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The size of Earth light helps in turns introspective as we come to appreciate that our very is like—snap! Now follow along in scale of the universe. The study of the chemical evolution of the universe is Figure 1. Theory and observation tells us that the uni- ward into the universe. As a result of both observation and theoretical work, we ings that make analogies between astronomical phenomena now understand that the only chemical elements found in and everyday objects more concrete.

Yet we ing at the speed of light. Crossing from one phosphorus, and a host of other chemical elements. If these side of the orbit of Neptune, the outermost classical planet elements that make up Earth and our bodies were not pres- in our Solar System, to the other takes about 8. Think ent in the early universe, where did they come from? Let it sink in. Comparing the size of The answer to this question lies within the stars. When begun our journey. Many steps remain. It takes us a bit over a star exhausts its nuclear fuel and nears the end of its life, four years to cover the distance from Earth to the nearest it often loses much of its mass—including some of the new star other than the Sun , or as much time as you spent in atoms formed in its interior— high school.

At this point, even our analogy using the travel by blasting it back into inter- We are stardust. We will talk human scale. Light takes about , years to travel across later about the life and death of stars. For now it is enough our galaxy—about the time that modern humans Homo sapi- to note that our Sun and Solar System formed from a cloud ens have walked the surface of Earth. Radius of the observable universe. Comparing the size of the universe to the size of Earth is like comparing three times the age of the planet to a snap of your fingers.

What combination of events—some probable, others much less likely—have led to our existence as sentient beings liv- ing on a small rocky planet orbiting a typical middle-aged star? Was this a unique happening, or are there others like us scattered throughout the galaxy? For life to exist around other stars in the galaxy, there must be life-sustaining planets to support it.

We will include the discovery of extrasolar planets and how they compare with the planets of our own Solar System. Discovery As we look at the universe through the eyes of astronomers, we will also learn something of how science works. It is esting chemical processes that go on around us—chemical beyond the scope of this book for us to provide a detailed processes such as life. Figure 1. However, we will try relationship between the world around us and our heritage to offer some explanation of where an idea comes from and in the stars. Look around you. The atoms that make up why we believe it to be valid.

We will not present some- everything you see were formed in the hearts of stars. We will be honest when we are on uncertain, specula- poetry. It is literal truth. This book is not a compendium of nings. Who or what is responsible for our existence? How revealed truth or a font of accepted wisdom. Rather, it is were Heaven and Earth created? In the modern world, an introduction to a body of knowledge and understanding primitive creation myths have largely given way to scien- that was painstakingly built and sometimes torn down and rebuilt brick by brick.

It is almost impossible to overstate the importance of science in our civilization. Since the launch of Sputnik, the were formed. Five decades later, we have seen humans walk on the Moon Figure 1. Satellite observatories in orbit around Earth have also given us many new perspectives on the universe. The very atmosphere that shields us from harmful solar radiation. Irwin stands by the lunar rover during an excursion to explore and collect samples from the Moon. These spacecraft are all shown to the same scale.

Others are interplanetary explorers sent to investigate other worlds within our Solar System. Space at a computer screen than peering through the eyepiece of astronomy continues to show us vistas hidden from the gaze a telescope. As astronomers, we use computers to collect of groundbased telescopes by the protective but obscuring and analyze data from telescopes, calculate physical mod- blanket of our atmosphere.

Satellites capable of detecting els of astronomical objects, and prepare and disseminate the full spectrum of radia- the results of our work. This is the lowest-energy microwaves—have brought surprising dis- sky as we would see it if our eyes were sensitive to radio covery after surprising discovery. Since the closing years of the 20th century we have witnessed a renewed vigor in astro- nomical observations from the surface of Earth. The view of the sky seen by radio telescopes, as shown in Figure 1.

When we think of astronomy, telescopes—both on the ground and in space —immediately come to mind. But you may be surprised to learn that a great deal of frontline astronomy is now carried out in large physics facilities like the one shown in Figure 1. She presents the idea to her colleagues as a hypothesis.

Her colleagues then look for testable predictions capable of disproving her hypothesis. Faith serves its own purpose in society, but it is not science. Science is sometimes misunderstood because of the spe- cial ways that scientists use everyday words. An example is the word theory. A theory is a well-developed idea that is ready to be confronted by nature. A well-corroborated 1. Given the visible importance of technology in our lives, you A successful and well-corroborated theory is the pinnacle might be tempted to say that science is technology.

It is true of human knowledge about the world. In science, the word idea has its everyday in the other. Yet science is much more than technology. Sci- use: an idea is just a notion about how something might be. Scientists build theoretical models that are used to connect theories with the behavior of complex systems. When you took how the universe is that guides our construction of new science courses in high school, you probably had the scien- theories.

At the heart of Start with an modern astronomy is another principle: the cosmological observation or idea principle. Simply put, the cosmological principle states that on a large scale the universe looks the same every- where. That is, when we look Suggest a out around us, what we see is There is nothing hypothesis representative of what the special about our universe is generally like. In place in the universe. By extension, matter prediction and energy obey the same physical laws throughout space and time as they do today on Earth.

This premise means that the same physical laws that we learn about in terres- trial laboratories can be used to understand what goes on Test does not support Perform a test, Test supports hypothesis; revise experiment, or hypothesis; make in the centers of stars or in the hearts of distant galaxies. We will discuss the cosmological principle in either accepted as a tested theory or rejected, on the basis of more detail in Chapter This concept is so important to your ativity. Scientists speak of a beautiful theory in the same understanding of how science works that we should empha- way that an artist speaks of a beautiful painting or a musi- size it once again.

Nature is the verb to know. At the same time, however, there is prove things to be true, but an aesthetic to science that is as human and as profound actually it is a way of proving things to be false. Before sci- as any found in the arts, as Figure 1. Only after repeated attempts to disprove an idea have failed do scientists begin to accept its likely valid- ity.

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This is, after all, ing nature whether an idea is false, but it offers no insight what it means to learn. It is what keeps us honest. Occa- ing this understanding to new territory into which the ear- sionally, however, major shifts occur in the framework of an lier theory could not go. As we continue on our journey, we will encounter many other discoveries and successful ideas that forced scientists either to abandon their treasured notions or be left behind, hopelessly locked into a worldview that had ultimately failed the test of observation and experiment.

No theory, no matter how central or how strongly held, is immune from the rules. It is a way of relating to nature. Science is an exquisite arate things. Einstein actually helped to tion because science works. Yet Einstein was unable to accept the implications of the second revolution he helped start—quantum mechanics— Challenges to Science and he went to his grave unwilling to embrace the view of Science has not achieved its prominence without criticism the world it offered.

