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All Textbook Solutions for Astronomy
From where on Earth could you observe all of the stars during the course of a year? What fraction of the sky can be seen from the North Pole?Give four ways to demonstrate that Earth is spherical.Explain, according to both geocentric and heliocentric cosmologies, why we see retrograde motion of the planets.In what ways did the work of Copernicus and Galileo differ from the views of the ancient Greeks and of their contemporaries?What were four of Galileo’s discoveries that were important to astronomy?Explain the origin of the magnitude designation for determining the brightness of stars. Why does it seem to go backward, with smaller numbers indicating brighter stars?Ursa Minor contains the pole star, Polaris, and the asterism known as the Little Dipper. From most locations in the Northern Hemisphere, all of the stars in Ursa Minor are circumpolar. Does that mean these stars are also above the horizon during the day? Explain.How many degrees does the Sun move per day relative to the fixed stars? How many days does it take for the Sun to return to its original location relative to the fixed stars?How many degrees does the Moon move per day relative to the fixed stars? How many days does it take for the Moon to return to its original location relative to the fixed stars?Explain how the zodiacal constellations are different from the other constellations.The Sun was once thought to be a planet. Explain why.Is the ecliptic the same thing as the celestial equator? Explain.What is an asterism? Can you name an example?Why did Pythagoras believe that Earth should be spherical?How did Aristotle deduce that the Sun is farther away from Earth than the Moon?What are two ways in which Aristotle deduced that Earth is spherical?How did Hipparchus discover the wobble of Earth’s axis, known as precession?Why did Ptolemy have to introduce multiple circles of motion for the planets instead of a single, simple circle to represent the planet’s motion around the Sun?Why did Copernicus want to develop a completely new system for predicting planetary positions? Provide two reasons.What two factors made it difficult, at first, for astronomers to choose between the Copernican heliocentric model and the Ptolemaic geocentric model?What phases would Venus show if the geocentric model were correct?Describe a practical way to determine in which constellation the Sun is found at any time of the year.What is a constellation as astronomers define it today? What does it mean when an astronomer says, “I saw a comet in Orion last night”?Draw a picture that explains why Venus goes through phases the way the Moon does, according to the heliocentric cosmology. Does Jupiter also go through phases as seen from Earth? Why?Show with a simple diagram how the lower parts of a ship disappear first as it sails away from you on a spherical Earth. Use the same diagram to show why lookouts on old sailing ships could see farther from the masthead than from the deck. Would there be any advantage to posting lookouts on the mast if Earth were flat? (Note that these nautical arguments for a spherical Earth were quite familiar to Columbus and other mariners of his time.)Parallaxes of stars were not observed by ancient astronomers. How can this fact be reconciled with the heliocentric hypothesis?Why do you think so many people still believe in astrology and spend money on it? What psychological needs does such a belief system satisfy?Consider three cosmological perspectives-the geocentric perspective, the heliocentric perspective, and the modern perspective-in which the Sun is a minor star on the outskirts of one galaxy among billions. Discuss some of the cultural and philosophical implications of each point of view.The north celestial pole appears at an altitude above the horizon that is equal to the observer’s latitude. Identify Polaris, the North Star, which lies very close to the north celestial pole. Measure its altitude. (This can be done with a protractor. Alternatively, your fist, extended at arm’s length, spans a distance approximately equal to 10°.) Compare this estimate with your latitude. (Note that this experiment cannot be performed easily in the Southern Hemisphere because Polaris itself is not visible in the south and no bright star is located near the south celestial pole.)What were two arguments or lines of evidence in support of the geocentric model?Although the Copernican system was largely correct to place the Sun at the center of all planetary motion, the model still gave inaccurate predictions for planetary positions. Explain the flaw in the Copernican model that hindered its accuracy.During a retrograde loop of Mars, would you expect Mars to be brighter than usual in the sky, about average in brightness, or fainter than usual in the sky? Explain.The Great Pyramid of Giza was constructed nearly 5000 years ago. Within the pyramid, archaeologists discovered a shaft leading from the central chamber out of the pyramid, oriented for favorable viewing of the bright star Thuban at that time. Thinking about Earth’s precession, explain why Thuban might have been an important star to the ancient Egyptians.Explain why more stars are circumpolar for observers at higher latitudes.What is the altitude of the north celestial pole in the sky from your latitude? If you do not know your latitude, look it up. If you are in the Southern Hemisphere, answer this question for the south celestial pole, since the north celestial pole is not visible from your location.If you were to drive to some city south of your current location, how would the altitude of the celestial pole in the sky change?Hipparchus could have warned us that the dates associated with each of the natal astrology sun signs would eventually be wrong. Explain why.Explain