Download motion

ASTR 1101-001
Spring 2008
Joel E. Tohline, Alumni Professor
247 Nicholson Hall
[Slides from Lecture11]
Exam #1 Results
• Total score composed
of two parts:
– (g1) score on scalemodel homework
assignment is 25%
– (g2) score on in-class
exam is 75%
• Where to find scores:
– g1 at bottom of exam
– g2 at top-left of exam
'Exam #1 Grades'
D's
F's
C's
A's
B's
Example: g1 = 17/25; g2 = 23/27  grade = 25*g1 + 75*g2 = 89
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)
Observation/Explanation
• OBSERVATION #1: Stars (and Sun,
Moon, etc.) rise in east and set in west
once every 24 hours
– EXPLANATION (Ptolemy): Celestial sphere
spinning east-to-west about a stationary Earth
every 24 hours
– EXPLANATION (Copernicus): Earth spins
west-to-east once on its axis every 24 hours
Observation/Explanation
• OBSERVATION #2: Remove 24-hr diurnal
motion; Sun and Moon both move steadily
west-to-east at different rates (Moon =
month period; Sun = year period)
– EXPLANATION (Ptolemy): Sun and Moon
both orbit the Earth
– EXPLANATION (Copernicus): Moon orbits
Earth (monthly) but Earth orbits Sun (yearly)
Observation/Explanation
• OBSERVATION #3: 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
– EXPLANATION (Ptolemy): Planets orbit Earth
via a combination of circular motions (deferent
+ epicycle)
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances greater than 1 AU and
with velocities less than the Earth’s
Observation/Explanation
• OBSERVATION #3: 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
– EXPLANATION (Ptolemy): Planets orbit Earth
via a combination of circular motions (deferent
+ epicycle)
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances greater than 1 AU and
with velocities less than the Earth’s
Observation/Explanation
• OBSERVATION #3: 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
– EXPLANATION (Ptolemy): Planets orbit Earth
via a combination of circular motions (deferent
+ epicycle)
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances greater than 1 AU and
with velocities less than the Earth’s
Observation/Explanation
• OBSERVATION #4: Inferior planets
[Mercury, Venus] never wander very far (in
angular separation) from the Sun; only
seen shortly before or shortly after
sunrise/sunset
– EXPLANATION (Ptolemy): Planet deferents
rotate together with the Sun
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances less than 1 AU
Observation/Explanation
• OBSERVATION #4: Inferior planets
[Mercury, Venus] never wander very far (in
angular separation) from the Sun; only
seen shortly before or shortly after
sunrise/sunset
– EXPLANATION (Ptolemy): Planet deferents
rotate together with the Sun
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances less than 1 AU
Observation/Explanation
• OBSERVATION #4: Inferior planets
[Mercury, Venus] never wander very far (in
angular separation) from the Sun; only
seen shortly before or shortly after
sunrise/sunset
– EXPLANATION (Ptolemy): Planet deferents
rotate together with the Sun
– EXPLANATION (Copernicus): Planets orbit
the Sun at distances less than 1 AU