In the early 20th century, the notion that we live in a universe in which effect fol- astronomers were arguing bitterly among themselves over lows cause in lockstep. Together, these revolutions led to the size of the universe. For centuries, conventional wisdom the birth of what has come to be known as modern physics. For many, it came as a rude from culture. But in Nature is the a greatly larger universe! You might feel that science is sometimes arbitrary. For should be judged not by cul- example, the decision as to whether Pluto should be con- tural norms but by whether their predictions are borne sidered a planet or a dwarf planet may seem subjective.

But out by observation and experiment. As long as the results it is important to keep in mind that this controversial deci- of experiments are repeatable and do not depend on the sion, unsupported by many astronomers, was really more a culture of the experimenter—that is, as long as there is matter of semantics than of science. Science was. Furthermore, no other are funded.

For this rea- to serious ethical issues. Whether you want to design ing available for AIDS research during the decade or so after a building that will not fall over, consider the most recent its discovery. More recently, progress in stem cell research medical treatment for a disease, or calculate the orbit of a and solutions to human-caused climate change have been spacecraft on its way to the Moon, you had better consult hindered by widely distributed misinformation on the Web a scientist rather than a psychic—regardless of your cul- and by constant political bickering.

The biochemistry of the food we eat remains the same. The Sun rises, sets, and then rises again at pre- Our Lives and dictable times. Spring turns into summer, summer turns into autumn, autumn turns into winter, and winter turns Science Possible into spring. If nature did not behave according to regular comfort in them. But imagine what life would be like if patterns, then our lives—indeed, life itself—would not sometimes when you let go of an object it fell up instead be possible.

What if one day apples were essential nutrition, The patterns that make our lives possible also make sci- but when you bit into an apple the next day you discov- ence possible. The goal of science is to identify and char- ered that they were deadly? What if, unpredictably, one acterize these patterns and to use them to understand the day the Sun rose at noon and set at P.

Some of the most regular and easily iden- it rose at A. In fact, objects do fall toward sky. What in the sky will look different or the same a week. These and other patterns shape our lives. Changing patterns in the sky echo changing patterns on Earth. A month from now? A year from now? Most of or spoken word. It quite simply cannot be done, or at least us probably lead an indoor and in-town existence, removed cannot be done meaningfully. Away On the other hand, as the authors of this book we under- from the smog and glare of our cities, however, the patterns stand and as is humorously depicted in Figure 1.

Patterns in the sky mark the changing of word. Many people decide early in their education that they the seasons Figure 1. Patterns in the sky share the rhythms of our tion of the word causes their eyes to glaze over and their lives. It is no surprise that astronomy, which is the expres- palms to sweat. A distaste for mathematics is one of the most sion of our human need to understand these patterns, is the common obstacles standing between a nonscientist and an oldest of all sciences.

To move beyond this obstacle, patterns is mathematics. Mathematics is cle lies with us, the authors. It is our job to take on the role Arithmetic is about counting the science and of translators, using words to express as many concepts things. Algebra is about language of patterns. When we do use the relationships between things. Geometry is about shapes. Other types of mathematics equations mean and try to show you how equations express include topology the properties of surfaces and statistics concepts that you can connect to the world.

We will also groups of objects and their relationships. What do all these limit the mathematics to a few basic tools that all college branches have in common? Why do we consider all of them students should have been exposed to. All These basic mathematical tools, as described in Math share one thing: they deal with patterns. The best working Tools 1.

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Scientists use a basic the language of patterns, it should come as no surprise that set of units to distinguish among time, distance, mass, and mathematics is the language of science. Trying to study sci- energy. As the authors of this text, we are Algebra also lets us combine these ideas with other aware that mathematics is not a friend to many of you taking ideas to arrive at new relationships. Even so, there are a few tools that we will need Proportionality.

Much more than documents.

Rather twice as long to get here. Rather than writing out 0. For example, the distance to the Sun among speed, time, and distance. If you are traveling at a con- is ,, km, but astronomers usually express it as stant speed, then time is proportional to distance. We write 1. Ratios are the most common way that astronomers power. For example, a circle of radius r has an area A equal use to compare things. To describe and understand objects in astron- omy and physics, we use concepts such as distance, shape, This means that if you make the radius of a circle three area, and volume.

Apparent separations between objects in times as large, its area will grow by a factor of 32, or 9. In this book we use the metric system of units, more with the Sun at one focus. Geometry provides the tools for work- The United States remains one of very few countries in the ing with these concepts. Algebra provides a way of using and manipulat- acceptance of the metric system can be seen in everyday ing symbols that represent numbers or quantities. We will life. We buy milk in quart-sized containers, but our soft use algebra to express relationships that are valid not just drinks come in liter-sized bottles.

Bank signs often display for a single case, but for many cases. In some municipalities road signs is equal to the speed at which you are moving multiplied show distances in both miles and kilometers. Finally, some algebra—mostly a few ratios and propor- that we use. Do not concede defeat while still in the starting tionalities—will provide a way of expressing the patterns blocks.

It is likely that you know what it means to square a that relate one physical quantity to another. Basic does not number, or to take its square root, or to raise it to the third necessarily mean easy, but it does mean that we will use power. The mathematics in this book is on a par with what it the most accessible tools that will make our journey of dis- takes to balance a checkbook, build a bookshelf that stands covery as comfortable and informative as possible.

Whatever your expectations, the story in 21st Astronomer Century Astronomy can fascinate you if you open your mind to it. Not everyone is fascinated by mathematics or science, but Our best suggestion for a successful journey through almost everyone harbors a spark of interest in astronomy. When, spark as well. The prominence of the the secret to thinking like a scientist! Sun, Moon, and stars in cave paintings and rock drawings Reading the book and taking the journey with us are such as those in Figure 1. This is only a guidebook. It can lead tells us that these questions have long occupied the human you to the trailhead and tell you something of what you imagination.

If have grown over the years as you saw or read news reports you become an active participant in this adventure, rather about spectacular discoveries made in your lifetime. This book will take you to places you never imagined thinking, and to learn to view the world from new and unfa- going and will lead you to insights and understandings miliar perspectives. Knowledge and understanding have you never imagined having. We recognize that you may be in this course pri- Changing the way that you think takes more effort. It also marily because you need a science credit to graduate.

Above all, remember that building under- reminded of your interest in astronomy, and that led you standing is always an active process, never passive! Here to choose astronomy over other options. Or perhaps you are a few practical suggestions for how you might better study this text:. Think about each section after the Sun, Moon, and stars. What major concepts were discussed in the section? How are they related to what you have read so far? Have you run into similar concepts else- where?

Why are the contents of that section impor- tant enough to be included in the book? Many physical and mathematical concepts are most easily understood if they are visual- ized. Review the photos, diagrams, and charts in each chapter to study key concepts. Draw a picture of the con- cept. If you understand a concept well enough to draw a picture that expresses it, then you probably understand the concept fairly well. Trying to sketch a picture will also help you better identify what things you understand and what things you do not. What if Earth were more massive?