three lines of evidence that argue against the validity of astrology.What did Galileo discover about the planet Jupiter that cast doubt on exclusive geocentrism?What did Galileo discover about Venus that cast doubt on geocentrism?Suppose Eratosthenes had found that, in Alexandria, at noon on the first day of summer, the line to the Sun makes an angle 30° with the vertical. What, then, would he have found for Earth’s circumference?Suppose Eratosthenes’ results for Earth’s circumference were quite accurate. If the diameter of Earth is 12,740 km, what is the length of his stadium in kilometers?Suppose you are on a strange planet and observe, at night, that the stars do not rise and set, but circle parallel to the horizon. Next, you walk in a constant direction for 8000 miles, and at your new location on the planet, you find that all stars rise straight up in the east and set straight down in the west, perpendicular to the horizon. How could you determine the circumference of the planet without any further observations? What is the circumference, in miles, of the planet?State Kepler’s three laws in your own words.Why did Kepler need Tycho Brahe’s data to formulate his laws?Which has more mass: an armful of feathers or an armful of lead? Which has more volume: a kilogram of feathers or a kilogram of lead? Which has higher density: a kilogram of feathers or a kilogram of lead?Explain how Kepler was able to find a relationship (his third law) between the orbital periods and distances of the planets that did not depend on the masses of the planets or the Sun.Write out Newton’s three laws of motion in terms of what happens with the momentum of objects.Which major planet has the largest . . . A. semimajor axis? B. average orbital speed around the Sun? C. orbital period around the Sun? D. eccentricity?Why do we say that Neptune was the first planet to be discovered through the use of mathematics?Why was Brahe reluctant to provide Kepler with all his data at one time?According to Kepler’s second law, where in a planet’s orbit would it be moving fastest? Where would it be moving slowest?The gas pedal, the brakes, and the steering wheel all have the ability to accelerate a car-how?Explain how a rocket can propel itself using Newton’s third law.A certain material has a mass of 565 g while occupying 50 cm3 of space. What is this material? (Hint: Use Table 3.1.)To calculate the momentum of an object, which properties of an object do you need to know?To calculate the angular momentum of an object, which properties of an object do you need to know?What was the great insight Newton had regarding Earth’s gravity that allowed him to develop the universal law of gravitation?Which of these properties of an object best quantifies its inertia: velocity, acceleration, volume, mass, or temperature?Pluto’s orbit is more eccentric than any of the major planets. What does that mean?Why is Tycho Brahe often called “the greatest naked-eye astronomer” of all time?Is it possible to escape the force of gravity by going into orbit around Earth? How does the force of gravity in the International Space Station (orbiting an average of 400 km above Earth’s surface) compare with that on the ground?What is the momentum of an object whose velocity is zero? How does Newton’s first law of motion include the case of an object at rest?Evil space aliens drop you and your fellow astronomy student 1 km apart out in space, very far from any star or planet. Discuss the effects of gravity on each of you.A body moves in a perfectly circular path at constant speed. Are there forces acting in such a system? How do you know?As friction with our atmosphere causes a satellite to spiral inward, closer to Earth, its orbital speed increases. Why?Use a history book, an encyclopedia, or the internet to find out what else was happening in England during Newton’s lifetime and discuss what trends of the time might have contributed to his accomplishments and the rapid acceptance of his work.Two asteroids begin to gravitationally attract one another. If one asteroid has twice the mass of the other, which one experiences the greater force? Which one experiences the greater acceleration?How does the mass of an astronaut change when she travels from Earth to the Moon? How does her weight change?If there is gravity where the International Space Station (ISS) is located above Earth, why doesn’t the space station get pulled back down to Earth?Compare the density, weight, mass, and volume of a pound of gold to a pound of iron on the surface of Earth.If identical spacecraft were orbiting Mars and Earth at identical radii (distances), which spacecraft would be moving faster? Why?By what factor would a person’s weight be increased if Earth had 10 times its present mass, but the same volume?Suppose astronomers find an earthlike planet that is twice the size of Earth (that is, its radius is twice that of Earth’s). What must be the mass of this planet such that the gravitational force (Fgravity) at the surface would be identical to Earth’s?What is the semimajor axis of a circle of diameter 24 cm? What is its eccentricity?If 24 g of material fills a cube 2 cm on a side, what is the density of the material?If 128 g of material is in the shape of a brick 2 cm wide, 4 cm high, and 8 cm long, what is the density of the material?If the major axis of an ellipse is 16 cm, what is the semimajor axis? If the eccentricity is 0.8, would this ellipse be best described as mostly circular or very elongated?What is the average distance from the Sun (in astronomical units) of an asteroid with an orbital period of 8 years?What is the average distance from the Sun (in astronomical units) of a planet with an orbital period of 45.66 years?In 1996, astronomers discovered an icy object beyond Pluto that was given the designation 1996 TL 66. It has a semimajor axis of 84 