Chapter 1

Seeing the Light: The Art and Science of Astronomy

IN THIS CHAPTER

Observing the nature of astronomy

Focusing on astronomy's language of light

Weighing in on gravity

Recognizing how objects move through 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 caused by a tiny piece of space dust streaking through Earth's upper atmosphere.

Another pinpoint of light moves slowly and steadily across the sky. Is it an artificial satellite, such as the Hubble Space Telescope or International Space Station, or is it just a high-altitude airliner? If you have binoculars, you may be able to see the difference. Airliners have flashing 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 on a clear night. 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 Vincent 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. Everything that astronomy deals with

• Is seen from a distance
• Falls from afar like a meteorite, or is collected and brought to Earth with a spacecraft like a Moon rock
• Is discovered by studying light and particles of matter that come 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 we 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.

Astronomers also use telescopes positioned in certain regions about 1 million miles from Earth, including the James Webb Space Telescope and the Solar and Heliospheric Observatory.

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 us, your personal cosmic tour guides). Most have completed years of schooling to earn PhDs. Many study advanced physics, work with automated, robotic telescopes, or use supercomputers to simulate the history of the universe. They may never have studied the constellations (star patterns, 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 constellation Pegasus 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.)

Courtesy of Richard Tresch Fienberg

FIGURE 1-1: The Big Dipper, one of the most familiar asterisms (star patterns) in the night sky, comprises the brightest stars in the constellation Ursa Major, the Great Bear.

Amateur astronomers, who vastly outnumber professionals, 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 enjoy total eclipses of the Sun or 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 (or with telescopes) through Citizen Science projects, which we describe in Chapter 2 and elsewhere throughout the book.

Many amateur astronomers do educational outreach in their communities, explaining astronomy to school groups and public gatherings.

In this and the next two chapters, we provide you with information on how to observe the skies effectively and enjoyably.

What You See: The Language of Light

Light brings us information about the planets, moons, and comets in our solar system; the stars, star clusters, and nebulas in our galaxy; and the objects beyond.

In ancient times, skygazers didn't think about the physics and chemistry of the stars; they absorbed and passed down folk tales and myths: the Great Bear, the Demon star, the Man in the Moon, the dragon eating the Sun during a solar eclipse, and more. The tales varied from culture to culture. But many people did discover the patterns of the stars. In Polynesia, skilled navigators sailed across hundreds of miles of open ocean with no landmarks in view, no compass, and certainly no GPS. They navigated by the stars, the Sun, and their knowledge of prevailing winds and currents.

Gazing at the light from a star, the ancients noted its brightness, position in the sky, and color. This information helps people distinguish one sky object from another, and the ancients got to know them like old friends. Now you can, too. Some basics of recognizing and describing what you see in the sky are

• Distinguishing planets from stars
• Identifying constellations, individual stars, and other sky objects by name
• Observing brightness (measured in magnitudes)
• Understanding the concept of a light-year
• Charting sky positions (measured in special units called RA and Dec)

They wondered as they wandered: Understanding planets versus stars

The term planet comes from the ancient Greek word planetes, meaning "wanderer." The Greeks (and other ancient people) noticed that five spots of light move with respect to the stars in the sky. Sometimes they go steadily in one direction; at other times they loop back on their own paths. Nobody knew why. And these spots of light don't twinkle like stars do; no one understood that difference, either. Every culture had a name for those five spots of light - what we now call planets. Their English names are Mercury, Venus, Mars, Jupiter, and Saturn. These celestial bodies aren't wandering among the stars; they orbit around the Sun, our solar system's central star.

Today astronomers know that planets can be smaller or bigger than Earth, but they all are much smaller than the Sun. The planets in our solar system are so close to Earth that they have perceptible disks - at least, when viewed through a telescope - so we can see their shapes and sizes. The stars are so far away that even if you view them through a powerful telescope, they show up only as points of light. (For more about the planets in the solar system, flip to Part 2. We cover the planets of stars beyond the Sun in Chapter 14.)

The essential physical difference between stars and planets is that stars are made of gas all the way through and shine by their own light, whereas planets have rock and perhaps ice inside and shine by reflecting light from their host star (in our solar system, that's the Sun). You can read about the planets of our solar system in Chapters 6, 8, and 9; about the Sun in Chapter 10; about other stars in Chapter 11; and about planets around other stars (exoplanets) in Chapter 14.

So, why do the planets in our solar system sometimes appear to change direction in the sky as they wander across the starry background? They orbit the Sun in concentric circles (actually in slightly out-of-round ellipses), like the lanes of a running track, going counterclockwise if we treat north as "up." Here on Earth, we're in lane 3; Mercury and Venus run in lanes 1...