Kepler correctly determined the motion of the planets giving his 3 Laws which still hold today for the planets and other orbital motion: moons around

Transcription

1 Kepler correctly determined the motion of the planets giving his 3 Laws which still hold today for the planets and other orbital motion: moons around planets, exoplanets around other stars, stars in the Milky Way about its center Kepler did not address WHY. Simply what curve best matched orbits and some arithmetical relationships The WHY was determined by physicists like Galileo and Newton. They needed to develop Physics as a science: understand motion, forces, and gravity 1

2 Kepler s Laws of Planetary Motion (1630) #1: The orbit of a planet is an ellipse with the Sun at one focus #2: The line joining a planet and the sun sweeps equal areas in equal time. #3:The square of a planet s period is proportional to the cube of the length of the orbit s semimajor axis. (semimajor axis is about the same as the distance to the Sun) 2

3 Kepler s Laws of Planetary Motion (1630) FIRST LAW: The orbit of a planet is an ellipse with the sun at one focus. A line connecting the two foci in the ellipse always has the same length. In an ellipse, the planet is sometimes closer to the Sun and sometimes further away. 3

4 Kepler s Second Law The line joining a planet and the sun sweeps equal areas in equal time. The planet moves slowly here when it is further away. The planet moves quickly here when it is closer to the Sun 4

5 Kepler s Third Law The square of a planet s period is proportional to the cube of the length of the orbit s semimajor axis. Same as how long it takes to make one orbit depends on how far away the planet is from the Sun Mathematically, T 2 /a 3 = constant (=1 if use 1 Earth year and 1 AU as units) The constant is the same for all objects orbiting the Sun same process determines all planets motions semimajor axis: a direction of orbit The time for one orbit is one period: T 5

6 Third Law Example Jupiter compared to Earth If we measure that it takes Jupiter 11.9 years to orbit the Sun then: distance 3 (Jupiter-Sun) = period(jupiter) 2 distance = period 2/3 distance = (11.9*11.9) 1/3 distance = (142) 1/3 = 5.2 AU 6

7 Mean Distance from Sun Sidereal Orbital Period AU P e Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune D 3 P

8 Galileo Born in Pisa, Professor of art, mathematics, natural philosophy, astronomy in Pisa and Padua with occasional contract work for various Dukes especially in Florence Very strong proponent of the scientific method use of observations to test theories. Possibly the greatest physicist/astronomer in history. Early work:motion, and practical elements like hydrostatics 1609: first person to use a telescope for astronomy became the most famous scientist/celebrity in Europe Last 25 years of life was often in trouble with the Catholic Church over saying Sun at center of solar system. His celebrity helped to save him 8

9 Galileo Telescope Jupiter had at least 4 moons which circled it (something besides Earth could be the center of motion); there were many more stars (saw 89 in Orion compared to 9 by eye), mountains on moon Book Starry Messenger Venus had definite phases and clearly orbiting Sun Observed sunspots (patches on Sun). Sun revolved on own axis. Wasn t perfect and changes in unpredictable manner Observed Saturn s rings but was confused as to what they were Wrote book on Copernican vs Ptolemaic models in 1632, nominally with Church s permission. But it offended Church. Dialogue Concerning the Two Chief World Systems. In Italian so everyone could read. Character in book Simplicio defended Church s position spent last 10 years of his life in house arrest. Catholic church said contrary to scriptures Church admits in error in

10 Jupiter s Moons (1610) Jupiter s 4 largest moons can be seen with a small telescope. Change position with time how long it takes to go around vs distance from Jupiter More discovered: Pluto now has 4 moons 10

11 Phases of Venus (1610) Venus has phases like the Moon which are easily explained by having it orbit the Sun and impossible to explain if the Earth was at the center of the Solar System 11

12 Sunspots Sunspots are now known to be magnetic storms. Allows the rotation of the Sun to be readily observed. Seen without telescopes before Galileo but his observations were of higher quality and showed Sun s 25 day rotation Sun isn t perfect and rotates like Earth 12

13 Lecture Feedback me a few sentences describing one topic you learned from this set of presentations. Please include the phrase Gallileo s telescopic observations clearly showed the planets all orbited the Sun in your mini-report but do not use that as your one topic. 13