AU. What is its orbital period according to Kepler’s third law?Discuss how latitude and longitude on Earth are similar to declination and right ascension in the sky.What is the latitude of the North Pole? The South Pole? Why does longitude have no meaning at the North and South Poles?Make a list of each main phase of the Moon, describing roughly when the Moon rises and sets for each phase. During which phase can you see the Moon in the middle of the morning? In the middle of the afternoon?What are advantages and disadvantages of apparent solar time? How is the situation improved by introducing mean solar time and standard time?What are the two ways that the tilt of Earth’s axis causes the summers in the United States to be warmer than the winters?Why is it difficult to construct a practical calendar based on the Moon’s cycle of phases?Explain why there are two high tides and two low tides each day. Strictly speaking, should the period during which there are two high tides be 24 hours? If not, what should the interval be?What is the phase of the Moon during a total solar eclipse? During a total lunar eclipse?On a globe or world map, find the nearest marked latitude line to your location. Is this an example of a great circle? Explain.Explain three lines of evidence that indicate that the seasons in North America are not caused by the changing Earth-Sun distance as a result of Earth’s elliptical orbit around the Sun.What is the origin of the terms “a.m.” and “p.m.” in our timekeeping?Explain the origin of the leap year. Why is it necessary?Explain why the year 1800 was not a leap year, even though years divisible by four are normally considered to be leap years.What fraction of the Moon’s visible face is illuminated during first quarter phase? Why is this phase called first quarter?Why don’t lunar eclipses happen during every full moon?Why does the Moon create tidal bulges on both sides of Earth instead of only on the side of Earth closest to the Moon?Why do the heights of the tides change over the course of a month?Explain how tidal forces are causing Earth to slow down.Explain how tidal forces are causing the Moon to slowly recede from Earth.Explain why the Gregorian calendar modified the nature of the leap year from its original definition in the Julian calendar.The term equinox translates as “equal night.” Explain why this translation makes sense from an astronomical point of view.The term solstice translates as “Sun stop.” Explain why this translation makes sense from an astronomical point of view.Why is the warmest day of the year in the United States (or in the Northern Hemisphere temperate zone) usually in August rather than on the day of the summer solstice, in late June?When Earth’s Northern Hemisphere is tilted toward the Sun during June, some would argue that the cause of our seasons is that the Northern Hemisphere is physically closer to the Sun than the Southern Hemisphere, and this is the primary reason the Northern Hemisphere is warmer. What argument or line of evidence could contradict this idea?Where are you on Earth if you experience each of the following? (Refer to the discussion in Observing the Sky: The Birth of Astronomy as well as this chapter.) A. The stars rise and set perpendicular to the horizon. B. The stars circle the sky parallel to the horizon. C. The celestial equator passes through the zenith. D. In the course of a year, all stars are visible. E. The Sun rises on March 21 and does not set until September 21 (ideally).In countries at far northern latitudes, the winter months tend to be so cloudy that astronomical observations are nearly impossible. Why can’t good observations of the stars be made at those places during the summer months?What is the phase of the Moon if it . . . A. rises at 3:00 p.m.? B. is highest in the sky at sunrise? C. sets at 10:00 a.m.?A car accident occurs around midnight on the night of a full moon. The driver at fault claims he was blinded momentarily by the Moon rising on the eastern horizon. Should the police believe him?The secret recipe to the ever-popular veggie burgers in the college cafeteria is hidden in a drawer in the director’s office. Two students decide to break in to get their hands on it, but they want to do it a few hours before dawn on a night when there is no Moon, so they are less likely to be caught. What phases of the Moon would suit their plans?Your great-great-grandfather, who often exaggerated events in his own life, once told your relatives about a terrific adventure he had on February 29, 1900. Why would this story make you suspicious?One year in the future, when money is no object, you enjoy your birthday so much that you want to have another one right away. You get into your supersonic jet. Where should you and the people celebrating with you travel? From what direction should you approach? Explain.Suppose you lived in the crater Copernicus on the side of the Moon facing Earth. A. How often would the Sun rise? B. How often would Earth set? C. During what fraction of the time would you be able to see the stars?In a lunar eclipse, does the Moon enter the shadow of Earth from the east or west side? Explain.Describe what an observer at the crater Copernicus would see while the Moon is eclipsed on Earth. What would the same observer see during what would be a total solar eclipse as viewed from Earth?The day on Mars is 1.026 Earth-days long. The martian year lasts 686.98 Earth-days. The two moons of Mars take 0.32 Earth-day (for Phobos) and 1.26 Earth-days (for Deimos) to circle the planet. You are given the task of coming up with a martian calendar for a new Mars colony. Would a solar or lunar calendar be better for tracking the seasons?What is the right ascension and declination of the vernal equinox?What is the right ascension and declination of the autumnal equinox?What is the right