The Motion of Planets

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The Motion of Planets

• History of Astronomy

Theory? Hypothesis? Law? Scientific Method?

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How is science done?

• Observations
• Experiments
• Explanations
• Theories
• Laws
• Repeat

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Assumptions of Early Models

• Geocentric - Earth in the middle
• Everything orbits the Earth
• Stars are located on the Celestial Sphere
• Everything moves in uniform circular motions

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Epicycle
Mars
Equant
Claudius Ptolemy (87-165)
Earth
Deferent
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The Dark Ages
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Nicolaus Copernicus (1473-1543)
Errors building up Must be a better way! Lets
try a Heliocentric system! Not any better though
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Tycho Brahe (1546-1601)
Comet beyond the Moon Supernova far
away Naked eye observations of planets Accuracy
through repetition Best observations of planetary
positions Hired nerd to help calculate model
Died.
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Johannes Kepler (1571-1630)
Worked for Brahe Took data after his death Spent
years figuring out the motions of the
planets Came up with
Three Laws of Planetary Motion
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1. Planets move in elliptical orbits with the Sun
at one foci
Sun
Perihelion
Aphelion
Foci (sing. Focus)
Average distance from the Sun 1 Astronomical
Unit (1 A.U.)
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2. Planets move faster at perihelion than at
aphelion.
1 Month
1 Month
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3. Period is related to average distance
P period of the orbit a average distance P2
k a3 Longer orbits - greater average
distance Need the value of k to use the formula k
depends upon the situation Can be used for
anything orbiting anything else
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Special version of Keplers third Law If the
object is orbiting the Sun P measured in years,
a measured in A. U., then. P2 a3
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Galileo Galilei (1564-1642)
Knew of Copernicuss Keplers work Used a
telescope to look at the sky What did he see?
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The Moon was an imperfect object
Venus has phases
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Jupiter has objects around it
Saturn is imperfect
The Sun is imperfect
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Isaac Newton (1642-1727)
The ultimate nerd Able to explain Keplers
laws Had to start with the basics - The Three
Laws of Motion
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1. Law of Inertia - Objects do whatever they are
currently doing unless something messes around
with them.
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2. Force defined Fma Fforce mmass aacceleratio
n (change in motion)
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3. For every action there is an equal and
opposite reaction.
The three laws of motion form the basis for the
most important law of all (astronomically
speaking) Newtons Universal Law of Gravitation
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Fforce of gravity Gconstant M1, M2
masses Rdistance from centers Gravity is the
most important force in the Universe
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Planetary Configurations Superior Planets (beyond
Earths orbit)
Eastern Quadrature
Conjunction
Earth
Opposition
Western Quadrature
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Inferior Planets (inside Earths orbit)
Greatest Eastern Elongation
Superior Conjunction
Earth
Inferior Conjunction
Greatest Western Elongation