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Chapter 08 - The Earth
CHAPTER 8
THE EARTH
CHAPTER OUTLINE AND LECTURE NOTES
1. Rotation and Revolution
The aberration of starlight is omitted from many introductory textbooks but I have included
it as the very first proof that the Earth actually moves about the Sun. Two proofs of the
Earth’s rotation are shown in Figure 8.4 (the Foucault pendulum) and Figure 8.6 (the
Coriolis effect). If you have a rotating air table, you probably can rig up a demonstration
like the one my colleagues and I used to show the Coriolis effect. We had a TV camera
mounted above the center of the air table so that it rotates with the air table (be careful that
you don’t overwind the cable from the camera). A spring and timer are used to propel a
ball bearing outward from near the center of the air table after the table is set in rotation.
The camera (in the rotating frame of reference) shows the ball bearing following a curved
path while the students (in an inertial frame) can clearly see that the ball bearing moves in a
straight line path toward the rim of the rotating air table. I have used quotation marks
around “Centrifugal Force” at the beginning of its subsection to emphasize that it is a
nonexistent force that we are led to employ if we adopt a non-inertial coordinate system.
2. Surface
Just as students (or maybe anyone else) don’t really fathom the immense distances to
astronomical bodies, they don’t have any experience with astronomical time scales either. I
have used Figure 8.10 to try to illustrate just how old the Earth is in terms of human history
and return to the difficulty of understanding astronomical time scales elsewhere in the
book, including the opening discussion of stellar evolution.
3. Interior
In describing the interior of the Earth (Figure 8.15), it might be a good idea to keep in mind
the following caution from geophysicist F. Birch: “Unwary readers should take warning
that ordinary language undergoes modification to a high pressure form when applied to the
interior of the earth.”
Some samples of equivalents are:
High-pressure form
Ordinary meaning
certain
undoubtedly
positive proof
unanswerable argument
pure iron
dubious
perhaps
vague suggestion
trivial objection
uncertain mixture of all the elements
Many of the differences between the Earth and the other terrestrial planets can be attributed
to the presence of full-blown plate tectonics (Figures 8.19, 8.20, 8.22) on Earth and either
alternative forms of crustal motion or no crustal motion on other terrestrial planets.
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4. Atmosphere
I always have the complete attention of the students when I talk about human influences on
the Earth’s atmosphere, such as an enhanced greenhouse effect (Figure 8.24).
5. Magnetosphere
There is more here about the Earth’s magnetosphere than in many textbooks partly because
so much of the exploration of the magnetosphere has been carried out by space scientists at
the University of Iowa, where I spent 30 years as a faculty member. I think that this aspect
of astronomy gets too little attention in many books, possibly because the space physics
community seldom publishes its research results in journals (such as The Astrophysical
Journal and Astronomy and Astrophysics) that most astronomers regularly read.
6.
Evolution of the Earth
KEY TERMS
aberration of starlight — The angular shift in the apparent direction of a star due to the orbital
motion of the Earth.
aerosol — Liquid droplets and solids suspended in the atmosphere of a planet or satellite.
aesthenosphere — A layer of plastic, deformable rock located in the upper mantle of a planet
directly below the lithosphere.
aurora australis — Light emitted by atoms and ions in the upper atmosphere near the south
magnetic pole. The emission occurs when atoms and ions are struck by energetic particles
from the Sun.
aurora borealis — Light emitted by atoms and ions in the upper atmosphere near the north
magnetic pole. The emission occurs when atoms and ions are struck by energetic particles
from the Sun.
basalt — An igneous rock often produced in volcanic eruptions.
bow shock — The region where the solar wind is slowed as it impinges on the Earth’s
magnetosphere.
core — The innermost region of the interior of the Earth or another planet.
Coriolis effect — The acceleration that a body experiences when it moves across the surface of
a rotating body. The acceleration results in a westward deflection of projectiles and
currents of air or water when they move toward the Earth’s equator and an eastward
deflection when they move away from the equator.
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crust — The outermost layer of the interior of a planet or satellite.
differentiation — The gravitational separation of the interior of a planet into layers according to
density. When differentiation occurs inside a molten body, the heavier materials sink to the
center and the light materials rise to the surface.
dynamo — A process in which electric currents within a rotating, convective body produce a
magnetic field.
greenhouse effect — The blocking of infrared radiation by a planet’s atmospheric gases.
