ASTR 1101-001
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 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)
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 (1564-1642)
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 “a”
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 (Plong & Pshort)
start out “in alignment” with one another,
they will return to an aligned arrangement
after a time that is identified by their socalled “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