How would www. What if the Sun were hotter? Your instructor is also there to help. How would that affect the color of the light from the Sun or the amount of energy that the Sun radiates? At the end of a reading assignment, talk understanding of the physical nature of the universe.

Ask each other lots of questions—the harder the better! Getting the most from to reach out with our minds and our science to touch the this journey will come from facing and mastering these fabric of the universe itself. It is probably safe to say that few challenging concepts. You have to know the facts, but the facts the universe and embracing what we found, or whether we are the starting point, not the end. Use the key concepts, will instead be remembered for a loss of spirit—for step- study questions, and other study aids to identify and ping back and turning away from the frontier of explora- concentrate your effort on the most important ideas.

Mathematical formulas are not magical incan- science—will play a part. Rather, they are expressions of logical ideas. The journey of discovery on which we are embarking is We will always present a plain-English discussion of not always easy, but few worthwhile journeys are. A hike the idea behind any mathematics that we use.

Begin by in the mountains can at times be an easy stroll and at other focusing on this discussion. After you grasp the idea, times a more strenuous climb; but when you arrive, the view then look at the math and any equations presented in the from the top is hard to beat. Try to see how the relation- will ask you to exercise your mental muscles in different, ships between the quantities in the equation embody the possibly unaccustomed ways.

If you need help with basic math skills, worth the investment. Student Questions This progress comes courtesy of a great many Seeing the Forest for the Trees advances both in our technology and in the sophistica- tion of our physical understanding of matter and energy At the end of each chapter of 21st Century Astronomy, you and of space and time themselves.

Building on the analogy of a hike in the The insights that this age of exploration and discovery mountains, we will spend a lot of time looking in detail have brought are often far more profound and startling at the rocks and the trees, but every so often we need to than dreamed of even a few decades ago. There can be step back and look around at the forest as a whole. That revolution fundamentally Like a hike through the mountains, 21st Century altered our way of thinking about the world, as well as Astronomy will not be an effortless journey.

A new spirit of rational in ways they are not used to, and they may even have inquiry was turned on the heavens, dislodging Earth to face an old adversary or two on the trail. But muscles and humankind from the center of the cosmos. The heavens became a realm not of mysticism and In Chapter 2 we begin the journey in earnest and, as magic, but instead of physical law—the same physical with most journeys, our starting point is home.

What law that governs the behavior of matter and energy in patterns do we see in the skies of our planet Earth, and laboratories here on Earth.

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  6. This is not an easy or The closing years of the 20th century saw our knowl- gentle slope on which to begin our trek, but our vistas edge of the universe charge ahead at an ever-accelerat- will change rapidly as we climb. Suppose you lived on imaginary planet Zorg orbiting Alpha Centauri, a nearby star. Without looking back in the text, laws that shape our lives here on Earth. How many dwarf planets?

    Assuming that the number of stars in all the dwarf gal- or website without regard to your own sign. Repeat number of stars in our Milky Way Galaxy, estimate the the experiment every day for a week and keep records. Was your horoscope sign consistently the best descrip- tion of your experiences?

    A scientist on television states that it is a known fact that life does not exist beyond Earth. Would you consider 5.

    21st Century Astronomy.pdf

    If a star exploded in the Andromeda Galaxy they some- this scientist reputable? Explain your answer. Some astrologers use elaborate mathematical formulas and procedures to predict the future. It is said that we are made of stardust. Explain why this that astrology is a science? Why or why not? Imagine yourself living on a planet orbiting a star in a 7.

    A friend tells you that the reason astronomers put tele- very distant galaxy. What does the cosmological prin- scopes in space is to get closer to the planets and stars. How do you explain the real reason verse from this distant location? You run across an old newspaper with the headline 8. Explain why not. List patterns in your own life that repeat regularly. How do these patterns affect you? Which patterns are of your 9. Name some supermarket items that are sold by the liter theory that Earth was visited by extraterrestrials in instead of by the quart.

    Can you think of any tests that could support or refute the theory? Explain how the word theory differs in meaning when waves travel at the speed of light. What does this fact used in common everyday language and when used by imply about the problems you would have if you tried a scientist. What is the difference between the terms hypothesis and theory as used by scientists? The tabloid newspaper at your local supermarket theo- basis for discussing astronomical distances.

    Use the rizes that, compared to average children, children born travel time of light and more familiar units, such as miles under a full Moon become more intelligent students. If so, how could it be tested? Galaxy and the distance to the Andromeda Galaxy. Imagine the Sun to be the size of a grain of sand and years for light to reach us from the Andromeda Galaxy. Earth a speck of dust 83 millimeters mm away.

    On this In this book we say that it takes 2,, years. What scale, each light-minute of distance equals 10 mm. What would be the distance from Earth knowledge evolves with time? From the Sun to Neptune? Astrology makes testable predictions. For example, it From Earth to the nearest star? To the nearest large gal- predicts that the horoscope for your star sign on any day axies? Read each of the daily horoscopes in a newspaper distances? The average distance from Earth to the Moon is , example, the price of a bag of apples is proportional to the km.

    How many days would it take, traveling at weight of the bag of apples. How are these constants determined? The surface area of a sphere is proportional to the square How does the surface area of the a. Are both offerings equally economical? Moon compare with that of Earth? If not, which is the better deal? Show why you think so. Write 86, the number of seconds in a day and 0. Write 1. You can approxi- standard notation.

    The time t it takes for light to reach us from a distant b. Noting that there are 8, hours in a year, how fast, galaxy is equal to the distance d of the galaxy divided in kilometers per hour, does Earth move in its orbit? Use algebra to describe this c. How far along in its orbit does Earth move in one relationship more simply. A remote Internet Web page may sometimes reach your Gasoline is sold by the gallon in the United States and computer by going through a geostationary satellite by the liter nearly everywhere else.

    There are approxi- orbiting approximately 3. What is the minimum delay, in seconds, that how much would it cost per liter?

    There are 4 quarts in a gallon. What is the constant of pro- freeze more quickly than a tray of cold water when both portionality, C? Does this theory make sense to you? StudySpace is a free and open Web site that pro- c. Do the experiment yourself. Note the results. Was vides a Study Plan for each chapter of 21st Century your intuition borne out? Study Plans include animations, read- For the ebook.

    The Moon rising over the ruins of the Greek Temple of Poseidon. Patterns in the Sky— Motions of Earth 2. The weather is comfortable and the days at the Sun, Moon, and stars and tried to understand are long, with none of the hardships that accompany the time of cold and snow and long, dark nights. It is a time of what they saw. You feel a sense of contentment and are thankful to appearances to the underlying motions that cause the gods for this time when life is good.

