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Astronomy For Dummies®

To view this book's Cheat Sheet, simply go to and search for “Astronomy For Dummies Cheat Sheet” in the Search box.


Astronomy is the study of the sky, the science of cosmic objects and celestial happenings. It’s nothing less than the investigation of the nature of the universe we live in. Astronomers carry out the business of astronomy by using backyard telescopes, huge observatory instruments, radio telescopes that detect celestial radio emissions, and satellites orbiting Earth or positioned in space near Earth or another celestial body, such as the Moon or a planet. Scientists send up telescopes in sounding rockets and on unmanned balloons, some instruments travel far into the solar system aboard deep space probes, and some probes gather samples and return them to Earth.

Astronomy can be a professional or amateur activity. About 25,000 professional astronomers engage in space science worldwide, and an estimated 500,000 amateur astronomers live around the globe. Many of the amateurs belong to local or national astronomy clubs in their home countries.

Professional astronomers conduct research on the Sun and the solar system, the Milky Way galaxy, and the universe beyond. They teach in universities, design satellites in government labs, and operate planetariums. They also write books like this one (but maybe not as good). Most hold PhDs. Nowadays, many professional astronomers study abstruse physics of the cosmos or work with automated, remotely controlled telescopes, so they may not even know the constellations.

Amateur astronomers know the constellations. They share an exciting hobby. Some stargaze on their own; many others join astronomy clubs and organizations of every description. The clubs pass on know-how from old hands to new members, share telescopes and equipment, and hold meetings where members tell about their recent observations or hear lectures by visiting scientists.

Amateur astronomers also hold observing meetings where everyone brings a telescope (or looks through another observer’s scope). The amateurs conduct these sessions at regular intervals (such as the first Saturday night of each month) or on special occasions (such as the return of a major meteor shower each August or the appearance of a bright comet like Hale-Bopp). And they save up for really big events, such as a total eclipse of the Sun, when thousands of amateurs and dozens of pros travel across Earth to position themselves in the path of totality and witness one of nature’s greatest spectacles.

About This Book

This book explains all you need to know to launch into the great hobby of astronomy. It gives you a leg up on understanding the basic science of the universe as well. The latest space missions will make more sense to you: You’ll understand why NASA and other organizations send space probes to planets like Saturn, why robot rovers land on Mars, and why scientists seek samples of the dust in the tail of a comet. You’ll know why the Hubble Space Telescope peers out into space and how to check up on other space missions. And when astronomers show up in the newspaper or on television to report their latest discoveries — from space; from the big telescopes in Arizona, Hawaii, Chile, and California; or from radio telescopes in New Mexico, Puerto Rico, Australia, or other observatories around the world — you’ll understand the background and appreciate the news. You’ll even be able to explain it to your friends.

Read only the parts you want, in any order you want. I explain what you need as you go. Astronomy is fascinating and fun, so keep reading. Before you know it, you’ll be pointing out Jupiter, spotting famous constellations and stars, and tracking the International Space Station as it whizzes by overhead. The neighbors may start calling you “stargazer.” Police officers may ask you what you’re doing in the park at night or why you’re standing on the roof with binoculars. Tell ’em you’re an astronomer. They probably haven’t heard that one (I hope they believe you!).

Foolish Assumptions

You may be reading this book because you want to know what’s up in the sky or what the scientists in the space program are doing. Perhaps you’ve heard that astronomy is a neat hobby, and you want to see whether the rumor is true. Perhaps you want to find out what equipment you need.

You’re not a scientist. You just enjoy looking at the night sky and have fallen under its spell, wanting to see and understand the real beauty of the universe.

You want to observe the stars, but you also want to know what you’re seeing. Maybe you even want to make a discovery of your own. You don’t have to be an astronomer to spot a new comet, and you can even help listen for E.T. Whatever your goal, this book helps you achieve it.

Icons Used in This Book

Throughout this book, helpful icons highlight particularly useful information — even if they just tell you to not sweat the tough stuff. Here’s what each symbol means.

remember The Remember icon points out information you should file away for future reference.

technicalstuff This nerdy guy appears beside discussions that you can skip if you just want to know the basics and start watching the skies. The scientific background can be good to know, but many people happily enjoy their stargazing without knowing about the physics of supernovas, the mathematics of galaxy chasing, and the ins and outs of dark energy.

tip This lightbulb puts you right on track to make use of some inside information as you start skywatching or make progress in the hobby.

warning How much trouble can you get into watching the stars? Not much, if you’re careful. But some things you can’t be too careful about. This icon alerts you to pay attention so you don’t get burned.

Beyond the Book

In addition to the book you’re reading right now, be sure to check out the free Cheat Sheet online. It offers a timeline of notable astronomical events and a list of famous female astronomers. To get this Cheat Sheet, simply go to and enter “Astronomy For Dummies” in the Search box.

If you want to test your astronomy knowledge, check out the practice quizzes online. Each chapter has a corresponding quiz consisting of multiple choice and true/false questions. I’ve also turned the glossary into flashcards that let you test your knowledge of astronomy terms.

To gain access to the online content, all you have to do is register. Just follow these simple steps:

  1. Find your PIN access code.
    • Print book users: If you purchased a hard copy of this book, turn to the inside front cover to find your PIN.
    • E-book users: If you purchased this book as an e-book, you can get your PIN by registering your e-book at Go to this website, find your book and click it, and answer the validation questions to verify your purchase. Then you’ll receive an email with your PIN.
  2. Go to and click Activate Now.
  3. Find your product (Astronomy For Dummies, 4th Edition) and then follow the on-screen prompts to activate your PIN.

