Download Rotation and Revolution of Earth

Rotation and Revolution of Earth
• Legend has it that Galileo muttered the words
“Eppur si muove” (It still moves) under his breath
while being tried for heresy during the Inquisition
– While probably false, this touches on principle objection to
heliocentric model
– How do you prove that Earth rotates about its axis and
rotates about the Sun?
• One of the main conceptual barriers was the large
speeds required
• Speed of rotation about Earth’s axis at the Equator:
– Circumference of Earth at Equator ≈ 40,000 km
– Time to complete one rotation = 24 hours
– Speed of rotation = Distance / Time = 40,000 km / 24 h
1670 km/h = 1037 mph
=
Rotation and Revolution of Earth
• Speed of revolution around the Sun:
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Radius of Earth’s orbit = 1 AU ≈ 150,000,000 km
Circumference of Earth’s orbit = 2pr ≈ 942,000,000 km
Time to complete one orbit = 365.2422 days = 8766 hours
Speed of revolution = Distance / Time
= 942,000,000 km / 8766 h = 107,000 km/h = 30 km/s =
18.6 miles/s
– In about 15 seconds, Earth moves through space a
distance about the width of Ohio
• As you move north or south of the Equator toward
the poles:
– East–west parallel of constant latitude narrows
– Distance covered in 24 hours is less, so speed is less
– In Delaware (40°N) speed of axial rotation is 1280 km/h
Rotation and Revolution of Earth
• Evidence for a rotating Earth: the Coriolis Effect
– Gives appearance of a force, although force is fictitious
– Causes deflection of projectile paths
• Fire a cannonball due north from a cannon at the Equator
• The cannon is moving east with the Earth’s rotation at a speed of
1670 km/h
• The cannonball retains its initial, faster, eastward speed as it flies
north (Newton’s 1st Law)
• The further north it flies, the slower the eastward motion of the
Earth’s surface beneath its flight
• Result is a slight eastward deflection of the projectile from its
original northward path
– Same eastward deflection occurs if you fire the projectile
toward the south
– Projectiles deflect toward the right (left) in Northern
(Southern) Hemisphere
Rotation and Revolution of Earth
• Coriolis effect responsible for spiral-like currents of
air around low- and high-pressure regions
Rotation and Revolution of Earth
• Evidence for a rotating Earth: the Foucault Pendulum
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67–m long pendulum with a 25–kg weight
Built by Jean Foucault in 1851
Hung from the dome of the Pantheon in Paris
Ball joint allowed pendulum to swing freely in all directions
• Direction of the swing appears to change over time
– Think of a Foucault Pendulum hung at the North Pole
• Here the pendulum’s swing rotates once every 24 hours
– At middle latitudes, the pendulum takes longer than 24
hours to complete one revolution
• In Delaware, it takes about 37 hours
– At the Equator, the pendulum never changes the direction
of its swing
– If the Earth were not rotating, the pendulum would never
change the direction of its swing at any latitude above the
Equator
Rotation and Revolution of Earth
• Evidence for a revolving Earth around the Sun:
aberration of starlight
• Aberration is the apparent change in the position of a
star whenever the Earth’s motion carries it in any
direction except directly toward or away from the star
– Analogous to tilting an umbrella when moving during a
rainstorm
– Tilt of telescope would be much smaller (about 20 seconds
of arc)
– Tilt is in the direction the Earth is moving
• Discovery of aberration was the first direct evidence
for the revolution of the Earth about the Sun
– James Bradley, 1728
Rotation and Revolution of Earth
• Evidence for a revolving Earth around the Sun:
stellar parallax
(courtesy of Ohio State University)
• As the distance to a star increases, stellar parallax
decreases
(courtesy of Ohio State University)
Rotation and Revolution of Earth
• Copernicus and heliocentric supporters were right:
stellar parallaxes were not easily observed because
stars are much more distant than was expected
– All stellar parallaxes are less than 1 arcsecond
– The nearest star with the largest parallax is Alpha Centauri