    As the embers die those patterns. To survive, you must learn the subtle patterns of your world. Above you now are the stars that rule the seasons we feel through the year. These are the stars that bring sum- mer and lead the herds to this pleasant place. The Moon and the spectacle of eclipses. Moon sometimes turns blood red, and the Sun is consumed by an ominous beast. But as long as the tribe remembers them, the gods and spirits of the sky will continue to bring the seasons and send the stars to guide the tribe. Egyptian— B. Chinese— A. European— A. Constellations are groupings of stars whose images, such as a dipper or bear, exist only in the human mind.

    The coming of night and day, the changing of the cultural traditions in human history. But where are these seasons, the rising and falling of the tides, the movement winged horses, dragons, chained maidens, and other imagi- of the herds—all of these march in lockstep with the changes nary images formed from patterns of stars? The answer may that we see in the sky. Instead there is only the random lives of humans since before have always been pattern of stars—about 5, of them visible to the naked the beginning of recorded his- important to our eye—spread out across the sky. By watching the patterns species.

    They are the in the sky, our ancestors found ideas and pictures that humans impose on the lights in the that they could predict when the seasons would change and sky in an effort to connect our lives on Earth with the work- the rains would come and the herds would move.

    Knowl- ings of the heavens. Hemisphere draw heavily from the list compiled 2, It was a small step from here to thinking of the unreach- years ago by the Alexandrian astronomer Ptolemy. As writing came to replace oral tra- lations as a kind of road map of the sky. The entire sky is ditions and legends, mythologies of the sky became more broken up into 88 different constellations, much as conti- elaborate as well.

    And as humans invented numbers and nental landmasses are divided into countries by invisible mathematics to describe and predict and account for things lines. Every star in the sky lies within the borders of a in the world, predictions of the motions of the stars and single constellation, and the names of constellations are planets were among their greatest successes. Some of our used in naming the stars that lie within their boundar- ancestors came to look upon the orderly and predictable ies. From Africa to Asia, from Europe to Central America, from North 1 Although stars may appear close together in the sky, most are not America to the British Isles, the archaeological record holds close together in space.

    As illustrated much farther away. Appendix 6 provides airplane high overhead and are caught off guard by the sky maps showing the constellations. For a moment, the ling to be missed. The idea of the sky as a realm of mysti- thousands of years separating you from a long-dead tribal cism and magic is deeply rooted in the traditions and beliefs nomad vanish as you share the same sense of wonder and and history of our species. At a time when the causes of things were unknown, and humans existed at It is here that we begin the journey of 21st Century the seeming whim of forces they could not comprehend, the Astronomy—with the changing patterns in the sky that cap- sky seemed to offer a window into a mystical and powerful tured the attention and imagination of that long-ago nomad world of spirits, gods, devils, and angels.

    Yet unlike that nomad, we look on those chang- few remarkable individuals, with names such as Coperni- ing patterns with the perspective of centuries of hard-won cus and Kepler and Galileo and Newton, tugged at the knowledge. And just as hap- on its axis, giving the stars, Science shattered pened over the history of our species, curiosity about the planets, Sun, and Moon the ancient mystical views changing patterns in the sky will show us the way outward appearance of following daily of the heavens.

    The subtle com- plexity of the changing patterns we see in the sky results from the motions of planets and moons as they step through 2. Even the Sun itself, whose radiant energy makes our world what it is, is only When our remote ancestors first noticed the sky with one of countless stars, adrift in a universe whose full extent something approaching human awareness, it was doubt- is unknown even today.

    Today it is a what they saw. The most successful was the street on an autumn day. Today the sky has become a win- in grade school, Christopher Columbus did not discover dow of knowledge on the physical world. This knowledge that the world is round. Long before his famous or possi- has proven worth the wait. His intention was to not only stop the orcs' corruption and enslavement to the Burning Legion which ultimately lead to the destruction of their home world , but to also influence an outcome and path where the new Iron Horde pure-blooded and uncorrupted orcs could lead a new era of conquest.

    Garrosh equipped the Iron Horde with technology from his time. His intent was to re-align the Dark Portal and bring this army into the present to crush his enemies on Azeroth. Some time after Garrosh's death at the hands of Thrall , Grommash again refused to accept the blood of Mannoroth when Gul'dan once again urged him to drink it. When he refused, Gul'dan imprisoned him and assumed control of the Iron Horde. Kilrogg Deadeye was the first to drink the blood of Mannoroth and become corrupted. The foundation of all Iron Horde technology is a primitive, coal-fired centrifugal engine which can be used to turn wheels, belts, and chains, allowing for the construction of crude vehicles and siege weapons, including tanks, cannons, and chariots.

    The Kor'kron Iron Star, an early, unmounted version of this engine, was first unleashed by orcish siege engineers during the final battle of the Siege of Orgrimmar , and it is this technology that Garrosh smuggles into the past. It was suggested at BlizzCon that some of the Iron Horde may join the Horde at the end of the expansion to level out the Horde and Alliance power. However, in the southern hemisphere they have no special distinction apart from their relative brilliance.

    Constellations of the Southern Spring Now imagine that it is around P. New constellations have risen in the east, and old ones have set in the west. Lyra, the lyre; Cygnus, the swan; and Aquila, the eagle. These constellations are marked by the bright stars Vega, Deneb, and Altair.

    In the north-northeastern sky is Aries, the winged ram Fig. The fish of Pisces are, according to mythology, joined at their tails, and Aries has fleece of gold. The variable star Mira is sometimes visible in this constellation. The star Tau Ceti is thought to be a candidate for having a solar system similar to ours and possibly an earthlike planet. Near the northern horizon, Andromeda, representing a princess, rides the horse alongside the Milky Way Fig. Andromeda, as it was originally called, is too near the horizon, as viewed from the southern temperate latitudes, to present itself well to casual observers.

    Aquarius supposedly brings love and peace. In ancient mythology, this constellation was seen as a person pouring water from a jug. Aquarius, the water-bearer, traverses the northwestern sky on spring evenings in the southern hemisphere. Piscis Austrinus, the southern fish, and Grus, the crane, are near the zenith on southern-hemisphere spring evenings. Here are the prominent new constellations of the southern-hemispheric summer as they appear from the latitude of Sydney, Buenos Aires, or Cape Town around P. Sirius is also called the Dog Star and is the brightest star in the sky except for the Sun.

    Orion contains two bright stars, Betelgeuse also spelled Betelgeux , a red giant, and Rigel, a blue-white star. This constellation contains the bright star Aldebaran. Below Taurus is a group of several stars known as the Pleiades. At this latitude, the Pleiades are less spectacular than they are as seen from the northern hemisphere, but on an especially dark night, with a good wide-angle telescope, their splendor shines through.

