1. 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

2. 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 = 2pr ≈ 942,000,000 km
Time to complete one orbit = 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

3. 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

4. Rotation and Revolution of Earth
Coriolis effect responsible for spiral-like currents of air around low- and high-pressure regions

5. 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

6. 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

7. 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)

8. 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

9. 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
(shape exaggerated for clarity)
Polar diameter

10. 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

11. 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

12. 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

13. 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)

14. 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)

15. 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

16. 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)

17. 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)

18. 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 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 Indian Ocean tsunami was in one of these deep subduction zones
(courtesy of nationalatlas.gov)

19. 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
(courtesy of USGS)
(Himalayas)
(Andes, Sierra Nevada)
(Japan, Indonesia)
(courtesy of USGS)

20. 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 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)

21. 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.)

22. 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!)

23. 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!)

24. Earth’s Atmosphere Structure of the atmosphere Low density region
(courtesy of USGS)
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

25. 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

26. Recent Data on Antarctic Ozone Hole
(courtesy of NOAA)

27. Recent Data on Antarctic Ozone Hole
(courtesy of NOAA)