NARRATOR：

Listen to part of a lecture in a history of science class.

FEMALE PROFESSOR：

Ok, we've been talking about how throughout history, it was often difficult for people to give up ideas,which had long been taken for granted as scientific "truths," even if those ideas were false.

In astronomy, for example, the distinction between the solar system and the universe wasn't clear until modern times.

The ancient Greeks believed that what we call the solar system was in fact the entire universe... and that the universe was geocentric.

Geocentric" means Earth centered, so the geocentric view holds that the Sun, the planets, and the stars all revolve around the Earth, which is stationary，Of course, we now know that the planets, including Earth, revolve around the Sun, and that the solar system is only a tiny part of the universe.

So, why did the ancient Greeks believe that the Earth was the center of the universe?

Well, it made sense to them—observations of the sky made it appear as if the Sun, the Moon, and the stars all revolved around the Earth every day, while the Earth itself stayed in one place.

And this view was also supported by their philosophical and religious beliefs about the origin and structure of the universe.

It was presented in the works of well-known Greek philosophers as early as the fourth century B.C.E.

And the geocentric theory continued to prevail in Western thought for almost 2,000 years, until the seventeenth century.

Now what's especially interesting is that when astronomical observations were made that seemed to be inconsistent with the geocentric view, the ancient Greeks did not really consider alternate theories.

It was so intuitive, so sensible that the Earth was the center of the universe ，that astronomers found ways to explain those seemingly inconsistent phenomena within the geocentric view.

For example, Greek astronomers made excellent, very accurate observations of the movements of the planets.

But the observations revealed a bit of a problem.

The geocentric theory said that the planets would move around Earth in one direction.

However, astronomers noticed that at times, several planets seemed to stop moving in one direction and start moving backward in their orbits around Earth.

And they came up with the theory that these planets themselves moved in smaller circles, called epicycles, as they traveled around the Earth.

Here's a picture of what they imagined.

You see how this epicycle theory could account for the seeming backward motion of a planet.

Of course, today we know that this appearance of backward motion is caused by the fact that Earth as well as other planets all move in their own orbits around the Sun, and the relative movements of the planets with respect to each other can get quite complex.

However, there were a few astronomers in Greece and other places ,who didn't agree with the geocentric view, for example, a-a Greek astronomer who lived in the third century B.C.E..

He proposed the theory that our planetary system might be heliocentric.

His name was Aristarchus.

"Heliocentric" means Sun centered-that the Earth revolves around the Sun.

Aristarchus recognized from his calculations that the Sun was much larger than the Earth and other planets.

It was probably this discovery that led him to conclude that the universe is heliocentric.

I mean, isn't it more sensible to think that a smaller heavenly body would orbit a larger one, rather than the opposite?

However, his proposition was rejected, largely based on other scientific beliefs held at the time- which all made sense in a way, even if they were incorrect.

Let me mention two objections Greeks made to Aristarchus' theory.

First, they believed that everything that moves creates its own wind, so to speak- everyone has this experience when you're running, right?

So they thought that if the Earth itself was moving, there would have to be a constant wind blowing, sweeping them off their feet- and of course there wasn't.

And second, the idea of an Earth that moved didn't fit in with the ancient Greeks' understanding of gravity.

They thought that gravity was basically a natural tendency of all things to move towards the center of the universe- which was the Earth, or the center of the Earth.

So that explained why apples and other falling objects were falling straight down.

If the Sun was at the center of the universe, things would fall toward the Sun-and away from the Earth. Which of course they didn't. So these were some of the reasons they rejected the heliocentric theory.