ASTR 1101-001 Spring 2008

1. ASTR 1101-001Spring 2008 Joel E. Tohline, Alumni Professor 247 Nicholson Hall [Slides from Lecture12]

2. Contrasting Models of Planetary Motions • Greek Geocentric Model (Ptolemaic System) – §4-1 • Originated by Apollonius of Perga & Hipparchus in 2nd century B.C. • Expanded upon by Ptolemy in 2nd century A.D. • Copernican Heliocentric Model – §4-2 • Proposed in 1543 by Nicolaus Copernicus • (Also considered in 3rd century B.C. by Aristarchus)

3. Contrasting Models of Planetary Motions • Geocentric Model • Underlying hypothesis: Earth is unmoving and at “center of the universe” • Heliocentric Model • Preferred (by Copernicus) initially because of its relative simplicity • Eventually proven correct by new observations made possible by the aid of telescopes (see Galileo’s discoveries §4-5)

4. Naked-Eye Observations“explained” by both Ptolemy and Copernicus • Stars (and Sun, Moon, etc.) rise in east and set in west once every 24 hours • Remove 24-hr diurnal motion; Sun and Moon both move steadily west-to-east at different rates (Moon = month period; Sun = year period) • Superior planets [Mars, Jupiter, Saturn] usually wander slowly east-to-west (observed periods greater than 1 year) but periodically display retrograde (west-to-east) motion • Inferior planets [Mercury, Venus] never wander very far (in angular separation) from the Sun; only seen shortly before or shortly after sunrise/sunset

5. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Revolutionary discoveries not especially relevant to the “geo-” versus “helio-” centric debate: • Mountains and Craters on the Moon • Sunspots on the Sun (ouch!) • Rings of Saturn • The “Milky Way” is “a mass of innumerable stars”

6. Galileo Galilei (1564-1642)

7. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Revolutionary discoveries not especially relevant to the “geo-” versus “helio-” centric debate: • Mountains and Craters on the Moon • Sunspots on the Sun (ouch!) • Rings of Saturn • The “Milky Way” is “a mass of innumerable stars”

8. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Revolutionary discoveries not especially relevant to the “geo-” versus “helio-” centric debate: • Mountains and Craters on the Moon • Sunspots on the Sun (ouch!) • Rings of Saturn • The “Milky Way” is “a mass of innumerable stars”

9. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Revolutionary discoveries not especially relevant to the “geo-” versus “helio-” centric debate: • Mountains and Craters on the Moon • Sunspots on the Sun (ouch!) • Rings of Saturn • The “Milky Way” is “a mass of innumerable stars”

10. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Revolutionary discoveries not especially relevant to the “geo-” versus “helio-” centric debate: • Mountains and Craters on the Moon • Sunspots on the Sun (ouch!) • Rings of Saturn • The Milky Way is “a mass of innumerable stars”

11. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Discoveries useful in resolving the “geo-” versus “helio-” centric debate: • Venus exhibits phases like those of the Moon • The apparent size of Venus was related to the planet’s phase (as well as to the planet’s angular distance from the Sun) • Jupiter has four “Galilean satellites”; their orbital behavior is like a Copernican system in miniature

12. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Discoveries useful in resolving the “geo-” versus “helio-” centric debate: • Venus exhibits phases like those of the Moon • The apparent size of Venus was related to the planet’s phase (as well as to the planet’s angular distance from the Sun) • Jupiter has four “Galilean satellites”; their orbital behavior is like a Copernican system in miniature

13. Phases of Venus

16. Phases of Venus expected in the Coperican Heliocentric model

18. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Discoveries useful in resolving the “geo-” versus “helio-” centric debate: • Venus exhibits phases like those of the Moon • The apparent size of Venus was related to the planet’s phase (as well as to the planet’s angular distance from the Sun) • Jupiter has four “Galilean satellites”; their orbital behavior is like a Copernican system in miniature

19. Angular size “a” expressed in arcseconds.

20. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • Discoveries useful in resolving the “geo-” versus “helio-” centric debate: • Venus exhibits phases like those of the Moon • The apparent size of Venus was related to the planet’s phase (as well as to the planet’s angular distance from the Sun) • Jupiter has four “Galilean satellites”; their orbital behavior is like a Copernican system in miniature

23. Galileo’s New Observationsmade with the aid of a telescope (~ 1610) • First fundamentally new astronomical data in almost 2000 years! • Contradicted prevailing opinion (and religious belief) and strongly suggested a heliocentric (Copernican) structure of the “universe”

24. Scientific utility of theCopernican Heliocentric Model • Can deduce the true “sidereal” (as opposed to readily measured “synodic”) orbital periods of each of the planets [see textbook BOX 4-1 and Table 4-1] • Can deduce the distance that each planet is from the Sun, relative to the Earth’s distance from the Sun (1 AU); [see textbook discussion associated with Table 4-2]