Presentation on theme: "ASTRONOMY 161 Introduction to Solar System Astronomy Class 6."— Presentation transcript:
ASTRONOMY 161 Introduction to Solar System Astronomy Class 6
Tycho, Kepler, & Galileo Wednesday, January 17 “E pur si muove!” [It still moves!] - Galileo
Astronomical movies: The Phases of Venus http://antwrp.gsfc.nasa.gov/apod/ap060110.html When Moons and Shadows DanceWhen Moons and Shadows Dance (Jupiter) http://antwrp.gsfc.nasa.gov/apod/ap030227.html Large Sunspot GroupLarge Sunspot Group (Sun) http://antwrp.gsfc.nasa.gov/apod/ap010411.html
Tycho, Kepler, & Galileo: Key Concepts (1) Tycho Brahe made accurate measurements of planetary motion. (2) Planetary orbits are ellipses with the Sun at one focus. (3) A line between planet & Sun sweeps out equal areas in equal times. (4) The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun. (5) Galileo made telescopic observations supporting the heliocentric model.
(1) Tycho Brahe made accurate measurements of planetary motion. Tycho Brahe (1546-1601): Danish astronomer
Tycho discovered ‘new star’, or ‘nova’, upsetting ancient notion of perfect, unchanging heavens. Made very accurate measurements of planetary positions. Tycho’s contributions to astronomy
Was Tycho’s assistant. Used Tycho’s data to discover Three Laws of Planetary Motion. Johannes Kepler (1571-1630): German
(2) Kepler’s First Law of planetary motion The orbits of planets around the Sun are ellipses with the Sun at one focus.
Ellipse = an oval built around two points, called focuses (or foci). SIZE of ellipse: Major axis = longest diameter of ellipse. Semimajor axis = half the major axis.
SHAPE of ellipse: Eccentricity = distance between foci divided by major axis. Foci close together: ellipse nearly circular, eccentricity close to zero. Foci far apart: ellipse very flattened, eccentricity close to one.
Example: Mars Semimajor axis = 1.524 A.U. Eccentricity = 0.093 (much smaller than one)
Ellipse comes from the family of Conic Sections
(3) Kepler’s Second Law of planetary motion A line from the Sun to a planet sweeps out equal areas in equal time intervals.
Consequences of Kepler’s Second Law: Planets move fastest when closest to the Sun. Example: Mars Perihelion: 206,600,000 km (1.381 A.U.) Max. Orbital Speed: 26.5 km/s Aphelion: 249,200,000 km (1.666 A.U.) Min. Orbital Speed: 22.0 km/s
(4) Kepler’s Third Law of planetary motion The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun*: *A planet’s average distance from the Sun is equal to the semimajor axis of its orbit.
Kepler’s Third Law in mathematical form: P = orbital period (in years) a = semimajor axis (in A.U.) Example: The orbit of Mars
(5) Galileo made telescopic observations supporting the heliocentric model. Galileo Galilei (1564- 1642): Italian Galileo was among the first to observe the sky with a telescope (1609).
Flashback to Class 1: What is Science? The SYSTEMATIC study of the Universe Gather facts Modify hypothesis Guess an explanation (Guess=hypothesis) Test hypothesis
1) Mountains on the Moon Aristotle & Ptolemy said the Moon is a perfect, smooth sphere. In fact, the Moon is no more “perfect” than the Earth.