A、How the natural world was described in Greek mythology. B、What they observed directly. C、The writings of philosophers from oth

In ancient times, many people believed the earth was a flat disc. Well over 2,000 years ago, the ancient Greek philosophers were able to put forward two good arguments proving that it was not. Direct observations of heavenly bodies was the basis of both these arguments. First, the Greeks knew that during eclipses of the moon, the earth was between the sun and the moon, and they saw that during these eclipses, the earth’s shadow on the moon was always round, they realized that this could be true only if the earth was spherical. If the earth were a flat disc, then its shadow during eclipses would not be a perfect circle, it would be stretched out into a long ellipse. The second argument was based on what the Greeks saw during their travels. They noticed that the North Star, or Polaris, appeared lower in the sky when they traveled south, in more northernly regions, the North Star appeared to them to be much higher in the sky. By the way, it was also from this difference in the apparent position of the North Star that the Greeks first calculate the approximate distance around the circumference of the earth, a figure recorded in ancient documents says 400,000 stadia, that’s the plural of the world stadium. Today, it’s not known exactly what length one stadium represents, but let’s say it was about 200 meters. It’s a very good estimate for those writing so long before even the first telescope was invented.
Questions 19 and 21 are based on the passage you have just heard.
19. According to the speaker, what were the beliefs of the Greek philosophers based upon?
20. What does the speaker say about ancient Greeks who traveled south?
21. What does the term stadium refer to?