Planetary Motion History

Chapter 28 – The Solar

System

​Nebular Theory

nebula consisted of hydrogen, helium, and
microscopic dust grains.
•solar nebula slowly contracted and began to
rotate.
•flat, disk shape with the protosun (pre-Sun) at the
center.
•Inner planets began to form from metallic and
rocky substances.
•Larger outer planets began forming from
fragments of ices (H2O, CO2, and others).

Open Ended

In your own words and based on what we have just discussed, explain why the planets orbit the sun in the same ecliptic plain.

Retrograde Motion?

Ptolemy & Aristotle

28.1 Models of the Solar System

•Aristotle (350 B.C.) - believed in a
geocentric solar system
•Ptolemy (200 A.D.) - Stated that
all planets have two motions.

✔1. Revolution around earth
✔2. Epicycles - little circles, like

links in a chain
•These two motions helped
explain retrograde motion, or the
apparent backward motion of the
planets.

Multiple Choice

What did Ptolemy's idea of "epicycles" try to support?

• A geocentric solar system model.

• A heliocentric solar system model.

• A flat Earth model.

Nicolaus Copernicus

The Copernican Model

• Nicolaus Copernicus (1500) Polish Cleric &

Astronomer

• 1. Proposed heliocentric, sun centered, solar system

idea.

• 2. Planets orbit in same direction around sun.
• 3. Each planet moves at a different speed. This

explained retrograde motion.

• Galileo – uses telescope observes moons orbiting

Jupiter.

Multiple Choice

Which of the following is not one of Copernicus' seven axiums?

• The planets orbit the sun at the same speed.

• The planets orbit the sun in the same direction.

• The planets are at different distances from the sun.

True reason for Retrograde Motion

•Mars retrogrades for 72 days every
25.6 months.
•Jupiter for 121 days every 13.1
months.
•Saturn for 138 days every 12.4
months.
•Uranus for 151 days every 12.15
months and
•Neptune for 158 days every 12.07
months.

Kepler’s Laws

•Kepler (German) - used observations
of Dutch astronomer Tycho Brahe
•Three Laws of Planetary Motion
•1. Law of Ellipses
•- all planets have elliptical orbits
•- ellipses have 2 foci (focus)
•- perihelion = closest to sun
•- aphelion = farthest from sun
•Most planets orbits are nearly a circle
•astronomical unit = the average
distance between the sun and earth
(149.5 million Km.)

Dropdown

According to Kepler's first law of planetary motion, planets have ​

Select answer

orbits. ​

Select answer

is when a planet is closest to the sun and ​

Select answer

is when a planet is farthest from the sun.

2. Kepler’s Law of Equal Areas

•Earth moves faster
during perihelion
slower during aphelion
•Within the elliptical
orbit of earth:

- area of sector with

fast movement = area
of sector with slow
movement

Drag and Drop

Kepler's second law of equal areas states that during ​

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planets move slower and during ​

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planets move faster when as they orbit.

Drag these tiles and drop them in the correct blank above

aphelion

perihelion

3. Kepler’s Law of Periods

•Orbit period of a planet = 1 revolution
around sun
•Orbital period squared = distance from
sun cubed.

•P2=a3

•Newton applied Kepler’s laws to explain

•Inertia - objects want to stay at
present state of motion
•Gravity - force that changes the inertia
of planets giving them elliptical orbits.

PERIOD
SQUARED
EQUALS
DISTANCE
CUBED

*this graph shows a direct relationship*

Draw

Draw a simple graph show an exponential relationship that would be a correct way of showing Keppler's Third Law. Just label one axis "period" and the other "area"