ascension and declination of the Sun at noon on the summer solstice in the Northern Hemisphere?During summer in the Northern Hemisphere, the North Pole is illuminated by the Sun 24 hours per day. During this time, the temperature often does not rise above the freezing point of water. Explain why.On the day of the vernal equinox, the day length for all places on Earth is actually slightly longer than 12 hours. Explain why.Regions north of the Arctic Circle are known as the “land of the midnight Sun.” Explain what this means from an astronomical perspective.In a part of Earth’s orbit where Earth is moving faster than usual around the Sun, would the length of the sidereal day change? If so, how? Explain.In a part of Earth’s orbit where Earth is moving faster than usual around the Sun, would the length of the solar day change? If so, how? Explain.If Sirius rises at 8:00 p.m. tonight, at what time will it rise tomorrow night, to the nearest minute? Explain.What are three lines of evidence you could use to indicate that the phases of the Moon are not caused by the shadow of Earth falling on the Moon?If the Moon rises at a given location at 6:00 p.m. today, about what time will it rise tomorrow night?Explain why some solar eclipses are total and some are annular.Why do lunar eclipses typically last much longer than solar eclipses?Suppose Earth took exactly 300.0 days to go around the Sun, and everything else (the day, the month) was the same. What kind of calendar would we have? How would this affect the seasons?Consider a calendar based entirely on the day and the month (the Moon’s period from full phase to full phase). How many days are there in a month? Can you figure out a scheme analogous to leap year to make this calendar work?If a star rises at 8:30 p.m. tonight, approximately what time will it rise two months from now?What is the altitude of the Sun at noon on December 22, as seen from a place on the Tropic of Cancer?Show that the Gregorian calendar will be in error by 1 day in about 3300 years.What distinguishes one type of electromagnetic radiation from another? What are the main categories (or bands) of the electromagnetic spectrum?What is a wave? Use the terms wavelength and frequency in your definition.Is your textbook the kind of idealized object (described in section on radiation laws) that absorbs all the radiation falling on it? Explain. How about the black sweater worn by one of your classmates?Where in an atom would you expect to find electrons? Protons? Neutrons?Explain how emission lines and absorption lines are formed. In what sorts of cosmic objects would you expect to see each?Explain how the Doppler effect works for sound waves and give some familiar examples.What kind of motion for a star does not produce a Doppler effect? Explain.Describe how Bohr’s model used the work of Maxwell.Explain why light is referred to as electromagnetic radiation.Explain the difference between radiation as it is used in most everyday language and radiation as it is used in an astronomical context.What are the differences between light waves and sound waves?Which type of wave has a longer wavelength: AM radio waves (with frequencies in the kilohertz range) or FM radio waves (with frequencies in the megahertz range)? Explain.Explain why astronomers long ago believed that space must be filled with some kind of substance (the “aether”) instead of the vacuum we know it is today.Explain what the ionosphere is and how it interacts with some radio waves.Which is more dangerous to living things, gamma rays or X-rays? Explain.Explain why we have to observe stars and other astronomical objects from above Earth’s atmosphere in order to fully learn about their properties.Explain why hotter objects tend to radiate more energetic photons compared to cooler objects.Explain how we can deduce the temperature of a star by determining its color.Explain what dispersion is and how astronomers use this phenomenon to study a star’s light.Explain why glass prisms disperse light.Explain what Joseph Fraunhofer discovered about stellar spectra.Explain how we use spectral absorption and emission lines to determine the composition of a gas.Explain the results of Rutherford’s gold foil experiment and how they changed our model of the atom.Is it possible for two different atoms of carbon to have different numbers of neutrons in their nuclei? Explain.What are the three isotopes of hydrogen, and how do they differ?Explain how electrons use light energy to move among energy levels within an atom.Explain why astronomers use the term “blueshifted” for objects moving toward us and “redshifted” for objects moving away from us.If spectral line wavelengths are changing for objects based on the radial velocities of those objects, how can we deduce which type of atom is responsible for a particular absorption or emission line?Make a list of some of the many practical consequences of Maxwell’s theory of electromagnetic waves (television is one example).With what type of electromagnetic radiation would you observe: A. A star with a temperature of 5800 K? B. A gas heated to a temperature of one million K? C. A person on a dark night?Why is it dangerous to be exposed to X-rays but not (or at least much less) dangerous to be exposed to radio waves?Go outside on a clear night, wait 15 minutes for your eyes to adjust to the dark, and look carefully at the brightest stars. Some should look slightly red and others slightly blue. The primary factor that determines the color of a star is its temperature. Which is hotter: a blue star or a red one? ExplainWater faucets are often labeled with a red dot for hot water and a blue dot for cold. Given Wien’s law, does this labeling make sense?Suppose you are standing at the exact center of a park surrounded by a circular road. An ambulance drives completely around this road, with siren