    From extreme southern latitudes, the Pleiades never rise above the northern horizon. Orion, the hunter, contains a nebula and two well-known bright stars. Pleiades Taurus Figure Taurus, the bull, and the Pleiades, also known as the Seven Sisters although there are really far more than seven of them. This constellation is Eridanus, the river. It, like Cetus, the whale, contains a star, Epsilon Eridani, that is thought by many scientists to have a solar system like ours.

    This constellation has the general shape of a long, thin, backward letter C if you stand facing north and look up at it Fig. At the right-hand extreme are two relatively bright stars, Castor and Pollux, named after the twin sons of the mythological Greek god Zeus. It contains the bright star Capella Fig. From extreme southern latitudes, Capella Pollux Castor Figure Gemini contains the stars Castor and Pollux, and appears low in the north-northeast sky in the southern-hemispheric summer.

    Auriga, the charioteer, contains the bright star Capella. In the southern hemisphere, elevation is measured in the same way as it is in the northern hemisphere, with the following exception: a It is a negative angle rather than a positive angle. The Sky c Sirius d None of the above The spring equinox in the southern hemisphere occurs in which month?

    Orion is a landmark constellation primarily in which season south of the equator? As the night progresses for an observer in Buenos Aires, Argentina, the south circumpolar stars seem to a revolve counterclockwise around the south celestial pole. If there were no Moon, it would have been more difficult. If the telescope had never been invented, the puzzle would have been tougher still. The Moon goes through obvious changes even to the most casual observer, but the reasons for these changes were not obvious to most people 50 generations ago.

    Neither the Sun nor the Moon is a smooth globe. Both have complicated surfaces. The Moon has craters, mountains, plains, and cliffs. The Sun has a mottled surface that is often strewn with spots. The Sun and Moon together perform a cosmic dance that, once in a while, puts on a show to rival anything else in nature.

    The Moon Earth has countless natural satellites—meteors captured by gravity and orbiting in all manner of elliptical paths. The only natural satellite of significance and the only one that can be detected without powerful observing aids, however, is the Moon. The Moon orbits Earth at a distance of about , kilometers , miles. The Earth-Moon system is sometimes considered a double planet, and some astronomers think the pair formed that way. But Earth is 81 times more massive than the Moon, and the Moon has essentially no atmosphere.

    Thus, in planetary terms, the Moon is a dull place. Perhaps you have seen drawings of the Earth-Moon system and have come to envision the Moon as much closer to Earth than is actually the case. The drawings in this chapter, except for Fig. There is a reason for this distortion. If the EarthMoon system were always drawn true to scale, the illustration would be of little use for most instructive purposes.

    Earth is a bit less than 12, kilometers 7, miles in diameter, and the Moon is about , kilometers , miles away on average. If drawn to scale, the Earth-Moon system would look like Fig. Think of the Earth and the Moon as pieces of fruit. Suppose that Earth is a centimeter-diameter grapefruit and the Moon is a millimeter-diameter plum 4 inches and 1 inch across, respectively. To make a scale model, you must set the two fruits 3 meters 10 feet apart.

    Earth-moon system, drawn to true scale. The exact synodic sun-based lunar orbital period varies slightly from one orbit to the next because the orbit of the Moon around Earth is not a perfect circle and the orbit of Earth around the Sun is not a perfect circle either. The synodic and sidereal lunar orbital periods differ for the same reason the synodic day is longer than the sidereal day. Every time the Moon makes one trip around Earth, our planet has moved approximately one-twelfth of the way around the Sun. The Moon has to travel further to come into line with the Sun from one orbit to the next than it must travel to come into line again with some distant star Fig.

    The synodic lunar period is longer than the sidereal lunar period. This drawing is not to scale. Have you ever looked at the Moon, especially the full Moon, and imagined it to be closer than you remember previous full Moons to have been? The Moon orbits Earth in an elliptical path, with Earth at one focus. The Moon can get as close as , kilometers , miles and as distant as , kilometers , miles from Earth. This is a difference of This is enough to make a difference, especially when the Moon passes precisely between an observer and the Sun.

    Libration can give rise to interesting phenomena. For example, when amateur radio operators bounce their signals off the Moon to communicate with their fellows on the opposite side of the world, libration produces multiple signal paths whose lengths vary constantly, making the radio waves add and cancel in a manner so complicated that precise analysis would challenge any computer.

    The resulting received signals sound like someone babbling or hooting underwater. The wavelengths of light are too short for this effect to be observed visually. If we could see at radio wavelengths, the Moon would seem to sparkle and scintillate as if fireworks were constantly being set off all over its surface. When the Earth, the Moon, and the Sun are in line or nearly in line, the Moon is said to be new, and its existence is not visually apparent unless there happens to be a solar eclipse.

    Three or four days after the new Moon, it is a waxing crescent. About a week after the new Moon, we see half its globe illuminated by the Sun; this is first quarter. Two weeks and 18 hours after the new Moon, it is entirely illuminated for us unless a lunar eclipse happens to be taking place. This is the full Moon. Phases proceed in timely fashion after the full Moon through waning gibbous, last quarter, waning crescent, and finally, back to new again.

    Almost nobody lives in this world without getting to know the lunar phases before they get into kindergarten. The waxing crescent is visible just after sunset; the first-quarter Moon can be seen until midnight. The waxing gibbous Moon stays in the sky into the wee hours of the morning, and the full Moon is above the horizon all night, setting as the Sun rises. After the full phase, the waning gibbous Moon rises a couple of hours after sunset; the last-quarter Moon rises around midnight; the waning crescent Moon PART 1 92 The Sky waits until the predawn hours to rise.

    This is natural because there are more people living north of the equator than south of it. As far as Earth itself is concerned, however, this is only half the story. Figure shows the way the Moon looks at various stages in its orbit around Earth as seen from a midlatitude northern location such as Kansas City, Missouri, or Rome, Italy.

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    There is some variance in the tilt, depending on the season of the year; moonrise and moonset occur somewhat north New Waxing crescent First quarter Waxing gibbous Full Waning gibbous Last quarter Waning crescent Figure Lunar phases as seen from middle latitudes in the northern hemisphere. The waxing crescent appears in the southwestern or western sky just after sunset and sets 2 to 4 hours after the Sun.

    The Moon at first quarter is in the southern sky at sunset, moves generally westward, and sets around midnight. The full Moon is opposite the Sun, rising at sunset and setting at or near sunrise. The waning gibbous Moon rises some time after sunset and sets after sunrise the next day. The last-quarter Moon rises around midnight and sets around noon. The waning crescent waits until the predawn hours to rise and sets in the afternoon. Figure illustrates the appearance of lunar phases as seen from a midlatitude southern location such as Perth, Australia, or Napier, New Zealand.

