CHAPTER 2: EARTH IN SPACE
SUMMARY
This chapter discusses how our understanding of Earth in space serves as a good example
of how scientific ideas change with time. Early astronomers believed in a geocentric view of
Earth’s position in the solar system (in this view, the Earth lies at the center of our Solar System
and the Sun and other planets revolve around the Earth). After nearly two thousand years,
scientists were able to analyze sufficient data from observations to propose the heliocentric view
of the solar system (in this view, the correct view, the Earth and other planets revolve around the
Sun). More recent evidence is interpreted to suggest that we are just a tiny speck in an expanding
universe that sprang into being with the Big Bang 10-20 billion years ago. Some of the debris
from the Big Bang came together to form galaxies, stars, and planets. A discussion of the
characteristics of the life cycle of stars follows. The formation and characteristics of terrestrial
and Jovian planets is covered in detail, with emphasis on our solar system.
The chapter continues with discussion why Earth is unique among the known planets.
The well-developed biosphere is a consequence of having plenty of water, an oxygen-rich
atmosphere, and a magnetic field that shields us from damaging solar radiation. Incoming solar
radiation powers activities in the hydrosphere, atmosphere, and biosphere. Earth’s position in
space provides us with just enough solar energy to sustain life. Annual variations in solar
radiation are represented by the seasons and are a consequence of the tilt of Earth’s axis relative
to the Sun.
Figure 2.3 (in book and shown above) shows the phases of Venus in the Heliocentric and
Geocentric concepts. The phases of Venus during 2004 are also shown under diagram (a.) above
the two models. The size and appearance of Venus shown under diagram (a.) above cannot be
explained by the geocentric model (where Venus lies between the Earth and the Sun, in which
case the phases would be crescents). In the heliocentric model, Venus is either in front of or
behind the Sun relative to Earth. This is what Galileo observed (and what we observe today).
2.2 Origin of the Universe (page 28): the Universe is billions of years old (our best scientific
estimate today is about 14 billion years old). The age and size of the Universe are based on
such observations as the luminosity of stars and the change in frequency of light of stars (the
Doppler Effect). Repeated measurments of pulsating stars (called cephied variables) shows the
Universe is expanding.
Page 29 shows a diagram of the electromagnetic spectrum with visible light broken into
segments of the colors we see (like the colors of a rainbow). Notice the wavelengths of light
increase as you look from the blue to red end of visible light. If an object in space shows a shift
in color from the blue end (shorter wavelenght) to the red end of the spectrum, this is called a
red shift, and means the object is moving away from Earth (the frequency of light is attenuated
relative to an observer on Earth-the Doppler Effect-as shown in Figure 2.4 c.).
2.3 Stars and Planets (page 31): Nuclear fusion creates stars. Stars can be various sizes and go
through various stages. Our Sun is an ordinary star (see Hertsprung-Russell figure 2.9 and note
where the Sun plots on the diagram) powered by hydrogen fusion. The lifecycle of our Sun is
described in this section; you will notice that larger stars have shorter lifespans than our Sun,
which is approximately halfway through its life at 5 billion years old. Notice on figure 2.9 that
luminosity increases up the diagram, and temperature increases from right to left across the
diagram. In general, are white dwarfs hotter or cooler than our Sun?
2.4 Our Solar System (page 34): Eight planets (Pluto not a planet anymore, but in its own class
termed a dwarf planet). Notice that distances in our Solar System are measured using the AU
(astronomical unit, where 1 AU=the average distance from the Sun to Earth). How are
distances measured in the Universe? (see light-year in previous section of this chapter). Note
table 2.1 outlines characteristics of the 8 planets. When you work on Checkpoint 2.14 make sure
that each generalization you make includes the 8 planets (for example, don't state that Earth's
orbital period is longer than Venus' and leave it at that; generalize about ALL of the planets in
each statement you make (the book gives an example: "planets closer to the Sun are smaller
than those farther from the Sun"; please don't use this!!).