blaring. How does the pitch of the siren change as it circles around you?How could you measure Earth’s orbital speed by photographing the spectrum of a star at various times throughout the year? (Hint: Suppose the star lies in the plane of Earth’s orbit.)Astronomers want to make maps of the sky showing sources of X-rays or gamma rays. Explain why those X-rays and gamma rays must be observed from above Earth’s atmosphere.The greenhouse effect can be explained easily if you understand the laws of blackbody radiation. A greenhouse gas blocks the transmission of infrared light. Given that the incoming light to Earth is sunlight with a characteristic temperature of 5800 K (which peaks in the visible part of the spectrum) and the outgoing light from Earth has a characteristic temperature of about 300 K (which peaks in the infrared part of the spectrum), explain how greenhouse gases cause Earth to warm up. As part of your answer, discuss that greenhouse gases block both incoming and outgoing infrared light. Explain why these two effects don’t simply cancel each other, leading to no net temperature change.An idealized radiating object does not reflect or scatter any radiation but instead absorbs all of the electromagnetic energy that falls on it. Can you explain why astronomers call such an object a blackbody? Keep in mind that even stars, which shine brightly in a variety of colors, are considered blackbodies. Explain why.Why are ionized gases typically only found in very high-temperature environments?Explain why each element has a unique spectrum of absorption or emission lines.What is the wavelength of the carrier wave of a campus radio station, broadcasting at a frequency of 97.2 MHz (million cycles per second or million hertz)?What is the frequency of a red laser beam, with a wavelength of 670 nm, which your astronomy instructor might use to point to slides during a lecture on galaxies?You go to a dance club to forget how hard your astronomy midterm was. What is the frequency of a wave of ultraviolet light coming from a blacklight in the club, if its wavelength is 150 nm?What is the energy of the photon with the frequency you calculated in Exercise 5.43?If the emitted infrared radiation from Pluto, has a wavelength of maximum intensity at 75,000 nm, what is the temperature of Pluto assuming it follows Wien’s law?What is the temperature of a star whose maximum light is emitted at a wavelength of 290 nm?What are the three basic components of a modern astronomical instrument? Describe each in one to two sentences.Name the two spectral windows through which electromagnetic radiation easily reaches the surface of Earth and describe the largest-aperture telescope currently in use for each window.List the largest-aperture single telescope currently in use in each of the following bands of the electromagnetic spectrum: radio, X-ray, gamma ray.When astronomers discuss the apertures of their telescopes, they say bigger is better. Explain why.The Hooker telescope at Palomar Observatory has a diameter of 5 m, and the Keck I telescope has a diameter of 10 m. How much more light can the Keck telescope collect than the Hooker telescope in the same amount of time?What is meant by “reflecting” and “refracting” telescopes?Why are the largest visible-light telescopes in the world made with mirrors rather than lenses?Compare the eye, photographic film, and CCDs as detectors for light. What are the advantages and disadvantages of each?What is a charge-coupled device (CCD), and how is it used in astronomy?Why is it difficult to observe at infrared wavelengths? What do astronomers do to address this difficulty?Radio and radar observations are often made with the same antenna, but otherwise they are very different techniques. Compare and contrast radio and radar astronomy in terms of the equipment needed, the methods used, and the kind of results obtained.Look back at Figure 6.18 of Cygnus A and read its caption again. The material in the giant lobes at the edges of the image had to have been ejected from the center at least how many years ago? Figure 6.18 Radio Image. This image has been constructed of radio observations at the Very Large Array of a galaxy called Cygnus A. Colors have been added to help the eye sort out regions of different radio intensities. Red regions are the most intense, blue the least. The visible galaxy would be a small dot in the center of the image. The radio image reveals jets of expelled material (more than 160,000 light-years long) on either side of the galaxy. (credit: NRAO/AUI)Why do astronomers place telescopes in Earth’s orbit? What are the advantages for the different regions of the spectrum?What was the problem with the Hubble Space Telescope and how was it solved?Describe the techniques radio astronomers use to obtain a resolution comparable to what astronomers working with visible light can achieve.What kind of visible-light and infrared telescopes on the ground are astronomers planning for the future? Why are they building them on the ground and not in space?Describe one visible-light or infrared telescope that astronomers are planning to launch into space in the future.What happens to the image produced by a lens if the lens is “stopped down” (the aperture reduced, thereby reducing the amount of light passing through the lens) with an iris diaphragm-a device that covers its periphery?What would be the properties of an ideal astronomical detector? How closely do the actual properties of a CCD approach this ideal?Many decades ago, the astronomers on the staff of Mount Wilson and Palomar Observatories each received about 60 nights per year for their observing programs. Today, an astronomer feels fortunate to get 10 nights per year on a large telescope. Can you suggest some reasons for this change?The largest observatory complex in the world is on Mauna