(0.76 arcsec)
– Such small angles cannot be measured with naked eye
• First stellar parallax was observed in 1837 for star 61
Cygni
– Used a telescope to make the measurements
– Measured a parallax of about 0.3 arcsec
– Corresponds to a distance of ~ 10 light years for 61 Cygni
• Modern parallax measurements use photography or
digital imaging techniques
– Upcoming space missions will have resolution of 10–6 arcsec
Consequences of Rotation
• We experience effects on Earth’s surface similar to a
person riding in a car accelerating around a corner
– Due to the rotational inertia of the Earth
– Sometimes referred to as the fictitious “centrifugal force”
• Objects weigh less than they would if the Earth
weren’t rotating
• Earth is slightly deformed due to its rotation
– About 0.3% (40 km) larger along the Equator than along its
polar diameter
Equator
Polar diameter
(shape
exaggerated for
clarity)
Earth’s Surface
• The Earth’s surface is:
– 71% oceans
– 29% continents
• Much of the Earth’s crust is made up of minerals
– A mineral is a solid chemical compound
– Most common minerals are silicates (oxygen–silicon
compounds)
• Examples are basalts, granites, and quartzes
– Oxides, carbonates, and sulfides are also common
– The structure of most minerals consists of crystals
(regular arrangements of atoms in 3–D lattices)
• A rock is a solid combination of one or more
minerals
Earth’s Surface
• Rocks can be classified according to their origin
– Igneous rocks are formed from molten material resulting
from volcanic eruptions (e.g. basalt)
– Sedimentary rocks are formed by the deposition and
hardening of layers of silt and debris in lakes and oceans
(e.g. limestone, sandstone, shale)
– Metamorphic rocks are altered and shaped by heat and
pressure beneath the surface (e.g. marble)
– Rocks can be modified and converted from one type to
another in various geological processes
• Surface layers have been subjected to:
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Water and wind erosion
Volcanic repaving
Downward movement of crust into the mantle (subduction)
Upward movement of crust forming mountains
Earth’s Interior
• The Earth’s interior is hot and dense
– Weight of upper layers exerts high pressure on the interior
– Extreme pressure leads to extreme heating
• Process of differentiation led to this
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Began with a molten mix of metals and minerals
Heavier metals (iron and nickel) sank to the center
Lighter minerals (silicates) floated to the surface
Important process for all of the terrestrial planets
• Differentiation led to a layered structure of the
Earth’s interior
– Similar to other terrestrial planets and satellites in the solar
system
Earth’s Interior
• Cutaway view of the Earth’s interior:
(courtesy of USGS)
– Solid inner core (5100 – 6370 km deep)
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Solid iron and nickel at 7000 K
Kept solid by high pressure
2% of the mass of the Earth
Suspended in the middle of the molten outer core
– Molten outer core (2900 – 5100 km deep)
• Molten iron and nickel + dissolved sulfur and oxygen
• 30% of the mass of the Earth
Earth’s Interior
• Cutaway view of the Earth’s interior:
(courtesy of USGS)
– Mantle (100 – 2900 km deep)
• Central portion called the asthenosphere of softy, mushy silicate rock
(slow-flowing motions occur)
• Upper portion (+ crust) called the lithosphere (semi-rigid zone)
• About 67% of the mass of the Earth
– Crust (about 100 km thick, but varies greatly)
• Solid, relatively low density compared to rest of interior
• Only about 1% of the mass of the Earth
Earth’s Magnetic Field
• Convection currents get set up in the molten outer
core because of a temperature difference between
top and bottom
– Similar to convection currents that flow while
heating a lab beaker filled with water
– Inner core is hotter than the outer core
• Flowing charged iron ions produce an
electrical current  current produces a
magnetic field
– Magnetic field extends
into space forming the
magnetosphere
– Magnetosphere
deflects and traps
charged particles from
space
Seismology
• How do we learn about the Earth’s interior features?