    Lunar phases as seen from middle latitudes in the southern hemisphere. Depending on the season of the year, moonrise and moonset occur somewhat north or south of due east or west. The waxing crescent is in the northwestern or western sky just after sunset and sets 2 to 4 hours after the Sun. The Moon at first quarter is in the northern sky at sunset, moves generally westward, and sets around midnight.

    The waxing gibbous Moon is in the northeast at sunset, moves to the west, and sets a couple of hours before dawn. The full Moon rises around sunset and sets around sunrise, tracking across the northern half of the sky during the night. The waning gibbous Moon rises shortly after sunset and sets after sunrise the next day. The last-quarter Moon rises at about midnight and sets around noon. The waning crescent rises a couple of hours before dawn and sets in the afternoon.

    In fact, the true austerity of the Moon would surprise even the most pessimistic dreamers of old. The naked-eye Moon, especially the full Moon, has light and dark features. In absolute terms, the whole Moon is a rather dark object; it reflects only a few percent of the solar light that strikes it.

    If the Moon were as white as snow or powdered sugar, it would shine several times more brightly. However, these general ideas were as far as pretelescopic people got. Many people considered the dark areas to be liquid oceans made up of water. The light areas were assumed to be land masses, but few people supposed they were strewn with mountain ranges and crater fields. To them, Galileo was a troublemaker, and he was treated as one. He ended up spending his last years under house arrest. It was not a tyrannical government dictator that subjected him to this, but the Pope.

    Imagine the reaction the Pope would get today if he demanded that some scientist spend the rest of his life under confinement! Earth pulls on the Moon, keeping it from flying off into interplanetary space. The lunar day is about an hour longer than the synodic day—roughly 25 hours—but the Moon, like the Sun, appears to revolve around any stationary earthbound observer.

    The effects of gravity propagate through space at the speed of light, some , kilometers , miles per second. But this does not mean that the gravitational effect of the Moon can be disregarded, as any ocean beach dweller will attest. When considered to scale, the oceans form a thin, viscous coating on Earth.

    Even the deepest undersea trenches reach less than 0. Even so, the depth of the oceans is affected by the combined external gravitational effects of the Sun and the Moon. The effect is greatest when the Sun, the Moon, and Earth are all in line, that is, at the times of new and full Moon.

    In certain places, the rise and fall is dramatic, and people who live on the shore must take its effects seriously. In other places, these tides are much less extreme. There are two high tides and two low tides during the course of every lunar day; the reason for this can be envisioned by looking at Fig. However, these drawings represent an oversimplification. The actual tides are delayed by the fact that on a planetary scale water behaves more like molasses than the freely running liquid with which we are familiar.

    Also, the contours of the ocean floor and the continental shelves have an effect. This is not all: Land masses break the planetary ocean up, so wave effects cannot propagate unimpeded around the world. The tides are waves, although they are very long, having two crests and two troughs with the passage of every lunar day. Actually, the tides consist of two waves of different frequencies. Simplified diagrams showing why lunar tides occur. Superimposed on the lunar tidal waves, which have a period of about 25 hours, are solar tidal waves truly tidal in nature, unlike tsunamis, which are caused by undersea earthquakes, not by tides with a period of 24 hours, but whose crests and troughs have smaller magnitude.

    The Moon and Sun are not the only natural entities that affect sea level. Weather systems, especially ocean-going storms, have an effect, and in some places a storm surge can cause the sea to rise 10 times as much as the astronomical tide. A tsunami comes in and pounds away at the shoreline in a manner similar to that of a storm surge, except that the tsunami is caused by a jarring of the sea floor or, occasionally, by a volcanic eruption rather than by high winds piling the water up onto shore.

    Neither of these phenomena are true tides. This is so because in order for water to rise in one location, it must fall somewhere else where the lunar-solar gravitational composite is different. There has been some debate about the effects of the solar and lunar gravitational fields on the behavior of living cells. No one has yet come out with a respected scientific study that quantifies and defines exactly how such effects, if any, are manifest.

    For example, so-called Moon madness lunacy has not been explained on the basis of increased intracellular tidal effects during the full Moon. Because a similar loss of reasoning power does not seem to grip its perennial victims during the new Moon, it is almost certain that Moon madness if it really exists is not caused by gravitation. Pluto and its moon Charon are sometimes called a double-planet system, but both of them are hardly larger than asteroids and might better be classified as such.

    However, because the Moon practically qualifies as a planet, being almost as large as Mercury, there are competing theories about its origin. One theory holds that the Moon was once a planet in its own right, orbiting the Sun rather than Earth, but something, such as a collision with a large asteroid, deflected it from its solar orbit and caused it to pass so close to Earth that Earth captured it permanently. Few astronomers believe this. Another theory suggests that Earth originally had no Moon but that a Mars-sized object struck Earth a glancing blow and sent vast quantities of material into orbit.

    This would have produced rings around Earth, something like those around Saturn. These rings eventually would have congealed into the Moon. One problem with this theory is that an impact severe enough to blast that much matter off Earth might have shattered our planet. A major impact could have jarred Earth and caused its axis to shift by This is a popular theory and is fairly consistent with what we observe about the Moon.

    The Sun The Sun has been worshipped, despised, and feared by humans throughout history. Ideas about the Sun are as strange nowadays as they have ever been, but there are some statements about our parent star that we can make with confidence. The Sun has a radius of about , kilometers , miles , more than times the radius of Earth. If Earth were placed at the center of the Sun assuming the planet would not vaporize , the orbit of the Moon would fit inside the Sun with room to spare Fig. The commonly accepted mean distance from Earth to the Sun is ,, kilometers 93,, miles in round numbers.

    But the dayto-day distance varies up to a couple of million kilometers either way. Earth is farthest away from the Sun—aphelion—in July. Surprisingly enough, for those of us in the northern hemisphere, the Sun is closest in the dead of the winter. It is not Earth-Sun distance that primarily affects our seasons but the tilt of Earth on its axis. Earth-moon system would easily fit inside the Sun. The distance to the Moon had been measured by parallax, as well as the distances to Mars and Venus at various times, but the Sun defied attempts to measure its distance until someone thought of finding it by logical deduction.

    What follows is an example showing the sort of thought process that was used, and can still be used, to infer the distance to the Sun. Given a central body having a known, constant mass, such as the Sun, all its satellites obey certain physical laws with respect to their orbits. This is true no matter what the mass of the orbiting object; a small meteoroid obeys the rule just as does Earth, Venus, Mars, and Jupiter.

    The next step involves measuring the distance to Venus. If we could do this when Venus is exactly in line with the Sun, then we could figure out our own distance by simple mathematics. However, when Venus is at its maximum elongation its angular separation from the Sun is greatest either eastward or westward , the radar works because the Sun is out of the way. At maximum elongation, note Fig.

