CXECA08ARD302_ALR_CV.indd 1 2/23/06 6:16:59 PM Photo Credits: Cover: Digital Vision/PictureQuest; p. 2: Corbis Sygma; pp. 4–5: Royalty-Free/Corbis; p. 6: NASA/STSci Cornell/Corbis, Corbis; p. 7: NOAO; pp. 8–9: Getty Images; p. 10: NASA/Photo Researchers, Inc.; p. 11: Time Life Pictures/Getty Images; p. 12: David Ducros/Photo Researchers, Inc. Copyright © by Harcourt, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be addressed to School Permissions and Copyrights, Harcourt, Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887-6777. Fax: 407-345-2418. HARCOURT and the Harcourt Logo are trademarks of Harcourt, Inc., registered in the United States of America and/or other jurisdictions. Printed in the United States of America ISBN-13: 978-0-15-352396-0 ISBN-10: 0-15-352396-4 1 2 3 4 5 6 7 8 9 10 175 15 14 13 12 11 10 09 08 07 06 If you have received these materials as examination copies free of charge, Harcourt School Publishers retains title to the materials and they may not be resold. Resale of examination copies is strictly prohibited and is illegal. Possession of this publication in print format does not entitle users to convert this publication, or any portion of it, into electronic format. CXECA08ARD302_ALR.indd b 2/23/06 6:24:46 PM Visit The Learning Site! www.harcourtschool.com CXECA08ARD302_ALR.indd 1 2/23/06 6:14:43 PM Time or Distance? “Steer the ship to the moons of Ptolemy 5, Lieutenant.” “But those are 3 light years away, Captain!” “Exactly. Put on your seatbelt. It’s going to be a long trip.” You’ve probably heard this kind of conversation on TV or in a movie that takes place in outer space. What is a light year: time or distance? It’s sort of both. A li