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

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

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)

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

Galileo’s New Observations made 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”

Galileo Galilei ( )

Galileo’s New Observations made 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”

Galileo’s New Observations made 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”

Galileo’s New Observations made 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”

Galileo’s New Observations made 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”

Galileo’s New Observations made 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

Galileo’s New Observations made 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

Phases of Venus

Phases of Venus expected in the Coperican Heliocentric model

Galileo’s New Observations made 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

Angular size “  ” expressed in arcseconds.

Galileo’s New Observations made 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

Galileo’s New Observations made 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”

Scientific utility of the Copernican 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]

Synodic versus Sidereal Planetary Orbital Periods When two systems that have different natural oscillation periods (P long & P short ) start out “in alignment” with one another, they will return to an aligned arrangement after a time that is identified by their so- called “beat” period, “B”.

Synodic versus Sidereal Planetary Orbital Periods For two planets: Sidereal periods

Synodic versus Sidereal Planetary Orbital Periods For two planets: Synodic period