Transcript cosmogony
Chapter 2 Gravitation and the Waltz of the Planets What do you think? • What is the shape of the Earth’s orbit around the Sun? • Do the planets orbit the Sun at constant speeds? Cosmogony • A cosmogony is theory about Earth’s place in the universe. • A geocentric cosmogony is a theory that proposes Earth to be at the center of the universe. • A heliocentric cosmogony is a theory that proposes the Sun to be at the center of the universe. Which is the geocentric cosmogony and which is the heliocentric cosmogony? geocentric (Earth-centered) heliocentric (Sun-centered) Planets were often called wandering stars because they seem to move from one constellation to the next. Copernicus devised the first comprehensive heliocentric cosmology to successfully explain retrograde motion sidereal period is the time it takes a planet to orbit the Sun once. synodic period is the time that elapses between two successive identical configurations as seen from Earth (e.g., time from opposition to opposition) Scientists use parallax to measure distances. Tycho Brahe measured distances using parallax that disproved ancient ideas about the heavens • A supernova in 1572 was shown to exist in the distant heavens; this troubled scholars who previously thought the heavens were unchanging. • He showed that comets were objects that occurred in the region of the planets, not in Earth’s atmosphere. Mathematician Johannes Kepler created laws of planetary motion that describe the orbital shapes, changing speeds, and the lengths of planetary years Kepler’s First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus. The distance between the two foci impact the eccentricity of the ellipse’s shape. Kepler’s Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time. Kepler’s Third Law: The square of a planet’s sidereal period is proportional to the cube of the length of its orbit’s semimajor axis (p2=a3). Italian scientist Galileo made discoveries that strongly supported a heliocentric cosmology Galileo’s telescope revealed phases of Venus which could only occur IF Venus orbits the Sun. Galileo’s telescope revealed that Jupiter had moons which orbited Jupiter instead of Earth. Isaac Newton formulated three laws that describe fundamental properties of physical reality Newton’s Three Laws of Motion • A body remains at rest or moves in a straight line at a constant speed unless acted upon by an unbalanced outside force. • Force = mass x acceleration • Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body. Newton’s description of gravity accounts for Kepler’s laws • Newton’s universal law of gravitation states: Two bodies attract each other with a force that is directly proportional to the square of the distance between them. – This law mathematically proves Kepler’s Third Law (p2=a3). – This law describes various orbits objects can take when moving near the Sun. – This law can be used to predict when comets, such as Halley’s comet, will pass near Earth. What did you think? • What is the shape of the Earth’s orbit around the Sun? All planets have elliptical orbits around the Sun. • Do the planets orbit the Sun at constant speeds? The speed varies inversely with distance from the Sun. The farther a planet is in its elliptical orbit from the Sun, the slower it moves. Self-Check 1: Compare and contrast the Ptolemaic and Copernican cosmologies by explaining a variety of naked-eye observations, using both models. 2: State Kepler’s three laws of planetary motion and describe the geometric content and observational consequences of each. 3: List Galileo’s telescopic observations and explain the success of failure of Ptolemaic and Copernican models in accounting for them. 4: State and identify examples of Newton’s three laws of motion. 5: State Newton’s law of universal gravitation and identify the characteristics that explain Kepler’s laws in terms of Newton’s laws.