Greek astronomy

1. Greek astronomy Much of our mathematics, philosophy, architecture, art is derived from ancient Greece and Rome The same is true about astronomy

2. Even if they had deities, Greeks placed their true faith in the hands of man Humanism: man is a thinker, a doer and a maker ~ 600 BC: discovery of deductive reasoning = the birth of science

3. The early Greeks regarded the sky as an actual sphere

4. 500 BC Pythagoras He believed spheres to be perfect forms and suggested the Earth should be a sphere

5. The hull of a sailing ship disappears before the mast

6. While if the Earth were flat: It would not be the case

7. Celestial bodies are spherical • They orbit the Earth at a constant speed • The Sun is further away than the Moon • The Earth is a sphere

8. Aristarchus (310-230 BC) was the first to put the Sun at the centre of the solar system (“heliocentric model”), but had no followers

9. Which astronomical distances where known to the Greeks? • Diameter of the Earth, Eratosthenes (12,800 km) • Moon-Earth distance, Hipparchus ( 416,000 km) • Sun-Earth distance, Aristarchus: 416,000 x 19

10. ~ 140 AD Ptolemy Wrote a mammoth compilation of astronomical knowledge: “The Almagest”

11. To a naked eye, seven objects move from day to day against the background of the stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn Ptolemy’s goal: predict the positions of the planets for any desired date and time

12. Geocentric model before Ptolemy Saturn Earth Jupiter Sun Moon’s orbit Mercury’s orbit Venus

13. But Ptolemy had a problem… 2 months How to explain this surprising behaviour?

14. Ptolemy introduced epicycles Mars

15. Retrograde motion of Mars described by Ptolemy’s model

16. Good prediction of the position of the planets, Sun and Moon Model used for the next 1000 years

17. To Ptolemy the size of the whole universe was 10,000 times the diameter of the Earth, that is: 10,000 x 12,800 km = 128,000,000 km

18. Size of Ptolemy’s universe Sun Actual orbit of the Earth

19. Renaissance The belief that the cosmos is ultimately understandable slowly emerges again

20. Nicolas Copernicus (1473 – 1543) Goal: to predict the position of the planets Proposed a simplified mathematicalmodel in which the Earth was orbiting the Sun

21. Mars Earth Moon Jupiter Saturn Venus Mercury Sun

22. He could reproduce the retrograde motion of Mars in very simple manner

23. Copernicus also deduced that the nearer a planet is to the Sun, the greater its orbital speed Mercury orbits at a higher speed than Mars

24. His model: no better accuracy than Ptolemy’s => not widely accepted until more than a century

25. People were not ready to believe that the Earth is moving They argued that if the Earth were rotating, we would feel its motion

26. Tycho Brahe (1546 – 1601)

27. Established a fine astronomical observatory on the North Sea island of Hven

28. He made a continuous record of the position of the Sun, Moon and planets for almost 20 years Found that the predictions of Ptolemy’s model were inaccurate Tycho measuring the altitude of a celestial object, 1587

29. Kepler (1571 - 1630) Brahe’s assistant Became convinced that all the planets are orbiting the Sun

30. Kepler’s first law: The orbit of a planet is an ellipse top view Sun planet

31. Kepler’s second law A planet: Speeds up as it comes closer to the Sun Slows down as it moves away from the Sun Lowest speed Highest speed Sun

32. Kepler’s third law P: orbital period (time needed for one revolution), in years a a: semimajor axis, in AU a3 = P2

33. The farther away a planet, the smaller its speed, and the longer it takes to orbit the Sun

34. This was a nice model, but yet it did not explain why the planets were orbiting the Sun Meanwhile, a genius was at work in Florence, Italy… Next week’s reading is: Chapter 2, Gravity and Motion