Energy and Human Ambitions on a Finite Planet, 2021a

4 Space Colonization 55
Figure 4.1: Earth <strong>and</strong> Moon (far right) to scale. On this scale, the sun would be larger than the page <strong>and</strong> about 400 pages away. Mars would
be 160 to 1,100 pages away. Since 1972, humans have not traveled beyond the black outline of the earth in this figure (600 km).
Sun
Earth (invisible)
Figure 4.2: Proving the point that textbooks are not conducive to correctly-scaled graphics of objects in space, by the time the Earth-Sun
distance spans the page, Earth (on far right) is too small to be visible in print, at less than 1% the diameter of the orange sun at far left. The
Earth–Moon distance is about the width of the arrow shaft pointing to Earth. <strong>Human</strong>s have never traveled more than the arrow shaft’s
width from Earth, <strong>and</strong> have not even gone 0.2% that far in about 50 years! Mars, on average, is farther from Earth than is the sun.
Let us first lay out some basic ratios that can help build suitable mental
models at whatever scale we choose.
Definition 4.1.1 Scale models of the universe can be built based on these
approximate relations, some of which appear in Table 4.1 <strong>and</strong> Table 4.2:
Table 4.1: Progression of scale factors.
Step
Factor
Earth diameter (start)
Moon distance 30×
Sun distance 400×
Neptune distance 30×
Nearest Star 9,000×
Milky Way Center 6,000×
Andromeda Galaxy 100×
Universe Edge 6,000×
1. The moon’s diameter is one-quarter that of Earth, <strong>and</strong> located 30
earth-diameters (60 Earth-radii) away from Earth, on average (see
Figure 4.1).
2. The sun’s diameter is about 100 times that of Earth, <strong>and</strong> 400 times as
far as the moon from Earth (see Figure 4.2).
3. Mars’ diameter is about half that of Earth, <strong>and</strong> the distance from Earth
ranges from 0.4 to 2.7 times the Earth–Sun distance.
4. Jupiter’s diameter is about 10 times larger than Earth’s <strong>and</strong> 10 times
smaller than the sun’s; it is about 5 times farther from the sun than is
the earth.
5. Neptune orbits the sun 30 times farther than does Earth.
2: The Oort cloud marks the outer influence
6. The Oort cloud 2 of comets ranges from about 2,000 to 100,000 times
of the sun, gravitationally.
the Earth–Sun distance from the sun.
3: . . . Proxima Centauri
7. The nearest star 3 is 4.2 light years from us, compared to 500 lightseconds
from Earth to the sun—a ratio of 270,000.
A light year is the distance light travels in a
year.
8. The Milky Way galaxy has its center about 25,000 light years away 4: That’s 6,000 times the distance to the
away, 4 <strong>and</strong> is a disk about four times that size in diameter.
closest star.
9. The next large galaxy 5 is 2.5 million light years away, or about 25 5: ...theAndromeda galaxy
Milky Way diameters away.
6: The “edge” is limited by light travel
10. The edge of the visible universe 6 is 13.8 billion light years away, or
time since the Big Bang (13.8 billion years
about 6,000 times the distance to the Andromeda galaxy.
ago), <strong>and</strong> is called our cosmic horizon. See
Sec. D.1 (p. 392) for more.
We will construct a model using the set of scale relations in Definition
4.1.1, starting local on a comfortable scale.
We’ll make Earth the size of a grain of s<strong>and</strong> (about 1 mm diameter).
The moon is a smaller speck (dust?) <strong>and</strong> the diameter of its orbit would
span the separation of your eyes. On this scale, the sun is 100 mm in
diameter (a grapefruit) <strong>and</strong> about 12 meters away (40 feet). Mars could
As we build up our model, pause on
each step to lock in a sense of the
model: visualize it or even recreate
it using objects around you!
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.