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Unit1: The Physics of Astronomy
This Week & Next: Astronomy in Motion
Today: Historical Background & Basic Refresher
•Kepler’s Laws
•Newton’s Laws
•Work & Energy
General Note: Please ASK if there are
terms/math/physics that are new to you!
Why is Astronomy Different?
Astronomy is the Science of Observation
rather than Experiment
X
What are the Most Basic Observable Quantities?
Motion
Light
What were the first objects routinely observed?
When?
A Little History...
Astronomy is an ancient science
•Maya
•Aztec
•Egyptians
•Chinese
•Greek
II. Early Scientific Astronomy
Plato (350 BC):
Geocentric Model of SS
-planets & Sun all orbit a
stationary Earth
- orbits are perfect spheres
vs.
X
Aristarchus (270 BC):
First Heliocentric Model
-planets orbit the Sun
-spherical orbits
REJECTED!
II. Early Scientific Astronomy
Ptolemy (100 AD):
added epicycles to better fit
observed motion of planets
Almagest
“The Great Treatise”
Medieval Manuscript,
Melk Abbey (Austria):
Geocentric Solar System!
(pic fr. 4/17/09)
15th Century!
Medieval Manuscript,
Melk Abbey (Austria):
Geocentric Solar System!
(pic fr. 4/17/09)
15th Century!
BUT
– Predicted
locations of
planets, by
late 1400s
PROVED
WRONG
III. Groundbreakers
Copernicus (1473 - 1543):
On the Revolution of Heavenly Spheres
New Heliocentric Model
BUT...
-still Spherical
-contained epicycles
III. Groundbreakers
Tycho Brahe (1546-1601):
Best set of astronomical positions to date
– * Incredible observer
– naked-eye observations
– Planetary & stellar positions
good to <1’
– Duel: who is a better
mathematician????
I sez
Copernicus not
gud yet - no
cheezburgers
III. Groundbreakers
Johannes Kepler (1571-1630)
• Recognized need for
model to match Tycho’s
observations
•New laws of Planetary
Motion (1609)
What are Kepler’s 3 Laws of Planetary Motion?
Kepler’s Laws of Planetary Motion
K1: Planets orbit the Sun in an ellipse, with the Sun
at one focus of the ellipse
K2: A line connecting a planet to the Sun sweeps
out equal areas in equal time intervals
K3 (Harmonic Law): P2 a3
The squares of the sidereal periods is proportional
to the orbital semi-major axis cubed
The Physics of Motion!
Galileo (1564-1642)
Isaac Newton (1642-1747)
Principia (1687)
Newton’s Laws of Motion
N1(Law of Inertia): An object at rest will remain at rest and
an object in motion will remain in motion in a straight line at
constant speed unless acted upon by an outside force
N2: The net force on an object is proportional to the object’s
mass & acceleration
N3: For every action there is an equal and opposite reaction
Newton
What is the physical explanation for
Kepler’s Laws of planetary motion??
Derivation of Universal Law of Gravity
What is the Nature of
this Force???