– They are probed by using seismic waves (vibrations caused
by earthquakes) that travel through the interior
• Three different kinds of seismic waves
– P waves: compression (pressure) waves
• Waves involving vibrations along the direction of travel
• Can travel through both solid and liquid zones
– S waves: transverse (shearing) waves
• Waves involving vibrations perpendicular
to the direction of travel
• Can pass through solid but not liquid zones
(liquids cannot be sheared since they flow
instead)
– Rolling transverse surface waves
(courtesy of Ohio State University)
• Seismic waves are used to map the interior much like
ultrasound or MRI is used to map interiors of people
Plate Tectonics
• Earth’s crust is broken into 16 rigid plates
(courtesy of USGS)
– Thin oceanic plates about 10 km thick
– Thick continental plates up to 50 km thick
• These plates float on the mantle above
a complex transition zone
– Region where basaltic lavas form
– Lubricates the bottoms of the crustal plates, allowing the
plates to slide
• Plate tectonics describes the
changing, dynamic structure of the
plates
– Plate motion averages about a few
cm/year
– Motion is driven by convection
currents in the mantle
(courtesy of USGS)
• Types of plate motion
– Lateral sliding between 2 plates at
transform boundaries
(courtesy of nationalatlas.gov)
Plate Tectonics
• Boundaries form transverse faults
• Example: San Andreas Fault between the
North American and Pacific plates
• Plates can stick at the boundary, building up
strain
• When the strain gives, the crust jumps many meters
• Source of strong near-surface earthquakes (e.g. 1906 quake that
leveled San Francisco)
– Two plates collide together at a convergent boundary
• Subduction occurs when one plate dives beneath another (sites of
deep, powerful earthquakes and volcanoes)
• The powerful earthquake that triggered the Dec. 2004 Indian
Ocean tsunami was in one of these deep subduction zones
Plate Tectonics
• Types of plate convergences: (courtesy of USGS)
(Himalayas)
(Andes, Sierra Nevada)
(Japan, Indonesia)
• Two plates move apart at a divergent boundary
– Magma wells up from below, filling the
gap and building new crust
– Older crust is dragged away from the
boundary
– Mid-Atlantic ridge
• Boundary of North American and Eurasian
plates
• Rocks older further from ridge
• Splitting Iceland into 2 parts
(courtesy of USGS)
Plate Tectonics
• Continental drift
– Movement of segments of the continental crust over long
periods of time
– Today’s continents are thought to
have once all belonged to the
supercontinent Pangea which
began breaking apart about 200
million years ago
• Moral of the story: Earth is a
dynamic, actively evolving planet
– It’s still active today because the interior
is still hot and molten
– Other terrestrial planets are less active
because they cooled down quicker after
formation (due to their smaller size)
(courtesy of USGS)
Earth’s Atmosphere
• After losing much of its original H and He, the
primordial atmosphere of Earth was built up by
outgassing of the crust by volcanoes
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Mostly H2O and CO2
Small amounts of sulfates and N2
No oxygen (O2)
Very different from today’s atmosphere
• Composition of the atmosphere today:
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77% N2 (molecular nitrogen)
21% O2 (molecular oxygen)
1% H2O (water vapor)
0.93% Ar (argon)
0.035% CO2 (carbon dioxide)
Traces of CH4 (methane), Inert gases (Ne, He, Kr, Xe)
Particulates (silicate dust, sea salt, etc.)
Earth’s Atmosphere
• Where did all the H2O and CO2 go?
– H2O vapor cooled and condensed to form liquid (oceans)
– CO2 dissolved into the oceans and precipitated out as
carbonates (e.g., limestone)
– Most of the present-day CO2 is contained in crustal rocks
and dissolved in the oceans
– N2 is inactive chemically, so it stayed in the atmosphere
(now the largest constituent)
• Where did O2 come from?
– Primarily from photosynthesis in plants and algae
– O2 content has increased by 20% over the past 20 My
• Ozone (O3)
– Forms in stratosphere from O2 interacting with UV radiation
from the Sun
– Blocks some UV rays from reaching the surface (pro life!)
Earth’s Atmosphere
• Why is the Earth as warm as it is?
– Temperature can be estimated by assuming Earth absorbs
and emits radiation as a blackbody
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Energy absorbed by surface from sunlight
Infrared radiation emitted into space by heated surface
Calculated temperature would then be about 248 K
Water freezes at 273 K  there would be no liquid water on Earth
– Molecules of H2O, CO2, CH4, and others absorb infrared
radiation from the Sun and that emitted from the surface
• This increases the temperature of the lower atmosphere as well as
the surface
• This process is called the greenhouse effect (not the same as
global warming)
• The greenhouse effect is responsible for making the Earth about 35
K warmer than it would be without an atmosphere (a good thing!)
Earth’s Atmosphere
• Structure of the
atmosphere
Low density region
Heated by x-ray and UV
radiation
Cooler intermediate region
Heated by UV absorption
Primarily in the ozone layer
Weather layer
(courtesy of USGS)
Human Impact on Earth’s Atmosphere
• Impacts on Earth’s atmosphere due to human
activity:
– Increasing amounts of greenhouse gas emissions
• Increased CO2 from burning fossil fuels
• Leading to global warming (more infrared radiation absorbed by
atmosphere)
• 0.60C global temperature increase over the past century probably
due to increase in greenhouse gasses in atmosphere
– Ozone layer destruction by industrial emission of
chlorofluorocarbons (CFCs)
• Refrigerants (freon) and aerosol propellants (spray cans)
• CFCs are also increasing the greenhouse effect
– Antarctic ozone hole
• Thinning of Antarctic ozone by 50% since the late 1970s
• NASA satellite monitoring over the past 20+ years
– Northern “ozone hole” over arctic
Recent Data on Antarctic Ozone Hole
(courtesy of NOAA)
Recent Data on Antarctic Ozone Hole
(courtesy of NOAA)