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:

– Radius of Earth’s orbit = 1 AU ≈ 150,000,000 km– Circumference of Earth’s orbit = 2r ≈ 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

– 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

• As the distance to a star increases, stellar parallax decreases

(courtesy of Ohio State University)

(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:– 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– 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:

– Solid inner core (5100 – 6370 km deep)• 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

(courtesy of USGS)

Earth’s Interior• Cutaway view of the Earth’s interior:

– 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

(courtesy of USGS)

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

• Seismic waves are used to map the interior much like ultrasound or MRI is used to map interiors of people

(courtesy of Ohio State University)

Plate Tectonics• Earth’s crust is broken into 16 rigid plates

– 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)

(courtesy of USGS)

Plate Tectonics• Types of plate motion

– Lateral sliding between 2 plates at transform boundaries

• 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 (c

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Plate Tectonics• Types of plate convergences:

• 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

(Himalayas) (Andes, Sierra Nevada) (Japan, Indonesia)

(courtesy of USGS)

(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– Mostly H2O and CO2

– Small amounts of sulfates and N2

– No oxygen (O2)– Very different from today’s atmosphere

• Composition of the atmosphere today:– 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

• 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

(courtesy of USGS)

Weather layer

Heated by UV absorption Primarily in the ozone layer

Cooler intermediate region

Heated by x-ray and UV radiation

Low density region

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)