experiments, which gave rise to such well-known misconceptions such as heavier objects falling faster than light ones (a conclusion which would only be overturned some 18 centuries later by Galileo).

However, even though Aristarchus was right about the Earth orbiting the Sun, his hypothesis was rejected for the right reasons. And therein lies a valuable lesson for all students of science: all scientific ideas and theories and conclusions are conditional by nature.

Would.
Could.
Maybe.
Should.
Potentially.
Possibly.

All of these words can be found at one time or another in scientific work. Scientists can never be absolutely sure, there is always the shadow of a doubt that new observations may force us to discard our previous conclusions – in the end the data must win out.

In science, a theory is much more than a guess or an idea. A theory is a proposed explanation for observed phenomena based upon all of the observations. A theory has to take into account all the results of the experiment, not just some of them. A theory must also be testable – it has to generate a prediction which can be tested. If a theory fails the test, then it is discarded. If it passes the test then the theory is provisionally accepted while new, different tests are generated. Theories and ideas in science are constantly being assailed and tested. Sometimes the theory is saved by making some modifications, other times the theory is tossed out entirely and a new one is developed.

For Aristarchus, his hypothesis that the Earth orbited the Sun explained the observed nightly and yearly motions of the stars. However, the hypothesis also made a prediction. As the Earth orbits about the Sun, the apparent positions of the stars should appear to shift slightly. If you go out tonight and carefully note the position of a star and go back in six months and repeat the observations, the star will have shifted slightly. This is a geometric shift because the Earth has moved – from one side of the Sun to the other.

This is called parallax, and you can see it quite easily for yourself. Hold your arm straight out in front of you and stick up your thumb. Close one eye and look at your thumb, then open that eye and close the other one and look again. You will notice that it appears as if your thumb has shifted with respect to the background. It's not your thumb that has moved, but the point where you measure from. By shifting from one eye to the other, you've changed your vantage point. The same thing happens as the Earth orbits the Sun. Observing a star's position when the Earth is on one side and then on the other is the equivalent of looking at your thumb with first one eye and then the other.

Image credit: http://www.noao.edu/outreach/nop/nophigh/steve9.html

Image credit: http://clyde.as.utexas.edu/parallax.GIF

When Aristarchus proposed the heliocentric hypothesis, other mathematicians realized that there should be a parallax. When none was observed, they rejected Aristarchus' model. So even though the rivals of Aristarchus were motivated to reject the idea of the Earth orbiting the Sun for non-scientific reasons, some of them actually had scientifically justified reasons for rejecting it.

So why didn't Aristarchus measure a parallax? He thought, correctly, that the stars were too far away to give a measurable parallax effect. Again you can try this yourself with just your own eyes and thumb. Do the parallax experiment while holding your thumb at different distances away from your face. You will notice that as you move farther and farther away, your thumb will appear to shift smaller and smaller amounts. If you could stretch your arms really far, eventually the shift would be so tiny that you wouldn't notice it. This is the case with the stars – they are so far away that we cannot measure the angular shift with the naked eye. Aristarchus apparently proposed this, but had no evidence, and it also went entirely against the Aristotelian view of the Universe. No one else could accept the notion of the stars being so far away – they couldn't even begin to conceive of such distances.

In the end, the heliocentric model was rejected because, while it did provide a working explanation for the appearance of the night sky and its changes, it failed when tested using what was then measurable.

There are a number of valuable lessons about how science is done. Scientific ideas have to stand and fall on their scientific merits alone – religious, political or philosophical reasons cannot be used to reject an idea. A successful theory has to explain what is observed, and also be testable. If it fails that test, it should be rejected or modified.

In the case of Aristarchus and his theory, his modification was to suggest that the stars were too far away to have measurable parallaxes with the naked eye. When he did this, his hypothesis could no longer be considered a theory because it was not testable. The annual parallax resulting from the Earth's motion would not be measured until the 19th century, well after the invention of the telescope. The observation of parallax would be considered to be the definitive evidence of the heliocentric model, although by that time nearly everyone accepted it based on other observational tests and the obvious faults and shortcomings of the geocentric model.

The idea of a Sun-centred Universe was not Aristarchus' only contribution. He also used geometry to estimate the relative sizes of and distances to the Sun and Moon. While his distances and sizes were off by a factor of 10, this was because of the poor quality of observations that were done at the time – his scientific and geometric reasoning was sound.

Not very much information about Aristarchus is known today – he remains a footnote. Almost all of his work has been destroyed over the years, most likely in one of the fires that consumed the last of the collection of the Great Library of Alexandria (to learn more about one of the greatest libraries in history go to: http://mediahistory.umn.edu/indextext/Alexandria.html and http://www.greece.org/alexandria/library/library16.htm).

It is possible that Copernicus was inspired by references to Aristarchus in some of the classical writings of Archimedes and Plutarch. Copernicus would have run across those references when he was a student in Renaissance Italy. If it weren't for the fires, we may have realized earlier in our history that we weren't the centre of the Universe – a shift in perspective which still echoes through our society today.