Kea, the tallest mountain on Earth. What are some factors astronomers consider when selecting an observatory site? Don’t forget practical ones. Should astronomers, for example, consider building an observatory on Denali (Mount McKinley) or Mount Everest?Suppose you are looking for sites for a visible-light observatory, an infrared observatory, and a radio observatory. What are the main criteria of excellence for each? What sites are actually considered the best today?Radio astronomy involves wavelengths much longer than those of visible light, and many orbiting observatories have probed the universe for radiation of very short wavelengths. What sorts of objects and physical conditions would you expect to be associated with emission of radiation at very long and very short wavelengths?The dean of a university located near the ocean (who was not a science major in college) proposes building an infrared telescope right on campus and operating it in a nice heated dome so that astronomers will be comfortable on cold winter nights. Criticize this proposal, giving your reasoning.What is the area, in square meters, of a 10-m telescope?Approximately 9000 stars are visible to the naked eye in the whole sky (imagine that you could see around the entire globe and both the northern and southern hemispheres), and there are about 41,200 square degrees on the sky. How many stars are visible per square degree? Per square arcsecond?Theoretically (that is, if seeing were not an issue), the resolution of a telescope is inversely proportional to its diameter. How much better is the resolution of the ALMA when operating at its longest baseline than the resolution of the Arecibo telescope?In broad daylight, the size of your pupil is typically 3 mm. In dark situations, it expands to about 7 mm. How much more light can it gather?How much more light can be gathered by a telescope that is 8 m in diameter than by your fully dark-adapted eye at 7 mm?How much more light can the Keck telescope (with its 10-m diameter mirror) gather than an amateur telescope whose mirror is 25 cm (0.25 m) across?People are often bothered when they discover that reflecting telescopes have a second mirror in the middle to bring the light out to an accessible focus where big instruments can be mounted. “Don’t you lose light?” people ask. Well, yes, you do, but there is no better alternative. You can estimate how much light is lost by such an arrangement. The primary mirror (the one at the bottom in Figure 6.6) of the Gemini North telescope is 8 m in diameter. The secondary mirror at the top is about 1 m in diameter. Use the formula for the area of a circle to estimate what fraction of the light is blocked by the secondary mirror. Figure 6.6 Focus Arrangements for Reflecting Telescopes. Reflecting telescopes have different options for where the light is brought to a focus. With prime focus, light is detected where it comes to a focus after reflecting from the primary mirror. With Newtonian focus, light is reflected by a small secondary mirror off to one side, where it can be detected (see also Figure 6.5). Most large professional telescopes have a Cassegrain focus in which light is reflected by the secondary mirror down through a hole in the primary mirror to an observing station below the telescope.Telescopes can now be operated remotely from a warm room, but until about 25 years ago, astronomers worked at the telescope to guide it so that it remained pointed in exactly the right place. In a large telescope, like the Palomar 200-inch telescope, astronomers sat in a cage at the top of the telescope, where the secondary mirror is located, as shown in Figure 6.6. Assume for the purpose of your calculation that the diameter of this cage was 40 inches. What fraction of the light is blocked? Figure 6.6 Focus Arrangements for Reflecting Telescopes. Reflecting telescopes have different options for where the light is brought to a focus. With prime focus, light is detected where it comes to a focus after reflecting from the primary mirror. With Newtonian focus, light is reflected by a small secondary mirror off to one side, where it can be detected (see also Figure 6.5). Most large professional telescopes have a Cassegrain focus in which light is reflected by the secondary mirror down through a hole in the primary mirror to an observing station below the telescope.The HST cost about $1.7 billion for construction and $300 million for its shuttle launch, and it costs $250 million per year to operate. If the telescope lasts for 20 years, what is the total cost per year? Per day? If the telescope can be used just 30% of the time for actual observations, what is the cost per hour and per minute for the astronomer’s observing time on this instrument? What is the cost per person in the United States? Was your investment in the Hubble Space telescope worth it?How much more light can the James Webb Space Telescope (with its 6-m diameter mirror) gather than the Hubble Space Telescope (with a diameter of 2.4 m)?The Palomar telescope’s 5-m mirror weighs 14.5 tons. If a 10-m mirror were constructed of the same thickness as Palomar’s (only bigger), how much would it weigh?Venus rotates backward and Uranus and Pluto spin about an axis tipped nearly on its side. Based on what you learned about the motion of small bodies in the solar system and the surfaces of the planets, what might be the cause of these strange rotations?What is the difference between a differentiated body and an undifferentiated body, and how might that influence a body’s ability to retain heat for the age of the solar system?What does a planet need in order to retain an atmosphere? How does an atmosphere affect the surface of a planet and the ability of life to exist?Which type of planets have the most moons? Where did these moons likely originate?What is the difference between