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Rotation and Revolution of Earth

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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

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)

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