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6-2 thru 6-3 The EarthMoon System © 2007 Jones and Bartlett Publishers Courtesy of NASA/JPL/Northwestern University Chapter 6 6-2 The Tides 1. The Moon exerts a gravitational force on each individual part of the Earth. This tidal force varies in strength and direction over the Earth causing it to deform. 2. A unit mass on the side of the Earth closest to the Moon feels a gravitational force from the Moon about 3% greater than the force on a unit mass at the Earth’s center, which in turn is 3% greater than the force on a unit mass at the far side of the Earth. 3. On the side of the Earth nearest the Moon, water feels a greater force and flows to the area under the Moon, causing a high tide. Figure 6.10b: In the Earth-Moon system, the gravitational forces result into a motion around the common center of mass and the differences in the forces result into tides. © Andrew J. Martinez/Photo Researchers, Inc. © Andrew J. Martinez/Photo Researchers, Inc. Figure 6.09b: High tide Figure 6.09a: Low tide 4. A high tide on the opposite side of the Earth (farthest from the Moon) occurs because the center of the Earth feels a greater force toward the Moon than water on that side, so the main body of the Earth is pulled away from the water, resulting in another high tide. 5. Differential gravitational pull on the various parts of the Earth results in two areas of the Earth experiencing high tides. On most days on the Earth there are two high tides and two low tides. 6. As the Earth rotates on its axis, the Moon revolves around the Earth. Because the Moon is not stationary, the Earth must turn for an additional 50 minutes each day before a spot on the Earth returns to the same position with respect to the Moon. This is what causes the high tides and the rising and setting of the Moon to occur about 50 minutes later each day. Figure 6.11 8. A spring tide is the greatest difference between high and low tide, occurring about twice a month when the lunar and solar tides correspond. 9. A neap tide is the least difference between high and low tide, occurring when the solar tide partly cancels the lunar tide (i.e., when the solar tides are 90° from the Moon’s). Figure 6.12a: During new or full moon, the tidal forces from the Moon and Sun add up to give us enhanced (spring) tides. Figure 6.12b: During first and third quarter Moon, the two tidal forces cancel partially, giving us smaller (neap) tides. Question 1 (6-2 thru 6-3 PPT Questions) The Earth has a high tide toward the Moon and away from the Moon. What is the reason for this? Rotation and Revolution of the Moon 1. The period of the Moon’s rotation exactly matches its period of revolution. This is caused by tidal forces, and as a result the Moon keeps the same face toward Earth at all times. 2. There are frictional forces between the solid Earth and its oceans. The Earth’s motion tends to drag the tides along with it, so that a high tide is not directly under the Moon but is farther to the east. 3. Tidal friction (i.e., friction forces that result from tides on a rotating object) has slowed Earth’s rotation over time. 4. As a result of tidal interactions, the Moon is pushed farther away from Earth. Precession of the Earth 1. Precession is the conical shifting of the axis of a rotating object, also known as wobbling. 2. The Earth is not a perfect sphere; its equatorial diameter is about 26 miles greater than its polar diameter. Earth’s spinning on its axis causes it to flatten slightly at the poles. 3. Oblateness is a measure of the “flatness” of a planet, calculated by dividing the difference between the largest and smallest diameter by the largest diameter. 4. The Earth precesses very slowly, requiring a period of about 26,000 years. 5. As the Earth precesses, stars different from Polaris (or no visible stars) occupy the position near the Earth’s north celestial pole. A corresponding effect is that the position of the vernal equinox changes over the centuries. Question 2 (6-2 thru 6-3 PPT Questions) Describe spring and neap tides. What are their differences and what causes them to occur? 6-3 Earth The Interior of the Earth 1. Density is the ratio of an object’s mass to its volume. Earth’s average density is 5.52 g/cm3. (The density of water is 1 g/cm3; of aluminum 2.7 g/cm3; of iron 7.8 g/cm3.) 