2 Extra Credit 1. Sat. Sept. Mt. Tam, Marin. Dr. Jill Tartar will give a talk on: Searching for Aliens, Finding Ourselves 2. SF State Observatory Open Mon, Wed & Thurs at 7:15 PM 3. Observe the Moon yourself starting tonight!
3 In Class Voting Sheet Print out from: physics.sfsu.edu/~chris/astro115 A B Bring your voting sheet to EVERY class. Its mandatory... and fun! C D
4 Test Question: What s your favorite class? A. Astronomy B. Astronomy C. Astronomy D. Astronomy
5 Question: Over the course of a year, how does the Sun s position appear to change? A. The sun appears in front of every one of the constellations B. The sun appears in front of just 13 of the constellations C. The sun appears to tilt D. The sun appears to shrink significantly.
6 The Moon is rising in this photo. What time of day is it? A. Sunrise B. Sunset C. Noon D. Midnight
7 This photo was taken just before Sunrise. The Moon is on the EASTERN Horizon. What phase is the Moon in? A. Full Moon B. First Quarter C. Crescent D. Gibbous
8 Chapter 2 The Motion of the Planets!What paths do the planets take through space?!why do they follow these paths?!the human quest to answer these questions led to the birth of modern science
9 Science & Theories Science does not provide absolute truth ; it is not always right. But it is very reliable. How reliable is a Scientific theory? Would you bet your life on a theory? Too late...you already have...and wisely. (eg. Aerodynamics tells us how airplanes work; Electromagnetism allows us to build traffic lights) A theory has been tested over and over & always passed the test. Nevertheless, theories that have been used for many years are sometimes refined or abandoned if disproven. So, science is self-correcting. How did humans develop this reliable way to understand the world?
10 Planets: Wanderers in The Sky! Ancient peoples noticed that five stars seemed to move...! They called them Planets which means wanderers.! Every ancient civilization was aware of these 5 planets.! Words for the days of the week derive from the 5 visible planets: Mercury, Venus, Mars, Jupiter, and Saturn.
11 How Planets Move! In one night, the planets move from East to West due to Earth s rotation.! But, Night after night, planets drift slowly from West to East with respect to the background stars.! This is called direct motion.! However sometimes a planet reverses direction!! We call this backward drift retrograde motion. ast West
14 Ptolemy s Geocentric Model! Greek scientist Ptolemy worked at the Great Library of Alexandria in Egypt (~140 AD)! His research included astronomy, astrology, geography & music.! Ptolemy used a geocentric model of the solar system explain retrograde motion.! Earth was at the center.! In Ptolemy s model, the planets travel along small circles (epicycles) that move on larger circles.
15 Ptolemy s Geocentric model
16 Geocentric Model Planets Circle the Earth on epicycles anim.
17 The Geocentric Model! Ptolemy s views were published in a book: the Almagest! Because it could explain retrograde motion, his theory was widely accepted by European & Middle Eastern astronomers.! When later observations did not agree with the Geocentric Model, many small changes were added.! The Model became quite cumbersome, but it was widely accepted for 1,600 years!
18 How do you Challenge an accepted theory?! Arab scientist Alhazan* ( ) came up with a way.! Like Ptolemy, he lived in Egypt and studied many subjects (optics, astronomy, math, geometry)! A hypothesis must be proved by experiments that can be confirmed, he said.! Later scientists in Europe read his books, and developed what we now call the scientific method. *a.k.a. Ibn al-haytham
19 Copernicus Heliocentric theory! We credit Nicolaus Copernicus, a 16th-century Polish astronomer, with our modern view of the Solar System! Copernicus suggested a heliocentric theory! Sun was at the center of the Solar System.! Planets moved in circles around the Sun. Copernicus could also explain retrograde motion.
20 I began to be annoyed that the movements of the world machine were not understood with greater certainty by the philosophers, who otherwise examined so precisely the most insignificant trifles of the world. Copernicus, Book on the Revolutions, 1543
21 it is just like someone taking from various places hands, feet, a head, and other pieces, each very well depicted, but not for the representation of a single person. In forcing these fragments together, they created a monster rather than a man. Copernicus, Book on the Revolutions, 1543
22 An inner planet (eg. Earth) catches up with an outer planet (eg. Mars) causing it to appear to move backward (retrograde motion)
23 Copernicus Heliocentric Theory! Copernicus new theory clearly stated planets positions relative to the Sun! It predicted the next appearances of planets in the sky.! These predictions proved correct, lending support to Copernicus Heliocentric Model.! However, his model was no more accurate than the Geocentric model
24 A Heretical Idea?! Copernicus Heliocentric theory caused problems with the Church.! The Church had accepted Ptolemy s geocentric universe.! It agreed with the Church s notion of heaven as a perfect place.! To question this model was to question the Church s authority.! Copernicus book was censored by the Catholic Church.! Copernicus died before the full effects of his model were felt! However, other astronomers supported his model, using their observations.