Because its atmosphere blocks the outward passage of infrared radiation emitted by the
ground and lower atmosphere, the planet cannot cool itself effectively and becomes hotter
than it would be without an atmosphere.
igneous rock — A rock formed by solidification of molten material.
ionosphere — The lower part of the thermosphere of a planet in which many atoms have been
ionized by ultraviolet solar photons.
lava — Molten rock at the surface of a planet or satellite.
lithosphere — The rigid outer layer, composed of the crust and upper mantle, of a planet or
satellite.
magma — Molten rock within a planet or satellite.
magnetopause — The outer boundary of the magnetosphere of a planet.
magnetosphere — The outermost part of the atmosphere of a planet, within which a very thin
plasma is dominated by the planet’s magnetic field.
magnetotail — The part of the magnetosphere of a planet stretched behind the planet by the
force of the solar wind.
mantle — The part of a planet lying between its crust and its core.
mesopause — The upper boundary of the mesosphere layer of the atmosphere of a planet.
mesosphere — The layer of a planet’s atmosphere above the stratosphere. The
mesosphere is heated by absorbing solar radiation.
metamorphic rock — A rock that has been altered by heat and pressure.
mineral — A solid chemical compound.
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oblate — A departure from the spherical shape of a body in which the body’s polar diameter is
smaller than its equatorial diameter.
outgassing — The release of gas from the interior of a planet or satellite.
ozone — A molecule consisting of three oxygen atoms. Ozone molecules are responsible for
the absorption of solar ultraviolet radiation in the Earth’s atmosphere.
plate tectonics — The hypothesis that the features of the Earth’s crust, such as mountains and
trenches, are caused by the slow movement of crustal plates.
plate — A section of the Earth’s lithosphere pushed about by convective currents within the
mantle.
primeval atmosphere — The original atmosphere of a planet.
rock — A solid aggregation of grains of one or more minerals.
sea floor spreading — The splitting of the oceanic crust where magma forces the existing crust
apart, creating new ocean floor.
secondary atmosphere — The atmosphere that forms after a planet has lost any original
atmosphere it had.
sedimentary rock — A rock formed by the accumulation of small mineral grains carried by
wind, water, or ice to the spot where they were deposited.
seismic wave — Waves that travel through the interior of a planet or satellite and are produced
by earthquakes or their equivalent.
seismometer — A sensitive device used to measure the strengths and arrival times of seismic
waves.
silicate — A mineral whose crystalline structure is dominated by silicon and oxygen atoms.
solar wind — The hot plasma that flows outward from the Sun.
stratosphere — The region of the atmosphere of a planet immediately above the troposphere.
subduction — The process through which lithospheric plates of a planet or satellite are forced
downward into the mantle.
thermosphere — The layer of the atmosphere of a planet lying above the mesosphere. The
lower thermosphere is the ionosphere. The upper thermosphere is the exosphere.
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transform fault — The boundary between two of the Earth’s crustal plates that are sliding past
each other.
tropopause — The upper boundary of the troposphere of the atmosphere of a planet.
troposphere — The lowest layer of the atmosphere of a planet, within which convection
produces weather.
Van Allen belts — Two doughnut-shaped regions in the Earth’s magnetosphere within which
many energetic ions and electrons are trapped.
zone of convergence — According to plate tectonics, a plate boundary at which the crustal
plates of a planet are moving toward one another. Crust is destroyed in zones of
convergence.
zone of divergence — According to plate tectonics, a plate boundary at which the crustal plates
of a planet are moving away from one another. Crust is created in zones of divergence
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
1. When the Earth is moving directly toward or away from the star, the telescope would be
pointed directly at the star. At other times, the telescope would have to point slightly ahead
of the direction to the star. The amount of tilt would be maximum when the Earth was
moving at right angles to the direction to the star.
2. The shift due to aberration would be twice as large.
3. The circulation is clockwise because air flowing in from the south is deflected westward
and air flowing in from the north is deflected eastward.
4. If there were no ozone, there would be no concentration of absorption of ultraviolet
radiation in the mesosphere and no local temperature maximum in the mesosphere.
5. Observers in Florida are farther from the north magnetic pole.
Problems
1. 199 pounds, the person’s mass would be the same
2. 16 days ago, 4 seconds before midnight on December 31
3. Into the ocean floor, about 0.15%
4. 33 K
5. 16%
6. 5 cm/yr
7. About 4000 km, 200 million years ago
8. 39 K cooler
9. The amount emitted by the planet would double (so that the same amount as before would
escape into space). Temperature would become 1.2 times as high.
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Figure-Based Questions
1. 5500 kg/m3, 11,500 kg/m3, 1.6 and 3.3 times as large
2. Zone of divergence, transform fault, zone of convergence
3. 280 K, 0.9 or 0.003 or 0.0003 or less than 0.00001 atmospheres
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