a meteor and a meteorite?Explain our ideas about why the terrestrial planets are rocky and have less gas than the giant planets.Do all planetary systems look the same as our own?What is comparative planetology and why is it useful to astronomers?What changed in our understanding of the Moon and Moon-Earth system as a result of humans landing on the Moon’s surface?If Earth was to be hit by an extraterrestrial object, where in the solar system could it come from and how would we know its source region?List some reasons that the study of the planets has progressed more in the past few decades than any other branch of astronomy.Imagine you are a travel agent in the next century. An eccentric billionaire asks you to arrange a “Guinness Book of Solar System Records” kind of tour. Where would you direct him to find the following (use this chapter and Appendix F and Appendix G): A. the least-dense planet B. the densest planet C. the largest moon in the solar system D. excluding the jovian planets, the planet where you would weigh the most on its surface (Hint:Weight is directly proportional to surface gravity.) E. the smallest planet F. the planet that takes the longest time to rotate G. the planet that takes the shortest time to rotate H. the planet with a diameter closest to Earth’s I. the moon with the thickest atmosphere J. the densest moon K. the most massive moonWhat characteristics do the worlds in our solar system have in common that lead astronomers to believe that they all formed from the same “mother cloud” (solar nebula)?How do terrestrial and giant planets differ? List as many ways as you can think of.Why are there so many craters on the Moon and so few on Earth?How do asteroids and comets differ?How and why is Earth’s Moon different from the larger moons of the giant planets?Where would you look for some “original” planetesimals left over from the formation of our solar system?Describe how we use radioactive elements and their decay products to find the age of a rock sample. Is this necessarily the age of the entire world from which the sample comes? Explain.What was the solar nebula like? Why did the Sun form at its center?What can we learn about the formation of our solar system by studying other stars? Explain.Earlier in this chapter, we modeled the solar system with Earth at a distance of about one city block from the Sun. If you were to make a model of the distances in the solar system to match your height, with the Sun at the top of your head and Pluto at your feet, which planet would be near your waist? How far down would the zone of the terrestrial planets reach?Seasons are a result of the inclination of a planet’s axial tilt being inclined from the normal of the planet’s orbital plane. For example, Earth has an axis tilt of 23.4° (Appendix F). Using information about just the inclination alone, which planets might you expect to have seasonal cycles similar to Earth, although different in duration because orbital periods around the Sun are different?Again using Appendix F, which planet(s) might you expect not to have significant seasonal activity? Why?Again using Appendix F, which planets might you expect to have extreme seasons? WhyUsing some of the astronomical resources in your college library or the Internet, find five names of features on each of three other worlds that are named after real people. In a sentence or two, describe each of these people and what contributions they made to the progress of science or human thought.Explain why the planet Venus is differentiated, but asteroid Fraknoi, a very boring and small member of the asteroid belt, is not.Would you expect as many impact craters per unit area on the surface of Venus as on the surface of Mars? Why or why not?Using Appendix G, complete the following table that describes the characteristics of the Galilean moons of Jupiter, starting from Jupiter and moving outward in distance. Table A This system has often been described as a mini solar system. Why might this be so? If Jupiter were to represent the Sun and the Galilean moons represented planets, which moons could be considered more terrestrial in nature and which ones more like gas/ice giants? Why? (Hint: Use the values in your table to help explain your categorization.)Calculate the density of Jupiter. Show your work. Is it more or less dense than Earth? Why?Calculate the density of Saturn. Show your work. How does it compare with the density of water? Explain how this can be.What is the density of Jupiter’s moon Europa (see Appendix G for data on moons)? Show your work.Look at Appendix F and Appendix G and indicate the moon with a diameter that is the largest fraction of the diameter of the planet or dwarf planet it orbits.Barnard’s Star, the second closest star to us, is about 56 trillion (5.61012) km away. Calculate how far it would be using the scale model of the solar system given in Overview of Our Planetary System.A radioactive nucleus has a half-life of 5108 years. Assuming that a sample of rock (say, in an asteroid) solidified right after the solar system formed, approximately what fraction of the radioactive element should be left in the rock today?What is the thickest interior layer of Earth? The thinnest?What are Earth’s core and mantle made of? Explain how we know.Describe the differences among primitive, igneous, sedimentary, and metamorphic rock, and relate these differences to their origins.Explain briefly how the following phenomena happen on Earth, relating your answers to the theory of plate tectonics A. earthquakes B. continental drift C. mountain building D. volcanic eruptions E. creation of the Hawaiian island chainWhat is the source of Earth’s magnetic field?Why is the shape of the magnetosphere not spherical like the shape of Earth?Although he did not present a mechanism, what were the key points of Alfred