    One of the oldest laws of geometry, credited to a Greek named Pythagoras, states that the square of the length of the longest side of a right triangle is equal to the sum of the squares of the other two sides. In Fig. However, even this will only give us an approximation because the orbits of Earth and Venus are not perfect circles. In recent decades, astronomers have made increasingly accurate measurements of the distance from Earth to the Sun using a variety of techniques. You know, if you have taken chemistry classes, that hydrogen is flammable and that it burns clean and hot.

    It might someday replace natural gas for heating if a method can be found to cheaply and abundantly produce it and safely distribute it. The enormous pressure deep in the Sun, caused by gravity, drives hydrogen atoms into one another.

    The earliest theories concerning the Sun involved ordinary combustion, the only question being what, exactly, was burning. Coal was suggested as a fuel for the Sun, but if this were the case, the Sun would have burned out long ago. Besides this, there was the little problem of how all that coal got up there into space. Another idea involved the direct combination of matter with antimatter, resulting in total annihilation. The hydrogenfusion theory accounts for what we see and is consistent with theories concerning the age of the Universe and the age of Earth.

    Should we worry about the possibility that the supply will run out soon and Earth will cool off and freeze over? In fact, most scientists believe that the Sun will continue to shine for at least 1 billion more years at about the same level of brilliance as it does today. There are a lot of hydrogen atoms in that globe.

    It has a radius, remember, of , kilometers, or 69,,, centimeters. Scientists would write that as 6. Using your calculator, you can figure out, using 3. The Sun will perish. The symptoms of aging will begin in or million years. As the supply of hydrogen runs out, the Sun will expand, and its surface will cool off. The climate will become intolerably hot; the polar ice caps will melt; wildfires will reduce all plant life to ashes. Sometime during this process, any remaining humans and other mammals will die off.

    The oceans, lakes, and rivers will boil dry. All living things, even the hardiest bacteria and viruses, will die. The atmosphere will be blown off into space. By then humanity will have colonized a couple of dozen other planets and will grieve no more about the fate of Earth than we do today about the buried houses of ancient cities. If our descendants remember us at all, it will be with fascination. Knowledge of our present society might be conveyed by legend, by stories told to children at bedtime, by tales about a place called Terra that sank beneath the surface of a stormy star after its inhabitants had fled, a place where people burned the decomposed by-products of dead plants and animals in order to propel surface transport vehicles.

    After the red-giant phase, the Sun will fuse helium into carbon, iron, and other elements, and will shrink as gravity once again gains dominance over PART 1 The Sky the pressure of nuclear heat. However, this process cannot continue forever. A point will be reached at which no further nuclear reactions can take place, and then gravitation will assert its ultimate power. The Sun will be crushed into an orb of planetary size and, as the last of its heat dissipates, will fade away and spend the rest of cosmic time as an incredibly dense, dark ball.

    Chances are good that you will see at least one dark spot on that bright disk. Before the time of Galileo in the seventeenth century, the idea of sunspots did not cross the minds of people in Western civilizations—or if it did, no one ever voiced their thoughts aloud. The Sun was regarded as perfect, and if someone had suggested otherwise, they would have been disciplined or put to death.

    There is reason to believe that the ancient Chinese knew about sunspots and accepted their existence, having seen them, most likely, when the Sun was rising or setting in a hazy sky. Sunspots enable astronomers to calculate the rotational period of the Sun because the blemishes seem to move across the solar disk, disappear over the edge, and then reappear a couple of weeks later on the opposite limb. The Sun rotates approximately once a month. Careful observation of spots has revealed that the Sun spins faster at lower latitudes than at higher latitudes.

    This fact has been used to prove that the Sun is not a solid body like Earth or the Moon. Sunspots have an appearance that reminds some observers of biological cells Fig. The darkest part, at the center, is called the umbra; it is surrounded by a brighter region called the penumbra. Sunspot sizes vary, but they can be, and often are, much larger in diameter than Earth. The spots tend to form in groups and are believed to be depressions in the solar surface resulting from magnetic disturbances. They are, in a sense, storms on the Sun.

    The overall average number of sunspots rises and falls in a cycle of roughly 11 years. The most recent sunspot maximum took place in late and early A sunspot can have a diameter greater than that of Earth. If sunspots are like hurricanes on the Sun, then solar flares are like nuclear explosions: sudden, bright eruptions that send high-speed, charged subatomic particles flying off into space. Solar flares are more difficult to see with an ordinary telescope than are sunspots, but astronomers constantly watch the Sun for signs of these outbursts.

    As the charged particles come near our planet, they accelerate toward the north and south geomagnetic poles. This acceleration produces its own magnetic field, which interacts with that of Earth. If you happen to live at a high latitude, especially in North America, you are familiar with this glow as the aurora borealis northern lights.

    Similar effects take place in the south polar regions, but the aurora australis southern lights are not spectacular, except as seen from Antarctica and from the far southern ocean on rare occasions when the sky over them is not socked in with grim overcast. The ionospheric disturbances affect radio communications and broadcast, especially on the so-called shortwave bands. In the extreme, even wire, cable, and satellite communications systems are disrupted by the powerful, erratic magnetic fields. Once in a while, however, shadow effects occur and are observed by people.

    These include the corona, a pearly white mane of glowing, rarefied gases, and solar prominences, which are bright red or orange and look like flames leaping thousands of miles up from the solar surface. Any object exposed to sunlight casts a shadow consisting of an umbra and a penumbra. The penumbra extends much further from the Moon and gets wider and wider with increasing distance, as shown in Fig.

    The closer to the umbra the observer gets, the slimmer the crescent Sun becomes. However, even a narrow crescent Sun is bright and has the same observed brilliance per unit area as the full Sun. Even with three-quarters of the Sun covered by the Moon, daylight still looks quite ordinary. This is where careless observers get into trouble. The exposed portion of the Sun is likely to cause permanent eye damage if a partial eclipse is viewed directly.

    This is true even if The only safe way to look at an eclipse is the hole-projection method. Never look at the Sun directly during an eclipse. If the Moon is at perigee and Earth is at aphelion when a solar eclipse takes place, the Moon is at its largest possible angular diameter, whereas the Sun is at its smallest. This results in a spectacular total solar eclipse, and, under ideal conditions, it can last about 7 minutes. The worst conditions for eclipses of the Sun occur when the Moon is at apogee and Earth is at perihelion. Then the Moon is at its smallest possible apparent size, and the Sun is at its largest.

