Introduction Cosmic View

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Secrets of the Deep Sky

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Strange is our situation here upon Earth.

Each of us comes for a short visit, not knowing why,

Yet sometimes seeming to divine a purpose.

Albert Einstein (1879-1955)

On a clear night in a place where the sky is really dark, you can see about 2000 stars with your unaided eye. You can look trillions of kilometers into space and peer thousands of years back into the distant past.

As you gaze at the stars you may wonder: What is the pattern or meaning of the starry heavens? What is my place in the vast cosmos? You are not alone in asking these questions. The beauty and mystery of space have always fascinated people.

Astronomy is the oldest science—and the newest. Exciting discoveries are being made today with the most sophisticated tools and techniques ever available. Yet dedicated amateurs can still make important contributions.

This book will teach you the basic concepts of astronomy and space exploration. You will more fully enjoy observing the stars as your knowledge and understanding grow. You will be better able to surf the Web and to read more on topics that intrigue you, from ancient astronomy to the latest astro-physical theories and spaceflights.

As you teach yourself astronomy, refer to:

The Star maps and Moon map at the back of this book. These special, easy-to-read maps will help you locate and identify particularly interesting objects in the sky.

Simple activities you can do that demonstrate a basic idea.

St a

^ Internet link to spectacular images and new reports.

Now, begin reading about the enormous tracts of space and time we call the universe, and stretch your mind!

Our home is planet Earth, a rocky ball about 13,000 km (8000 miles) in diameter suspended in the vastness of space-time (Figure I.1).

Figure I.1. Earth photographed from space. Sunshine dramatically spotlights Earth's blue ocean, reddish-brown land masses, and white clouds from the Mediterranean Sea area to the Antarctica polar ice cap.
Figure I.2. Planets orbiting the Sun in the solar system. (Drawing not to scale.)

Earth belongs to the solar system (Figure I.2). The solar system consists of one star—our Sun—plus planets, moons, small solar system bodies, and dust particles, all of which revolve around the Sun. The solar system is more than 15 trillion km (9 trillion miles) across.

The Sun and the solar system are located in one of the great spiral arms of the Milky Way Galaxy (Figure I.3). Our immense Milky Way Galaxy

Sun with its planets and other bodies

Sun with its planets and other bodies

Figure I.3. The solar system in the Milky Way Galaxy.

includes over 200 billion stars plus interstellar gas and dust, all revolving around the center. The Milky Way Galaxy is about 100,000 light-years across. (One light-year is practically 10 trillion km, or 6 trillion miles.)

Our Milky Way Galaxy is only one of billions of galaxies that exist to the edge of the observable universe, some 14 billion light-years away (Figure I.4).

Figure I.4. Nearly 10,000 distant gallaxies in a patch of sky just one-tenth as big as the full Moon, in the constellation Fornax. Each galaxy includes billions of stars.

And that inverted bowl we call the Sky Where under crawling coop't we live and die Lift not your hands to it for help—for It As impotently rolls as you and I. Rubaiyat of Omar Khayyam (1048-1131)

UNDERSTANDING THE STARRY SKY

And that inverted bowl we call the Sky Where under crawling coop't we live and die Lift not your hands to it for help—for It As impotently rolls as you and I. Rubaiyat of Omar Khayyam (1048-1131)

Objectives

^ Locate sky objects by their right ascension and declination on the celestial sphere.

^ Identify some bright stars and constellations visible each season.

^ Explain why the stars appear to move along arcs in the sky during the night.

^ Explain why some different constellations appear in the sky each season.

^ Explain the apparent daily and annual motions of the Sun.

^ Define the zodiac.

^ Describe how the starry sky looks when viewed from different latitudes on Earth.

^ Define a sidereal day and a solar day, and explain why they differ.

^ Explain how astronomers classify objects according to their apparent brightness (magnitude).

^ Explain why the polestar and the location of the vernal equinox change over a period of thousands of years.

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