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15B The Sizes of the Planets
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How big are the planets relative to Earth?
Compared to the distances between them, the planets are relatively tiny.
Although Earth seems large while standing on it, you could fit almost 12,000
Earths in the space between Earth and the sun! Earth is small compared to
some of the other planets, such as Jupiter or Saturn. But other planets are
smaller than Earth’s moon! This investigation will look at the relative sizes of
the planets.
A
Materials
• Metric tape measure
(sewing tape)
• Lacrosse ball
• Other sizes of balls
Determining scale sizes of the planets compared to the solar system
What would the planets look like in a 100 meter scale model of the solar system? For example, Mercury has
a diameter of 4,880 kilometers. How big would Mercury be in a 100-meter scale model? You can use the
same method to determine the scale diameter of Mercury that you used in the last investigation:
x
100 m
=
4,880 km 5,900,000,000 km
Cross-multiply and rearrange the variables to solve for x:
x=
100 m
× 4,880 km = 0.000083m
5,900, 000, 000 km
Based on the example above, the diameter of Mercury in a 100-meter scale solar system would be
0.000083 meters or 0.083 millimeters. This is about the thickness of a human hair! If the distance from the
sun to Pluto were 100 meters, Mercury would be smaller than the period at the end of this sentence. The
space between the planets is enormous compared to the size of the planets themselves.
B
Calculating the scaled sizes of the planets
Calculate the scaled diameters of the other planets as well as the sun and Earth’s moon. Write these values in
units of meters in the third column of Table 1.
Table 1: Diameters of the planets, Earth’s moon, and sun
Planet
Actual diameter (km)
Sun
Mercury
Venus
Earth
Moon
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto (dwarf)
1,391,980
4,880
12,100
12,800
3,475
6,800
142,000
120,000
51,800
49,500
2,300
70
Scale diameter (m)
0.000083
Scale diameter (mm)
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Investigation 15B The Sizes of the Planets
C
a.
Stop and think
How big is the sun in this model in cm?
b. How much larger is the sun’s diameter compared with Earth’s? How much larger is
Earth’s diameter compared with the moon’s?
c.
The smallest object that the human eye can see without magnification is 0.100
millimeters. Given this information, which planets would be visible to the human eye?
Would you be able to see the sun or the moon on this 100-meter scale model of the solar
system?
d. What is your impression of how the size of the planets and the sun compare with the size
of the solar system?
D
Modeling the sizes of the planets
To get a relative sense of the sizes of the planets, we
need to work at a much larger scale. Suppose Earth
were the size of a lacrosse ball. The diameter of a
lacrosse ball is 6.4 cm. The best way to measure the
diameter of a ball is to measure the circumference by
wrapping a tape measure around the widest part. The
diameter is the circumference divided by π (3.14).
Table 2: Diameters of the planets, Earth’s moon, and sun
E
a.
Planet
Actual diameter (km)
Sun
Mercury
Venus
Earth
Moon
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto (dwarf)
1,391,980
4,880
12,100
12,800
3,475
6,800
142,000
120,000
51,800
49,500
2,300
Scale diameter (cm)
6.4
Stop and think
See if you can find some spheres that are the approximate sizes of the scale diameters
from Table 2. Compare these to the size of the lacrosse ball that represented Earth.
b. Cut circles out of construction paper or cardboard to represent the size of each planet.
The circle for Earth should be 6.4 centimeters in diameter. Use the scale diameters from
Table 2 for the other planets.
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F
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Extension: Making a larger scale model of the solar system
In this part of the Investigation, you will use common objects to compare the diameters of
planets, the sun, and Earth’s moon in our solar system. For example, we could use an Earth
globe to represent the scale size of Earth. The diameter of the globe we will use is 30
centimeters.
1. If an Earth globe is used to represent the size of Earth, what would the sizes of the
sun and the other planets be? How big would the moon be? Use what you have learned
in this Investigation to calculate the scale diameters of the other planets, the moon,
and the sun. Fill in the third column of Table 3 with these values.
2. What objects could be used to represent each of the planets, the moon, and the sun?
Fill in the fourth column of Table 3 with your answers to this question.
3. Answer the questions that follow the table.
Table 3: A scale model of the solar system
Planet
Actual diameter
of planet
(km)
Sun
1,391,980
Mercury
4,880
Venus
12,100
Earth
12,800
Moon
3,475
Mars
6,800
Jupiter
142,000
Saturn
120,000
Uranus
51,800
Neptune
49,500
Pluto (dwarf)
2,300
Scale diameter
of sun or planet
(cm)
Representative object and its
diameter or length
(cm)
30 cm
Earth globe, 30 cm
a.
How many times bigger is 30 centimeters than 0.20 millimeters? These are the diameters
of Earth for the two scale models you created.
b.
Using your answer to question 5a, what would be the distance between the sun and Pluto
on this larger scale? Come up with a way to explain or model this distance.
c.
Why is it challenging to make a scale model of the solar system that includes the
distances between planets and the sun and the sizes of the planets?
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