Now you’re ready to go! You can come back to the program as often as you want — simply log on with the username and password you created during your initial login. No need to enter the access code a second time.

Tip: If you have trouble with your PIN or can’t find it, contact Wiley Product Technical Support at 877-762-2974 or go to

Your registration is good for one year from the day you activate your PIN. After that time frame has passed, you can renew your registration for a fee. The website gives you all the details about how to do so.

Where to Go from Here

You can start anywhere you want. Worried about the fate of the universe? Start off with the Big Bang (see Chapter 16 if you’re really interested).

Or you may want to begin with what’s in store for you as you pursue your passion for the stars.

Wherever you start, I hope you continue your cosmic exploration and experience the joy, excitement, enlightenment, and enchantment that people have always found in the skies.

Part 1

Getting Started with Astronomy


Discover the basic elements of astronomy, check out a list of constellations, and get a crash course on gravity.

Find out about the resources available to help you check out the night sky, including organizations, facilities, and equipment.

Get an introduction to astronomical and artificial phenomena that sweep across the night sky, such as meteors, comets, and artificial satellites.

Chapter 1

Seeing the Light: The Art and Science of Astronomy


check Understanding the observational nature of astronomy

check Focusing on astronomy’s language of light

check Weighing in on gravity

check Recognizing the movements of objects in space

Step outside on a clear night and look at the sky. If you’re a city dweller or live in a cramped suburb, you see dozens, maybe hundreds, of twinkling stars. Depending on the time of the month, you may also see a full Moon and up to five of the eight planets that revolve around the Sun.

A shooting star or “meteor” may appear overhead. What you actually see is the flash of light from a tiny piece of space dust streaking through the upper atmosphere.

Another pinpoint of light moves slowly and steadily across the sky. Is it a space satellite, such as the Hubble Space Telescope, the International Space Station, or just a high-altitude airliner? If you have a pair of binoculars, you may be able to see the difference. Most airliners have running lights, and their shapes may be perceptible.

If you live in the country — on the seashore away from resorts and developments, on the plains, or in the mountains far from any floodlit ski slope — you can see thousands of stars. The Milky Way appears as a beautiful pearly swath across the heavens. What you’re seeing is the cumulative glow from millions of faint stars, individually indistinguishable with the naked eye. At a great observation place, such as Cerro Tololo in the Chilean Andes, you can see even more stars. They hang like brilliant lamps in a coal black sky, often not even twinkling, like in van Gogh’s Starry Night painting.

When you look at the sky, you practice astronomy — you observe the universe that surrounds you and try to make sense of what you see. For thousands of years, everything people knew about the heavens they deduced by simply observing the sky. Almost everything that astronomy deals with

  • Is seen from a distance
  • Is discovered by studying the light that comes to you from objects in space
  • Moves through space under the influence of gravity

This chapter introduces you to these concepts (and more).

Astronomy: The Science of Observation

Astronomy is the study of the sky, the science of cosmic objects and celestial happenings, and the investigation of the nature of the universe you live in. Professional astronomers carry out the business of astronomy by observing with telescopes that capture visible light from the stars or by tuning in to radio waves that come from space. They use backyard telescopes, huge observatory instruments, and satellites that orbit Earth collecting forms of light (such as ultraviolet radiation) that the atmosphere blocks from reaching the ground. They send up telescopes in sounding rockets (equipped with instruments for making high-altitude scientific observations) and on unmanned balloons. And they send some instruments into the solar system aboard deep-space probes.

Professional astronomers study the Sun and the solar system, the Milky Way, and the universe beyond. They teach in universities, design satellites in government labs, and operate planetariums. They also write books (like me, your loyal For Dummies hero). Most have completed years of schooling to hold PhDs. Many of them study complex physics or work with automated, robotic telescopes that reach far beyond the night sky recognizable to our eyes. They may never have studied the constellations (groups of stars, such as Ursa Major, the Great Bear, named by ancient stargazers) that amateur or hobbyist astronomers first explore.

You may already be familiar with the Big Dipper, an asterism in Ursa Major. An asterism is a named star pattern that’s not identical to one of the 88 recognized constellations. An asterism may be wholly within a single constellation or may include stars from more than one constellation. For example, the four corners of the Great Square of Pegasus, a large asterism, are marked by three stars of the Pegasus constellation and a fourth from Andromeda. Figure 1-1 shows the Big Dipper in the night sky. (In the United Kingdom, some people call the Big Dipper the Plough.)


Photo © Jerry Lodriguss

FIGURE 1-1: The Big Dipper, found in Ursa Major, is an asterism.

In addition to the roughly 30,000 professional astronomers worldwide, several hundred thousand amateur astronomers enjoy watching the skies. Amateur astronomers usually know the constellations and use them as guideposts when exploring the sky by eye, with binoculars, and with telescopes. Many amateurs also make useful scientific contributions. They monitor the changing brightness of variable stars; discover asteroids, comets, and exploding stars; and crisscross Earth to catch the shadows cast as asteroids pass in front of bright stars (thereby helping astronomers map the asteroids’ shapes). They even join in professional research efforts with their home computers and smartphones through Citizen Science projects, which I describe in Chapter 2 and elsewhere throughout the book.

In the rest of Part 1, I provide you with information on how to observe the skies effectively and enjoyably.