Sunshine StateSTANDARDSSC.C.2.3.7: The studentknows that gravity is auniversal force thatevery mass exerts onevery other mass.SC.E.1.3.3: The studentunderstands that ourSun is one of manystars in our galaxy.SC.H.2.3.1: The studentrecognizes that pat-terns exist within andacross systems.

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KEY CONCEPT

Some space objects arevisible to the human eye.

EXPLORE Distance

How far is the Moon from Earth?

PROCEDURE

Tie one end of the string around the middle of the tennis ball. The tennis ball will represent Earth.

Wrap the string 9.5 times around the tennisball, and make a mark on the string at thatpoint. Wrap the aluminum foil into a ballaround the mark. The foil ball will represent the Moon.

Stretch out the string to put the model Moon and Earth at the right distance compared to their sizes.

WHAT DO YOU THINK?• How does the scale model compare with your previous

idea of the distance between Earth and the Moon?• How many Earths do you estimate would fit between

Earth and the Moon?

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MATERIALS• tennis ball• aluminum foil

(5 cm strip)• string (250 cm)• felt marker

We see patterns in the universe.For most of history, people had very limited knowledge of space. Theysaw planets and stars as points of light in the night sky. However, theydid not know how far those bodies were from Earth or from eachother. Early observers made guesses about planets and stars on thebasis of their appearance and the ways they seemed to move in the sky.Different peoples around the world connected the patterns they saw inthe sky with stories about imaginary beings.

BEFORE, you learned

• Earth is one of nine planetsthat orbit the Sun

• The Moon orbits Earth• Earth turns on its axis every

24 hours

NOW, you will learn

• How the universe is arranged• How stars form patterns in

the sky• How the motions of bodies in

space appear from Earth

MAIN IDEA WEBRecord details about patterns in space.

FCAT VOCABULARYsolar system p. 674galaxy p. 674universe p. 674constellation p. 676

VOCABULARYorbit p. 674

We still have much to learn about the universe. Within the last fewhundred years, however, new tools and scientific theories have greatlyincreased our knowledge. In this chapter you will learn about thearrangement of planets and stars. You will also learn about the ways in which astronomers explore and study space.

Arrangement of the UniverseIf you look up at the sky on a clear night, you will see only a tiny fraction of the planets and stars that exist. The number of objects inthe universe and the distances between them are greater than mostpeople can imagine. Yet these objects are not spread around randomly.Gravity causes objects in space to be grouped together in different ways.

The images on page 675 show some basic structures in the uni-verse. Like a camera lens zooming out, the images provide views ofspace at different levels of size.

Earth Our planet’s diameter is about 13,000 kilometers (8000 mi).This is almost four times the diameter of the Moon, which orbitsEarth. An is the path of an object in space as it movesaround another object because of gravity.

Solar System Earth and eight other major planets orbit the Sun.The Sun, the planets, and various smaller bodies make up the

The Sun is about 100 times greater in diameter than Earth. You could fit more than 4000 bodies the size of theSun between the Sun and the solar system’s outermost planet at itsaverage distance from the Sun. The Sun is one of countless stars inspace. Astronomers have detected planets orbiting some of theseother stars.

The Milky Way Our solar system and the stars you can see withyour bare eyes are part of a galaxy called the Milky Way. A is a group of millions or billions of stars held together by their owngravity. If the solar system were the size of a penny, the Milky Waywould stretch from Chicago to Dallas. Most stars in the Milky Wayare so far away that our galaxy appears to us as a hazy band of light.

The Universe The is everything—space and all the mat-ter and energy in it. The Milky Way is just one of many billions ofgalaxies in the universe. These galaxies extend in all directions.

Astronomers study space at each of these different levels. Somefocus on planets in the solar system. Other astronomers study distantgalaxies. To learn how the universe formed, astronomers even studythe smallest particles that make up all matter.

check your reading What is the relationship between the solar system and the Milky Way?

universe4

galaxy

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solar system.

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orbit

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674 Unit 6: Space Science

reading tip

The word orbit can be a noun or a verb.

Gravity causes objects to be groupedtogether in space.

Structures in the Universe

Chapter 19: Exploring Space 675

How are these structures different from oneanother? How are they similar?

Earth1

We live on Earth, a planet thatorbits the Sun.

Solar System2

The solar system contains the Sun, ninemajor planets, and many smaller objects.

Milky Way3

The Sun and billions of other stars are groupedtogether in a galaxy called the Milky Way.

Universe4

Billions of galaxies are spread throughout the universe.

VISUALIZATIONCLASSZONE.COM

View images of thenight sky takenthroughout the year.

ConstellationsIf you want to find a particular place in the United States, it helps toknow the name of the state it is in. Astronomers use a similar systemto describe the locations of objects in the sky. They have divided thesky into 88 areas named for the constellations.

