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“To boldly go where no man has gone before”
Introduction
Man has spent well over a thousand years investigating the sky above the earth. From
the geocentric (earth-centered solar system) model, proposed by Claudius Ptolemy
around 100 AD, to the heliocentric model (sun-centered solar system), proposed by
Nicholas Copernicus during 1400 AD, the characteristics of the solar system began to
emulate the present-day characteristics scientists observe today. By the 1600’s,
Johannes Kepler furthered defined solar system characteristics by developing three laws
of planetary motion. Kepler’s three laws of planetary motion revolutionized the concept
of planetary revolutions around the sun, planetary orbital speeds, and the understanding
of planetary distances from the sun. Typically, distances of planets from the sun are
measured using astronomical units (AU). One astronomical unit represents the sunearth distance or 93 million miles. For example, the earth is 1 AU, or Uranus is a
distance of 19.2 AU (19.2 x 93 million miles). Additionally, Kepler’s laws supported the
Copernican heliocentric model, and Kepler’s laws are used today in space travel and
predicting precise locations of various celestial bodies. By the late 1600’s to early
1700’s, Sir Isaac Newton had demonstrated the three laws of motion and how universal
gravitation governs bodies on earth as well as in our universe. Newton further showed
the consistency of universal gravitation with Kepler’s three laws of planetary motion,
thereby removing any doubts of a heliocentric-type solar system. In other words, by the
1700’s, the scientific world and societies accepted the heliocentric model. The evolution
and present-day understanding of the solar system represents several hundred years of
observations, hypotheses, and experimentation. What method does this sound like?
Objectives
 Students will learn the basic differences between terrestrial and outer planets
 Students will construct a scale model of the solar system and understand
planetary distances.
 Students will understand Kepler’s laws of planetary motion and the importance to
modern astronomy.
The Solar System
The solar system consists of a main sequence star (the sun), four terrestrial planets
(Mercury, Venus, Earth, Mars), four outer planets (Jupiter, Saturn, Uranus, Neptune),
approximately three “dwarf planets,” more than 130 satellites (orbiting moons), an
asteroid belt, and entering comets. As one ventures into space, the distances from one
planet to another are vast. In fact, the solar system is comprised of mostly empty space.
The planets are very small when compared to the sun. If one were to weigh the entire
solar system (sun + planets), the sun would represent 99.85% of the solar system mass.
From Kepler’s first law of planetary motion, the orbits of planets are elliptical with the sun
located at one focus. A side view observation of the solar system would reveal that all
the planets orbit the sun more or less along the same plane, or what scientists refer to
as the “elliptical plane.” The diagram below depicts the orbital planet configuration along
the elliptical plane.
Solar system elliptical
plane
The planets orbit the
sun along the same
plane or elliptical plane.
Planets orbit the sun along the elliptical plane counterclockwise looking downward from
above the sun’s north pole.
Pluto does not orbit along the elliptical plane like the other seven planets. In fact, Pluto’s
orbital plane is tilted approximately 18 degrees above the elliptical plane and is roughly
the size of our earth’s moon. Given the orbital characteristic and size of Pluto, the
science community recently reassigned Pluto as a dwarf planet.
The Terrestrial Planets
The terrestrial planets represent the four inner planets from the sun. The term
“terrestrial planet” defines a compact, rocky, dense planet. Terrestrial planets are similar
to the rocky characteristics of Earth. The terrestrial or inner planets from the sun are
Mercury, Venus, Earth, and Mars. The planets Venus, Earth, and Mars possess
significant types of atmospheres while the close proximity of Mercury to the sun results
in little to no atmosphere. Of the four terrestrial planets, Earth is the only planet that
contains copious amounts of water that exist in all three phases (solid, liquid, and vapor).
The Jovian Planets or Outer Planets
The Jovain, outer, or gas giants represent Jupiter, Saturn, Uranus, and Neptune.
These planets are enormous compared to the terrestrial planets. In fact, scientists
suggest that Jupiter (the largest solar system planet) represents at least 500 earths.
Jupiter and Saturn are composed almost entirely of hydrogen and helium and various
hydrogen compounds. Uranus and Neptune, however, are composed of hydrogen
compounds in addition to compounds of methane (CH4), ammonia (NH3), and water
(H20),
The Obliquity of Terrestrial and Jovian Planets
The diagram above depicts the obliquity of both terrestrial and Jovian planets.
Obliquity represents the angle between the planets equatorial plane and its orbital plane.
For example, the earth’s obliquity or angle of tilt is 23.5 degrees. By International
Astronomical Union (IAU) convention, a planet's north pole lies above the elliptic plane.
By this convention, Venus, Uranus, and Pluto have a retrograde rotation, or a rotation
that is in the opposite direction from the other planets.
The Sun and Planets to Scale
The diagram above depicts the approximate planetary sizes compared to the sun. Note
that the sun is only partially illustrated to accommodate the size of both terrestrial and
Jovian planets.
The table below lists various statistical information for the sun and planets. Note that
numbers displayed in distance, radii, and mass are comparisons to Earth. For example,
the sun’s radius is 10 times larger than that of earth. The table below will be used to
answer questions in parts B, C, and D.
Sun
Mercury
Distance
(AU)
Radius
compared
to Earth
Mass
compared
to Earth
0
109
332,800
0.39
0.38
Venus
0.72
0.95
Earth
1.0
1.0
Mars
1.5
0.53
Jupiter
Number of
Obliquity
(degrees)
Density
9
---
1.410
0.05
0
0.1
5.43
0.89
0
177.4
5.25
1.0
satellites
1
23.5
(g/cm3)
5.52
0.11
2
25.19
3.95
5.2
11
318
63
3.12
1.33
Saturn
9.5
9
95
47
26.73
0.69
Uranus
19.2
4
17
27
97.86
1.29
13
29.56
1.64
Neptune
30.1
4
17
Pluto
39.5
0.18
0.802
1
119.6
2.03
Part-A Vocabulary
geocentric model–
heliocentric model-
satellite –
elliptical plane
Terrestrial planets
Jovian planets
obliquity –
retrograde motion –
astronomical unit-
Part B – Scale model of the Solar System
1. Attach together 3-pieces of 8.5 x 11 inch paper length wise using tape.
2. On the left side of your attached 3-pieces, draw half of the sun.
3. Using the statistical chart located at the end of the text, plot a point that
represents the correct distance from the sun for each planet. The astronomical
unit distance is equal to centimeters. For example, Mercury would be .39 cm
from the sun, Venus would be .72 cm from the sun, and Earth would be 1.0 cm
from the sun.
4. Using an earth science reference book, locate a diagram that shows the size
comparison of each planet. Draw the scaled sized version along with the
accurate distance from the sun as outlined in #3.
5. Color each planet and label the following features on your scale model of the
solar system
 One astronomical unit (1AU)
 Outer planets
 Inner planets
 Name of each planet
 The location of the asteroid belt
6. How many miles from the sun are the planets? For each planetary distance
convert miles to kilometers.
1. Mercury =
2. Venus =
3. Earth =
4. Mars =
5. Jupiter =
6. Saturn =
7. Uranus =
8. Neptune =
9. Pluto (dwarf planet) =
Part C – Kepler’s laws of planetary motion.
Describe Kepler’s three laws of planetary motion (how do the planets move?) and
provide a diagram that illustrates each law.