Wegener’s proposal for the concept of continental drift?List the possible interactions between Earth’s crustal plates that can occur at their boundaries.List, in order of decreasing altitude, the principle layers of Earth’s atmosphere.In which atmospheric layer are almost all water-based clouds formed?What is, by far, the most abundant component of Earth’s atmosphere?In which domain of living things do you find humankind?Describe three ways in which the presence of life has affected the composition of Earth’s atmosphere.Briefly describe the greenhouse effect.How do impacts by comets and asteroids influence Earth’s geology, its atmosphere, and the evolution of life?Why are there so many impact craters on our neighbor world, the Moon, and so few on Earth?Detail some of the anthropogenic changes to Earth’s climate and their potential impact on life.If you wanted to live where the chances of a destructive earthquake were small, would you pick a location near a fault zone, near a mid ocean ridge, near a subduction zone, or on a volcanic island such as Hawaii? What are the relative risks of earthquakes at each of these locations?Which type of object would likely cause more damage if it struck near an urban area: a small metallic object or a large stony/icy one?If all life were destroyed on Earth by a large impact, would new life eventually form to take its place? Explain how conditions would have to change for life to start again on our planet.Why is a decrease in Earth’s ozone harmful to life?Why are we concerned about the increases in CO2 and other gases that cause the greenhouse effect in Earth’s atmosphere? What steps can we take in the future to reduce the levels of CO2 in our atmosphere? What factors stand in the way of taking the steps you suggest? (You may include technological, economic, and political factors in your answer.)Do you think scientists should make plans to defend Earth from future asteroid impacts? Is it right to intervene in the same evolutionary process that made the development of mammals (including us) possible after the big impact 65 million years ago?Europe and North America are moving apart by about 5 m per century. As the continents separate, new ocean floor is created along the mid-Atlantic Rift. If the rift is 5000 km long, what is the total area of new ocean floor created in the Atlantic each century? (Remember that 1km=1000m .)Over the entire Earth, there are 60,000 km of active rift zones, with average separation rates of 5 m/ century. How much area of new ocean crust is created each year over the entire planet? (This area is approximately equal to the amount of ocean crust that is subducted since the total area of the oceans remains about the same.)With the information from Exercise 8.25, you can calculate the average age of the ocean floor. First, find the total area of the ocean floor (equal to about 60% of the surface area of Earth). Then compare this with the area created (or destroyed) each year. The average lifetime is the ratio of these numbers: the total area of ocean crust compared to the amount created (or destroyed) each year.What is the volume of new oceanic basalt added to Earth’s crust each year? Assume that the thickness of the new crust is 5 km, that there are 60,000 km of rifts, and that the average speed of plate motion is 4 cm/y. What fraction of Earth’s entire volume does this annual addition of new material represent?Suppose a major impact that produces a mass extinction takes place on Earth once every 5 million years. Suppose further that if such an event occurred today, you and most other humans would be killed (this would be true even if the human species as a whole survived). Such impact events are random, and one could take place at any time. Calculate the probability that such an impact will occur within the next 50 years (within your lifetime).How do the risks of dying from the impact of an asteroid or comet compare with other risks we are concerned about, such as dying in a car accident or from heart disease or some other natural cause? (Hint: To find the annual risk, go to the library or internet and look up the annual number of deaths from a particular cause in a particular country, and then divide by the population of that country.)What fraction of Earth’s volume is taken up by the core?Approximately what percentage of Earth’s radius is represented by the crust?What is the drift rate of the Pacific plate over the Hawaiian hot spot?What is the percent increase of atmospheric CO2 in the past 20 years?Estimate the mass of the object that formed Meteor Crater in Arizona.What is the composition of the Moon, and how does it compare to the composition of Earth? Of Mercury?Why does the Moon not have an atmosphere?What are the principal features of the Moon observable with the unaided eye?Frozen water exists on the lunar surface primarily in which location? Why?Outline the main events in the Moon’s geological history.What are the maria composed of? Is this material found elsewhere in the solar system?The mountains on the Moon were formed by what process?With no wind or water erosion of rocks, what is the mechanism for the creation of the lunar “soil?”What differences did Grove K. Gilbert note between volcanic craters on Earth and lunar craters?Explain how high-speed impacts form circular craters. How can this explanation account for the various characteristic features of impact craters?Explain the evidence for a period of heavy bombardment on the Moon about 4 billion years ago.How did our exploration of the Moon differ from that of Mercury (and the other planets)?Summarize the four main hypotheses for the origin of the Moon.What are the difficulties with the capture hypothesis of the Moon’s origin?What is the main consequence of Mercury’s orbit being so highly eccentric?Describe the basic internal structure of Mercury.