The History of Astronomy II September 18, 2009

Taking Care of Business (TCB) Read textbook Unit 12 Read textbook Unit 12 Take Moon observations Take Moon observations 6 Observations due September 28 6 Observations due September 28 Test #1 – September 18 to September 21 Test #1 – September 18 to September 21 Sept 16 class – last day of test material Sept 16 class – last day of test material Reserve your test date/time ASAP Reserve your test date/time ASAP InQsit instructions on Blackboard InQsit instructions on Blackboard

Parallax Review A star located 25 parsecs from the Sun has a parallax angle of A star located 25 parsecs from the Sun has a parallax angle of A. 25 arcseconds A. 25 arcseconds B. 50 arcseconds B. 50 arcseconds C arcseconds C arcseconds D arcseconds D arcseconds E. None of the above E. None of the above 1/25= 1/d 1/25= 1/d

Review Question What is the distance to a star (in parsecs) that has a parallax angle of 0.1 arcseconds? What is the distance to a star (in parsecs) that has a parallax angle of 0.1 arcseconds? a) 0.01 a) 0.01 b) 0.1 b) 0.1 c) 1 c) 1 d) 10 d) 10 e) 100 e) 100 1/  = 1/0.1 1/  = 1/0.1

Ptolemy Earth-centered system Earth-centered system Earth located slightly off the center of orbiting circles Earth located slightly off the center of orbiting circles Ptolemaic model was inconsistent over many years Ptolemaic model was inconsistent over many years Epicycles adds to prove his method, instead it’s too complicated Epicycles adds to prove his method, instead it’s too complicated Geocentric Method Geocentric Method Ptolemy – 100 – 170 A.D.

Ptolemy’s Geocentric Model

Ptolemy’s Epicycles Epicycle Deferent Earth

Ptolemy’s Epicycles

Where is retrograde motion occurring? Planet’s Path

Ptolemy’s Epicycles Where is retrograde motion? Planet’s Path

Astronomy Revolution Idea of an Earth- centered system held to be true for close to 2000 years Idea of an Earth- centered system held to be true for close to 2000 years Believed that there must be a simpler explanation of the Solar System Believed that there must be a simpler explanation of the Solar System Usually true when it is the simplest Usually true when it is the simplest Placed the Sun at the center Placed the Sun at the center Heliocentric system Heliocentric system Nicholas Copernicus –

Discussion Why was the Copernican model not widely accepted at first? Why was the Copernican model not widely accepted at first?

Tycho Brahe Great interest in observing the motions of the sky Great interest in observing the motions of the sky Noticed that positions of planets were not accurate compared to available tables Noticed that positions of planets were not accurate compared to available tables Did his own detailed observations to mark positions Did his own detailed observations to mark positions Discovered a “new star” Discovered a “new star” Supernova Supernova Brahe –

Kepler Wanted to accurately match the observations of Mars – off by as much as 8 arcminutes Wanted to accurately match the observations of Mars – off by as much as 8 arcminutes Developed a new model that did not rely on circles, but instead used ellipses Developed a new model that did not rely on circles, but instead used ellipses Kepler’s 3 Laws of Motion Kepler’s 3 Laws of Motion Johannes Kepler –

Kepler’s First Law The orbit of each planet about the Sun is an ellipse with the Sun at one focus The orbit of each planet about the Sun is an ellipse with the Sun at one focus

Kepler’s Second Law As a planet moves around its orbit, it sweeps out equal areas in equal times As a planet moves around its orbit, it sweeps out equal areas in equal times To do this, a planet must move slower at aphelion (point furthest from sun) than it does at perihelion (point closest to the sun: fastest) To do this, a planet must move slower at aphelion (point furthest from sun) than it does at perihelion (point closest to the sun: fastest)

Post Tutorial Question During which part of the planet’s orbit (A, B, C, or D) would the planet move with the greatest speed? During which part of the planet’s orbit (A, B, C, or D) would the planet move with the greatest speed?

Post Tutorial Question During how many portions of the planet’s orbit (A, B, C and D) would the planet be speeding up the entire time? During how many portions of the planet’s orbit (A, B, C and D) would the planet be speeding up the entire time? a. Only during one of the portions shown. a. Only during one of the portions shown. b. During two of the portions shown. b. During two of the portions shown. c. During three of the portions shown. c. During three of the portions shown. d. During four of the portions shown. d. During four of the portions shown. e. None of the above. e. None of the above.

Post Tutorial Question According to Kepler’s Second Law, during which one of the portion of the planets orbit “B”, “C”, or “D”, would the planet take the same amount of time as it took for the portion of the orbit identified with letter “A”? If you think all the portions of the orbit take the same amount of time, answer “E”. According to Kepler’s Second Law, during which one of the portion of the planets orbit “B”, “C”, or “D”, would the planet take the same amount of time as it took for the portion of the orbit identified with letter “A”? If you think all the portions of the orbit take the same amount of time, answer “E”.