Ancient Astronomy

1. Ancient Astronomy

2. The Sky in Ancient Times • Mercury • difficult to see; always close to Sun in sky • Venus • very bright when visible — morning or evening “star” • Mars • noticeably red • Jupiter • very bright • Saturn • moderately bright • Other things • Comets • Stars

3. Why did the ancients observe? • Archeologists & anthropologists surmise: • to keep time • for agricultural purposes • for religious purposes • As far as we can tell, none of these ancient cultures tried to build a physical model based on their observations. • Instead, they created myths to explain the motions of the objects in the sky.

4. Observations don’t mean science • The ancients observed natural phenomena, but things did not always make sense…. • Fertile ground for myth. Things you don’t understand are caused by Gods, Giants, Titans, Demons……. • The Greeks were the first to really start applying science to observations. This did not stop them from creating a truly sociopathic mythology….

5. For Example… Apparent Retrograde Motion • Like the Sun, planets usually appear to move eastward relative to the stars. • We pass them in our orbit…. • Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us) • But very difficult to explain if you think that Earth is the center of the universe! • In fact, ancients considered but rejected the correct explanation…

6. Apparent motion of the sun and planets from the Earth

7. Why did the ancient Greeks reject the notion that the Earth orbits the sun? • It ran contrary to their senses. • If the Earth rotated, then there should be a “great wind” as we moved through the air. • Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

8. Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun The nearest stars are much farther away than the Greeks thought. Parallax angles of the star are so small, that you need a telescope to observe them.

9. Possible reasons why stellar parallax was undetectable: • Stars are so far away that stellar parallax is too small for naked eye to notice • Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

10. Eratosthenes (276 - 195 BC) • He measured the circumference of the Earth. • The Sun is at the zenith in the city of Syene at noon on the summer solstice. • But at the same time in Alexandria, it is 7 from the zenith. • Eratosthenes inferred that Alexandria was 7 of latitude north of Syene. • The distance between the two cities is 7/360 times the Earth’s circumference. • His result of 42,000 km is very close to the right number: 40,000 km.

11. Claudius Ptolemy (AD 100-170) Almagest (The Great Treatise) • Star catalogue (Hipparchus), instruments • A theory (model) of the motions of planets, Sun, Moon (Handy Tables) • Based on Babylonian observations, preserved by Arabs, translated into Latin in 12th century • Also wrote books on Geography and Astrology His theory fit the data, made accurate predictions!

13. Ptolemy’s Geocentric Model • Earth is at center • Sun orbits Earth • Planets orbit on small circles whose centers orbit the Earth on larger circles • the small circles are called epicycles • This explained retrograde motion

14. But once position predictions were written down, then observations could test the theory….and this was not good for the theory • What theorists did was add more and more epicyles to account for the differences between reality and theory… a common story in science

15. Occam’s Razor • Among competing theories, the best theory is the simplest theory-that is, the one with the fewest assumptions. • On seeing the complexity of the Ptolemaic epicycles, Alfonso X (el Sabio) is said to have remarked….. "If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler." • With better observations more problems with the Ptolemaic model became apparent…..and created more complex fixes (harmonic spheres, etc).

16. Copernicus • Polish scientist and founder of our modern view of the solar system (1473-1543). • His uncle was a Bishop and got him a position as Canon of Frauenburg Cathedral (which he held for the rest of this life) • He attended various Universities in Poland and Italy from 1491-1505 • Studied Greek, mathematics, canon law, medicine, and astronomy • One of the great polymaths of the Renaissance, a mathematician, astronomer, jurist, physician, quadrilingual polyglot, classics scholar, translator, artist, Catholic cleric, governor, diplomat and economist • Made astronomical observations all his life

17. His observations made it clear that Ptolemy theory did not work very well • His solution was to put the Sun at the center. • The math was much simpler and the results much better • The Sun was at the center of the solar system • The stars were very distant • Earth orbits like any other planet • Inferior planet orbits are smaller • Retrograde motion occurs when we “lap” Mars & the other superior planets • The annual cycle of the Sun is caused by the Earth’s orbit

18. He started circulating his ideas in 1514 but did not publish until 1543 • Which was also when he died • The publication was supervised by a friend who inserted a preface …..

19. Why wait to publish? • It is very hard for us to understand the passions that issues like this roused in Reformation Europe. • Take Giordano Bruno • In 1592 he was arrested by the Inquisition for a lecture tour he had taken around Europe • After an 8-year investigation/trial he was tried and burned at the stake • Note that Giordano had the distinction of being excommunicated by the Catholics, Lutherans, and Calvinists • In 1539, Martin Luther had called Copernicus "that fool [who would] reverse the entire art of astronomy” • In 1616, the Catholic church banned reading of Copernicus’ book De Revolutionibus "until corrected." (unbanned in 1758)

20. Tycho Brahe • Danish nobleman and Scientist (1546-1601) • He also was something of a professional student (college years 1559-1570) • Lost his nose in a duel…. • His contribution was as a matchless observer and recorder of his observations. • In his old age he quarreled with his patron and moved to Prague as Imperial Mathematician (i.e. astrologer) to the Holy Roman Emperor • He had a young assistant, Kepler

22. Johannes Kepler • German astronomer and mathematician (1571-1630) • Son of a soldier and an innkeeper’s daughter…. • Entered University to study for the Lutheran ministry. • Succeeded Brahe as Imperial astrologer • He was deeply religious and actually strongly believed in the predictive power of astrology • After a change of Emperors, Kepler found himself as a “field astrologer” casting horoscopes for the general leading the Catholic army during the Thirty Years War.

23. Kepler’s Laws • Kepler’s mathematical skill, together with Brahe’s observational records solved the problem of the shape of the solar system • Right from the start it was clear to Kepler that only a Sun-centered system would explain the observations. • BUT, his first insight was that orbits were not circular….they were ellipses

24. Kepler’s Laws • Law #1. Each planet moves in an ellipse with the Sun at one focus. • Law #2. The line between the Sun and the planet sweeps over equal areas in equal time intervals.

25. Kepler’s Laws Law #3: The square of the orbital period of a planet (in years) is directly proportional to the cube of the semi-major axis of its orbit (in AU). a3/ P2 = 1 a3 = P2

26. Galileo • Kepler nailed the mathematical description of planetary motion. • But, his books were not very…. accessible • The combination of writing in Latin and using lots of math is not the way to get a best-seller. • However, if the Bishops can’t understand it, they are unlikely to burn you…. • It was left to Galileo (1564-1642) to popularize the Copernican model and Kepler’s laws.

27. Galileo • Galileo was an interesting character, a brilliant scientist who knew how to turn a buck. • He used the development of the telescope to speculate on the arrival of cargo ships • And he was a great popularizer of science

28. Galileo • He had a huge list of discoveries and contributions. • First to use the telescope for astronomy • Discovered the moons of Jupiter • Discovered sunspots • Discovered the mountains of the Moon • Discovered the Milky Way was made of other stars

29. It was Galileo’s observation of the phases of Venus that buried the geocentric model. Geocentric Heliocentric No gibbous or full phases! All phases are seen! Galileo observed all phases!

30. Galileo • In addition to his observations Galileo wrote a book in Italian called “Dialogue of the Two Chief World Systems” • It featured a fictionalized debate between Copernican and Ptolemaic advocates • That got him in trouble with the Catholic church