25 Galileo Galileo Galilei ( ) was an Italian scientist who studied many fields. He tried to measure the speed of light, and studied gravity. He obtained the first observations to confirm the Heliocentric Model
26 More Evidence for the Copernican Model! Galileo was the first person to use a telescope for astronomy.! Several of his observations supported the Heliocentric model of Copernicus:! He discovered moons orbiting Jupiter (not the Earth)
28 Galileo Galilei! Galileo observed Venus with his telescope.! He reported seeing crescent and gibbous phases on Venus! Gibbous phases are only possible if Venus is on the other side of the Sun from us.! This can t happen in the Geocentric model of the Universe!
29 Geocentric model Of Ptolemy
30 Theory: Venus is always between the Sun and Earth Prediction: Venus will always be seen in crescent phase
31 Not to scale! Theory: Venus sometimes goes behind the Sun Prediction: Venus will sometimes be seen in gibbous phase. Telescope Observation: Venus does show gibbous phase!
32 Galileo Challenges the Geocentric Model! Heliocentric model challenged beliefs held for 2000 years.! The Church felt threatened by this.! Galileo was arrested and tried.! He had reason to fear: Giordano Bruno, another advocate of the Heliocentric Model, was tried by The Inquisition.! Bruno was burned at the stake for heresy!! Galileo was not allowed to advocate the Heliocentric Model.! However word of his discoveries spread.
33 Tycho Brahe! Tycho Brahe ( ) was a Danish nobleman! As a child, he had witnessed a Solar Eclipse! He was so impressed that eclipses could be predicted, that he dedicated his life to astronomy.! He built his own observatory on an island.! He observed a nova or new star, and found it was outside the solar system.
34 Tycho Brahe! Tycho made the most precise observations of the planets to date without using a telescope.! During his life, Tycho was reluctant to share his precise data.! He also had a wild lifestyle, and lost his nose in a duel.! He attended many royal parties! Perhaps one too many. He died after eating and drinking too much at a party.
35 Johannes Kepler! After Tycho died, his assistant, Johannes Kepler, took his place.! Unlike others, Kepler saw planets as real objects, which required a physical theory.! (He even wrote the first Sci-Fi novel, Somnium, about space travel) Kepler studied Tycho s precise observations of the planets! He saw patterns in their motion never seen before.
36 Platonic Solids Initially, Kepler thought the orbits of the planets had the same proportions as the 5 Perfect Solids of geometry. He studied this for years, but could not support it with observations.
37 Kepler! However, Kepler did not give up.! His next model matched 10 of Tycho s observations of Mars perfectly.!...but it mis-matched 2 of Tycho s observations... by only 0.1 degree.! So, he rejected that idea also.! He found that the planets motions could be explained if he gave up the idea of circular orbits.! In time he discovered that all planets follow the same three rules..! Kepler s Laws of Planetary Motion
39 Ellipse Shapes Circle
40 How to draw an Ellipse Ellipse Kit
41 Size of an Ellipse! An ellipse is a flattened circle.! We measure the size of a circle using: Diameter or Radius! But how would you measure the size of an ellipse?! It has two diameters :! The major axis of an ellipse is its long diameter! (The major axis passes through the two foci.)! The minor axis is perpendicular to the major axis.
43 Kepler s Second Law! When a planet is at its greatest distance from the Sun it moves the slowest! When a planet is at its smallest distance from the Sun it moves the fastest.! The point of closest approach to the Sun is called perihelion! The point of farthest distance from the Sun is called aphelion
44 Kepler s 3 rd Law... relates a planet s Orbit Period to its Semimajor axis Orbital Period (P) = time it takes to orbit the Sun once (in years) Semi-major axis (a) = distance from the planet to the Sun (in AU) The Astronomical Unit (AU) is a unit of distance a It is used to measure distances to planets. 1 AU = Earth s distance from the Sun = 93,000,000 miles
45 Kepler was the first to realize that P is related to a...in the same way for every planet. Kepler s 3 rd Law P 2 = a 3 P = orbital period in years a = semi-major axis in A.U. P A P 2 A 3
46 Kepler s Third Law An Example The Planet Uranus orbits the Sun at a distance of 19 AU. So its semimajor axis is: a= 19 AU Question: How long does it take Uranus to Orbit the Sun? P 2 =a 3 a 3 = a x a x a = 19 x 19 x 19 = 6859 P 2 = a 3 = 6859, so P = the square root of 6859 = 83 years
47 Kepler s Third Law An Example The Planet Saturn orbits the Sun in 29 years Question: How far is Saturn from Sun? P 2 =a 3 P 2 = 29 x 29 = 841. So a 3 = 841 What is a? a 3 = a x a x a Take a guess: a=10. Then a x a x a = 1000 too high Guess: a=9. Then a x a x a = 729 too low Answer: a= 9.5
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