1st Law: The Law of Ellipses

2nd Law: The Law of equal Areas

3rd Law: The Law of periods
Match the following:
_____ Solar system where the earth is in the center (planets revolve around the sun)
_____ All planets orbit in an elliptical pattern
_____ The sun centered solar system
_____ Planets orbit faster near the sun, and slower away from the sun
_____ A prediction of the time it takes a planet to orbit the sun one complete revolution
_____ The distance between the sun and the earth
_____ The outer or Jovian planets
_____ Terrestrial type planets (inner planets)
_____ Study of the universe
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
Kepler's 1st law
Geocentric
Astronomical unit
Saturn, Neptune
Astronomy
Mars, Mercury
Heliocentric
Kepler's 2nd law
Kepler's 3rd law
Retrograde motion
PART D – Critical Thinking Questions
1. Using the sun-planet statistical table at the end of the text, observe the density
column. What are the density differences between the terrestrial and Jovain
planets and why does this difference exist?
2. Which planet is the largest in the solar system and if you reduced the planets to
both baseball and basket ball sizes, which planet would float in water and why?
3. How would you explain the high occurrence of satellites with the gaseous planets
compared to the terrestrial planets?
4. Where is the asteroid belt and why do you think the asteroid belt exists?
5. Construct a time line starting at 100 AD to 1800 AD and plot the time span of the
following astronomers (Copernicus, Ptolemy, Brahe, Kepler, and Newton).
Also, briefly describe the major contribution to astronomy for each person.
6. Briefly explain how the scientific method is used throughout the last 1700 years
to decipher the current characteristics and configuration of our solar system ----Look how each astronomer contributed to the knowledge of the solar system.