    In this case, a total eclipse does not occur anywhere on Earth. During an annular eclipse, the landscape and sky appear as they would in ordinary daylight through dark sunglasses. The Moon might pass beneath or above the shadow core, so darkness never covers the Moon completely; in these cases we see a partial lunar eclipse. For this reason, there is a fair chance that the Moon will plunge entirely into the umbra for a time, causing a total lunar eclipse Fig. These happen more often than total solar eclipses occur on Earth.

    An observer on the Moon would see a total solar eclipse of a truly alien sort. Imagine it: a thin red, orange, and yellow ring hanging in a black sky filled with unblinking stars, the Moonscape aglow as if with energy of its own. Progress of a lunar eclipse. The rotational period of the Sun a is synchronized with the orbit of Earth.

    On a particular day, the Moon sets around high noon. What phase is the Moon in or near? The composite tidal pull of the Sun and Moon is greatest when a Earth, the Sun, and the Moon are at the vertices of an equilateral triangle. Earth is a about 81 times more massive than the Moon. In an annular eclipse of the Sun, a the path of totality is narrow. Which of the following statements is true? The sidereal lunar orbital period around Earth is a longer than the synodic lunar orbital period. Test: Part One Do not refer to the text when taking this test.

    A good score is at least 30 correct. Longitudes are not assigned values greater than degrees east or degrees west because a doing so would result in a redundant set of coordinates. Latitudes are commonly assigned values greater than degrees east or degrees west. PART 1 5. Which of the following constellations consists of a group of stars early in their lifespans, and still shrouded in the gas and dust from which they formed?

    Scattered artificial light, such as that produced by the lights of a large city, a obscures many of the dimmer stars and constellations, which can be seen easily from locations in the outback. If Orion, the hunter, appears to be standing upright in the southern sky on a January evening as seen from New Hampshire, then at the same time, to viewers in Santiago, Chile, the hunter is a standing upright in the northern sky.

    PART 1 Test b lying on his side in the northern sky. The constellation Octans, near the south celestial pole, can be seen rising in the east on evenings in the month of a April. Assuming that interstellar travel is possible and that humans will do it someday, the constellations will not be usable by the captains of interstellar spaceships because a stars are invisible at warp speeds. The constellations will be perfectly good navigational tools for long-distance interstellar travel.

    Meridians on the Earth a are circles centered at the equator. At which of the following times of year would the number of hours of daylight change the least rapidly from one day to the next? How many hours of right ascension are there in 30 degrees of arc, measured along the ecliptic? Which constellation, easily visible at temperate latitudes in both the northern and southern hemispheres, is also known as the hunter?

    The summer solstice in London, England, occurs within a day of a March At the time of the first-quarter moon, a the Earth is directly between the Sun and the Moon. Which of the following constellations lies in the same direction as the center of our galaxy? As seen from a midlatitude location in the southern hemisphere at around sunset, the first-quarter moon a would be in the northern sky. The brightest star in the sky, other than the Sun, is a Procyon. Suppose that we make a scale model of the Earth-Moon system.

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    The Moon would best be represented as: a a basketball 30 meters away. The Sky c another beach ball meters away. Azimuth is essentially the same thing as a celestial longitude. At the north celestial pole, a none of the observed constellations are circumpolar. Suppose that you are in Calcutta, India, on March For how long is the Sun above the horizon that day, measured with respect to the center of its disk? Test d years into the future. Magellanic Clouds are a high-altitude weather phenomena visible long after sunset.

    The Sun derives its energy primarily from a hydrogen combustion. From April 1 to July 1, as viewed at P. In a few minutes on a clear, starry night, a star at the zenith will move toward the a north celestial pole. The circumference of the ninetieth parallel in the northern hemisphere is a the same as that of the equator. The two closer-in planets are Mercury, named after the Roman messenger god, and Venus, named after the goddess of love and beauty. Both of these inferior planets, as they are called, suffer temperature extremes that make them forbidding places for life to evolve or exist.

    Conditions on both planets are so severe that future space travelers will dread the thought of spending time on either of them. The harsh environments might, however, serve as testing grounds for various machines and survival gear, using robots, not humans, as subjects. Mercury rarely shows itself to casual observers; you have to know when and where to look for it, and it helps if you have a good pair of binoculars or a small telescope. Venus, in contrast, is at times the third brightest object in the sky, surpassed only by the Sun and the Moon. Maximum elongations happen quite often with Mercury because it travels around the Sun so fast.

    But ideal observing conditions are rare. Suppose that you live at temperate latitudes in the northern hemisphere. If Mercury is at maximum eastern elongation the planet is as far east of the Sun as it ever gets near the March equinox, it can be spotted with the unaided eye low in the western sky about a half hour after sunset. If Mercury is at maximum western elongation near the September equinox, look for the planet low in the eastern sky about a half hour before sunrise.

    If you live in the southern hemisphere, the situation is reversed. If Mercury is at maximum eastern elongation near the September equinox, it can be spotted with the unaided eye low in the western sky about a half hour after sunset. If Mercury is at maximum western elongation near the March equinox, look for the planet low in the eastern sky about a half hour before sunrise.

    Venus shows itself plainly much more often than does Mercury. With a light haze or with high-altitude, thin cirrostratus clouds, the planet can be blurred and the effect exaggerated. Nevertheless, ideal observing conditions occur under the same circumstances as with Mercury; Venus can be seen for several hours after sunset when it is at maximum eastern elongation and for several hours before sunrise when it is at maximum western elongation.

    The phases occur for the same reason the Moon goes through phases, and they can range all the way from a thin sliver of a crescent to completely full. Figure shows the mechanism by which an inferior planet attains its phases. The half-illuminated phases occur, in theory, at the points of maximum elongation, that is, when the angle between the planet and the Sun is greatest as seen from Earth.

    In the case of Venus, this is not quite true because the thick atmosphere of that planet has a slight effect on the position of the twilight line. The full phase of an inferior planet takes place at and near superior conjunction. Phases of the inferior planets result from their relative positions with respect to Earth and the Sun. The new phase, which takes place at inferior conjunction, is usually invisible too, but not always. On rare occasions, the same thing happens with Venus. When this happens, Mercury or Venus is said to transit the Sun.

    This is one of six planets, other than Earth, that has been known since antiquity. It got its name from the fact that it moves fast, not only as we see it, but literally, because of its proximity to the Sun. At perihelion, Mercury is 46 million kilometers 29 million miles from the Sun; at aphelion, it is 70 million kilometers 44 million miles distant.

    That is, at perihelion, Mercury gets more than twice as much energy from the Sun as it does at aphelion. This does not matter in any practical sense; from a human point of view, the surface broils in daylight and freezes at night. The weather would be more violent, the tides would be greater, and things in general would be rougher than they are.

    Mercury is more dense than any other planet in the solar system except Earth and is considerably smaller, only about 4, kilometers 3, miles in diameter. Figure shows the relative sizes of Mercury and Earth.