A is a group of stars that form a pattern in the sky.In the constellation Cygnus, for example, a group of bright stars formthe shape of a flying swan. Any other objects in that area of the sky,such as galaxies, are said to be located in Cygnus, even if they are notparts of the swan pattern. The ancient Greeks named many of the constellations for animals and imaginary beings.

Unlike the planets in the solar system, the stars in a constellationare usually not really close to each other. They seem to be groupedtogether when viewed from Earth. But as the illustration above shows,you would not see the same pattern in the stars if you viewed themfrom another angle.

check your reading What relationship exists among the stars in a constellation?

constellation

676 Unit 6: Space Science

Constellation Patterns

The stars of a constellation are often farapart from one another, but they appeargrouped together when viewed from Earth.

Which two stars in Cygnus are farthest apart from each otherin space?

1500 light-years

1000 light-years

500 light-years

Delta Cygni

Sadr

DenebAlbireo

1 light-year = 9.5 trillion kilometers

Gienah Cygni

Cygnus (the Swan)

The sky seems to turn as Earth rotates.

You cannot see all of the constellations at once,because Earth blocks half of space from yourview. However, you can see a parade ofconstellations each night as Earth rotates. Assome constellations slowly come into view overthe eastern horizon, others pass high in the skyabove you, and still others set at the western horizon. Throughout the ages, many peoples haveobserved these changes and used them to help in navigation and measuring time.

If you extended the North Pole into space, itwould point almost exactly to a star called Polaris,or the North Star. If you were standing at the North Pole, Polariswould be directly over your head. As Earth rotates through the night,the stars close to Polaris seem to move in circles around it. Althoughnot the brightest star in the sky, Polaris is fairly bright and easy tofind. You can use Polaris to figure out direction and location.

check your reading What causes constellations to change positions during the night?

Constellation PositionConstellation Positions

The stars in this imagewere photographed overseveral hours to showhow they move across thenight sky.

How does time of day affect the positions of constellations?PROCEDURE

Cut out both diagrams on the Constellation Wheel Sheet and assemble them as shown.

Rotate the wheel so that the current month is aligned with 9 P.M. Observe the positions of the constellations.

Align the current month with other times to determine how the positions of the constellations change during the night.

WHAT DO YOU THINK?• How do the positions of the constellations change

during the night?

• In which direction does the northern sky seem to turn?

CHALLENGE Earth’s rotation makes the sky seem to turn. What does the model tell you about the direction of Earth’s rotation?

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SKILL FOCUSAnalyzing

MATERIALS• Constellation

Wheel Sheet• scissors• brass fastener

TIME20 minutes

Chapter 19: Exploring Space 677

The movements of planets and other nearbyobjects are visible from Earth.

A jet plane travels at a greater speed and altitude than a bird. Yet if abird and a plane flew overhead at the same time, you might think thatthe bird was faster. You would have this impression because the fartheraway a moving object is from you, the less it seems to move.

Stars are always moving, but they are so far awaythat you cannot see their movements. Observers haveseen the same constellation patterns for thousands of years. Only over a much longer period does themotion of stars gradually change constellation patterns.

By contrast, the Moon moves across the starbackground a distance equal to its width every houras it orbits Earth. The Moon is our closest neighbor.The planets are farther away, but you can see theirgradual movements among the constellations over a period of weeks or months.

Planet comes from a Greek word that means“wanderer.” Ancient Greek astronomers used thisterm because they noticed that planets move among

the constellations. It is easiest to see the movements of Venus andMars, the two planets closest to Earth. They change their positions in the sky from night to night.

The apparent movement of the sky led early astronomers tobelieve that Earth was at the center of the universe. Later astronomersdiscovered that Earth and the other planets orbit the Sun. The time-line on pages 736–739 introduces some of the astronomers whohelped discover how planets really move in the solar system.

KEY CONCEPTS1. What are the basic structures

in which objects are groupedtogether in space?

2. What is a constellation?

3. How does Earth’s rotationaffect our view of stars?

CRITICAL THINKING4. Compare and Contrast

How is the grouping of stars in a constellation differentfrom the grouping of planetsin the solar system?

5. Apply The planet Jupiter isfarther than Mars from Earth.Which planet seems to movefaster when viewed fromEarth? Explain.

CHALLENGE6. Predict Suppose that you

are standing at the North Poleon a dark night. If you keepturning clockwise at the samespeed as Earth’s rotation, howwould your movement affectyour view of the stars?

678 Unit 6: Space Science

The Motion of Mars

These illustrations show how Mars changes positions in the constellation Gemini over a period of two weeks.

Sept. 20th

Mars

Oct. 4th

Mars