2.5 Earth, The Sun and Seasons (page 42): what causes the seasons! Read on this to
understand the factors that contribute to producing our seasons. It is NOT how close we are to
the Sun (the Earth is closer to the Sun during our winter season in the northern hemisphere). The
Earth is tilted on its geographic axis at 23.5 degrees, and as the Earth orbits the Sun, the most
direct solar radiation (insolation) is received by Earth at the Equator throughout the year. In fact,
the equatorial area receives 2.5 times more incoming solar radiation than the poles (i.e., the
Sun's radiation is most direct at the equator because it is overhead for much of the year
and radiation takes a shorter path through the atmosphere)...the Sun's energy is diluted over
a larger area as it reaches the poles (see figure 2.23), so it is less effective in heating the Earth's
surface. It is interesting that at the north pole the Sun rises above the horizon during the spring
equinox and does not set until 6 months later (fall equinox)!! But again, the Sun's insolation is
less direct, travels farther and is less effective in heating the Earth's surface at the poles.
When viewing Figure 2.19 notice that during Summer Solstice in the northern hemisphere (June
21) the Earth is tilted directly toward the Sun (so the most direct sunlight strikes Earth at the
Tropic of Cancer, 23.5 degrees north of the equator); 3 months later during Fall Equinox
(September 22) the Earth is tilted neither away or toward the Sun (and the most direct sunlight
on this day strikes the equator); 3 months later during Winter Solstice (December 21) the Earth is
tilted at its maximum away from the Sun (in the northern hemisphere) and the most direct
sunlight now hits the Tropic of Capricorn (23.5 degrees south of the equator); and, 3 months later
during Spring Equinox (March 20) the Earth is tilted neither toward or away from the Sun and
the most direct sunlight on this day strikes the equator.
Keep in mind, everything is opposite in the southern hemisphere (for example winter solstice in
the northern hemisphere occurs during summer solstice in the southern hemisphere where the
Earth is tilted directly toward the Sun! See figure 2.19 b again.).
for more on the seasons, please click the link below from the National Oceanic and
Atomospheric Administration (NOAA):
www.wrh.noaa.gov/fgz/science/season.php
2.6 The Unique Composition of Earth (page 44): The Earth's interior can be separated into
three layers of differing composition: the crust, mantle and core. The core is composed mostly
of iron and nickle and is divided into the solid inner core and liquid outer core (the liquid core is
where the Earth's magnetic field is generated). Rocks in the mantle are denser than rocks in the
crust and contain more magnesium and heavier elements.
The asthenosphere and lithosphere are sections of the Earth's mantle and crust, and are defined
by their mechanical strength (i.e., brittle and ductile). The asthenosphere is a section of the
mantle from about 100-300 km beneath the Earth's surface where the rocks slowly flow (solid
but the flow, know as ductile behavior). This is the layer of the Earth that the tectonic plates
move on (more on this in the plate tectonics chapter). The lithosphere is a section of the upperrigid mantle and the crust (brittle behavior). It lies above the asthenosphere and it the section of
the Earth that is broken up into the plates. If the lithosphere is composed of oceanic crust (plus
the upper rigid mantle), it is denser than lithosphere composed of continental crust (plus the
upper rigid mantle)...more on this in chapter 4.
For more on the Earth's layers click on the link below from the United States Geological Survey:
pubs.usgs.gov/gip/dynamic/inside.html
When working on Checkpoint 2.25 you can find hints in this section also (for example, if the
Earth were smaller, its atmosphere would be...thicker or thinner? See bold writing on page 48).
LEARNING OBJECTIVES
1. Students will explain concepts related to Earth in space.
2. Students will compare and contrast the characteristics that are present in geocentric and
heliocentric models of the solar system.
3. Students will describe the Big Bang theory and key developments in the history of the
universe.
4. Students will describe components of the solar system and features of the Sun.
5. Students will analyze the similarities and differences between terrestrial and Jovian planets.
6. Students will explain how insolation varies with the seasons.
7. Students will describe the three main boundaries in Earth.
8. Students will explain why the Earth system allows life to flourish.
9. Students will give examples of how scientific ideas change with time.