2. Earth’s interior is made up of three layers. (a) Crust is the thin (<100 km) outermost layer of the Earth; it has a density of 2.5–3 g/cm3. (b) Mantle is the thick (2900 km) solid layer between the crust and the Earth’s core; it has a density of 3–9 g/cm3. The crust “floats” on top of the mantle. (c) Core is the central part of the Earth, composed of a solid inner core and a liquid outer core. The core is probably composed of iron and nickel and its density ranges from 9–13 g/cm3. Figure 6.16: The interior of the Earth, showing its primary layers. 3. This pattern of increasing density is called differentiation and is caused by the sinking of denser materials toward the center of planets or other objects. 4. We know about the makeup of the Earth’s interior by analyzing travel times of two types of waves generated by earthquakes: the P-waves (primary waves, analogous to waves produced by pushing a spring back and forth), and the S-waves (secondary waves, analogous to the waves produced by shaking a rope attached to a wall up and down). Plate Tectonics 1. Alfred Wegener is credited with first developing the idea of continental drift—the gradual motion of the continents relative to one another. 2. Rift zone is a place where tectonic plates are being pushed apart, normally by molten material being forced up out of the mantle. Figure 6.17 The arrangement of Earth's continents about 200 million years ago Figure 6.17: The arrangement of Earth's continents today 3. The theory of plate tectonics states that sections of the Earth’s crust move across the underlying mantle. There are about 12 tectonic plates that extend about 50–100 km deep. 4. Over millions of years, moving plates—often crashing into one another—have caused the continents to “drift,” mountains to be uplifted, ocean trenches to form, and earthquakes to be unleashed. Figure 6.18: The major tectonic plates of the Earth Earth’s Atmosphere 1. Earth’s atmosphere consists of about • 78% nitrogen (N2), • 21% oxygen (O2), • with minor amounts of water vapor (H2O), carbon dioxide (CO2), argon (Ar), and trace amounts of ozone (O3). 2. Troposphere is the lowest level of the Earth’s atmosphere • it contains 75% of the atmospheric mass • it is about 11 km (7 mi) deep • and is where weather occurs. Figure 6.20: The temperature of the atmosphere varies with altitude. 3. The troposphere receives most of its heat from infrared radiation emitted from the ground; thus, the temperature of the troposphere decreases as one goes higher. 4. About 50 km above the Earth’s surface is the ozone layer. • Ozone is an efficient absorber of the Sun’s UV radiation. • This absorption causes the temperature of the Earth’s atmosphere to peak at the ozone layer. 5. The ozone layer has protected life on Earth for billions of years. The release of chlorofluorocarbons during the 20th century has reduced, through molecular interactions, the amount of ozone available to protect us. This is an issue of international concern and underscores the difficulties encountered when competing interests— political, economic, scientific—collide. Earth’s Magnetic Field 1. A magnetic field exists in a region of space if magnetic forces can be detected there. 2. The magnetic poles of the Earth are not located at its poles of rotation. The location of the magnetic poles changes with time. 3. According to the dynamo model, the Earth’s (and other planets’) magnetic field is due to currents within a molten iron core. 4. The three main conditions for generating a magnetic field are: (i) a seed magnetic field, (ii) a conducting fluid, and (iii) an energy source to move the fluid in an appropriate pattern. 5. The Van Allen belts are doughnut-shaped regions composed of charged particles (protons and electrons) emitted by the Sun and captured by the magnetic field of the Earth. 6. Auroras are caused by charged particles trapped in the Earth’s magnetic field striking atoms and molecules in the upper atmosphere. Figure 6.24a: The Van Allen belts are regions where the magnetic field of the Earth traps charged particles from the Sun. Question 3 (6-2 thru 6-3 PPT Questions) Craters have been formed on the Earth and on the Moon by meteorite impact. The Earth has a much stronger gravitational field than does the Moon, and yet we find more craters on the Moon. Explain this apparent contradiction.