1. Lecture 2 The Solar System The Universal Gravitation Chapter 1.3  1.11 Homework: On-line quiz for Chapter 1 (due August 27th) Outline What we see in the sky The Ptolemaic system The Copernican system, Kepler’s laws Universal gravitation

2. Constellations Patterns of stars seen in the sky There are 88 constellations About 50 of them were named by ancient Greeks and Romans

3. Constellation of Orion

4. Winter triangle

5. Daily star paths

6. Seasonal changes in the sky

7. Annual change of the Sun altitude

8. Ancient Observations Relations of celestial objects to weather and agriculture Observations of the Sun and Moon Observations of stars and planets

9. Ptolemaic model of the Universe Developed by Claudius Ptolemy (A.D. 100-170) The Earth is in the center The Sun is at the third orbit from Earth after Mercury and Venus Epicycles are added to circular orbits of planets to explain retrograde motion

10. Ptolemy (A.D. 100  170)

11. Ptolemaic Epicycles The Ptolemaic model along with a catalog of positions of 1028 stars were published in his book Almagest

12. Nicolaus Copernicus (1473  1543) Copernicus is said to be the founder of modern astronomy

13. Kepler’s Laws 1.Planets move on elliptical orbits 2. The planet’s radius-vector sweeps out the same areas in equal times 3. The squares of the periods of the planets are proportional to the cubes of their semi- major axes Demonstration of Kepler's laws

14. The Solar System Content: Sun (the only star) 9 planets Nearly 100 moons Asteroids Comets Free-flying gas and ``dusty’’ particles http://solarviews.com/eng/homepage.htm

15. Planets of the Solar System ObjectDiameterDist. from SunDist. in A.U.Revol. Period Earth years Mercury 4,88057.9 million km 0.4 0.2 Venus 12,100108.2 0.7 0.6 Earth 12,760149.6 (1 A.U.) 1.0 Mars 6,790227.9 1.5 1.9 Jupiter143,000778.3 5.2 11.9 Saturn120,0001,427 9.5 29.5 Uranus 52,0002,87019.2 84.0 Neptune 48,4004,49730.1164.8 Pluto 2,2605,90039.4247.7

16. Sir Isaac Newton (1642-1727)

17. The Universal Law of Gravitation Every mass attracts every other mass through the force called gravity The force of attraction is directly proportional to the product of their masses The force of attraction is inversely proportional to the distance between the objects F g = G x M 1 x M 2 / d 2 gravity

18. The Universal Law of Gravitation

19. Support of Kepler’s Laws Newton found that Kepler’s first two laws apply not only to planets, but to any object going around another one under the force of gravity The orbits do not have to be elliptical They can also be parabolic or hyperbolic

20. Tides Tides are due to gravitational attraction between the Earth and the Moon

21. Spring Tides

22. Neap Tides

23. Tidal Friction and Synchronous Rotation The tidal bulges try to stay on the Earth – Moon line The Earth’s rotation tries to pull the bulges around The tidal friction slows down the Earth’s rotation The length of a day gets longer It makes the Moon move further away from Earth The Moon is in synchronous rotation with the Earth (always showing the same face)

24. The Discovery of Neptune In 1781, the planet Uranus was discovered telescopically from Britain by William Herschel.William Herschel In 1845, a Cambridge mathematician, John Couch Adams, based on the law of gravitation, predicted the existence of an unseen planet, to account for the fact that Uranus was being pulled slightly out of position in its orbit.John Couch Adams He sent the calculations to test to England’s Royal Astronomer, who set them aside. Shortly after that, a French mathematician, Urbain Leverrier, published a similar prediction and contacted astronomers at Berlin Observatory, who found the new planet on the night of 23 September 1846.

25. Urbain Leverrier (1811  1877)

26. Astronomy and Astrology Astronomy is a science It describes the real world, sets new problems and solves them, using methods of itself and other sciences (such as physics and mathematics) Astrology is interpreting apparent positions of the Sun, planets, and stars to predict human life. It does not set and solve any problem

27. Summary The Earth is not the center of the universe but instead is a planet orbiting a rather ordinary star in the Milky Way Galaxy. Newton’s discoveries showed that the same physical laws we observe on Earth apply throughout the Universe. Celestial bodies in the gravitational field of each other move according to Kepler’s laws.