the moon Kosmos A series exploring our expanding knowledge of the cosmos through science and technology and investigating historical, contemporary and future developments as well as providing guidance for all those interested in astronomy. Series Editor: Peter Morris Already published: Jupiter William Sheehan and Thomas Hockey The Moon Bill Leatherbarrow The Sun Leon Golub and Jay M. Pasachoff The Moon Bill Leatherbarrow reaktion books For Alex and Zoe Published by Reaktion Books Ltd Unit 32, Waterside 44–48 Wharf Road London n1 7ux, uk www.reaktionbooks.co.uk First published 2018 Copyright © Bill Leatherbarrow 2018 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publishers Printed and bound in China by 1010 Printing International Ltd A catalogue record for this book is available from the British Library isbn 978 1 78023 914 9 contents Preface 7 1 Our Companion Moon: From Mirror to Museum 9 2 The Moon as a World: Observation and Discovery in the Telescopic Age 23 3 The Moon in the Modern Age 62 4 The Moon in the Age of Spacecraft Exploration 94 5 Observing the Moon 125 References 167 Further Reading and Resources 172 Acknowledgements 174 Photo Acknowledgements 175 Index 177 The surface of the Moon. Preface The aim in writing this book has been not only to provide an accessible account of current lunar science, but to uncover the history of observation and discovery that gave rise to that science. What we know about the Moon today does not exist in isolation, but has emerged from an unfolding process that has seen awareness of our satellite pass through various stages and revolutions. We cannot fully understand our Moon unless we also understand that process of discovery. We shall see in the course of the following chapters just how radically our perception of the Moon has shifted over time as our knowledge and techniques have improved. Initially an inexplicable, and at times alarming, presence in our skies – one that gave rise to all kinds of superstitions and religious beliefs – the Moon then evolved into an embodiment of our increasing understanding of celestial mechanics, as we uncovered the mysteries of its movements and appearances. Subsequently, with the invention of the telescope it was revealed as another world, one that was perhaps similar to our own and possibly capable of supporting life. With the advent of the Space Age that dream was laid aside; our Moon turned out to be a rocky, airless and uninhabited world, but one that was no less fascinating. A geologist’s paradise, it provides for those willing to decipher it a dramatic account of much of our solar system’s history and an important key to the understanding of worlds elsewhere. 7 the moon That process of discovery is one that can be replicated by the amateur observer, and this book also sets out to provide practical guidance for the owner of a small telescope. It is possible to under­ stand our Moon by reading books about it, but nothing brings home the true meaning of science better than active participation in it. 8 one Our Companion Moon: From Mirror to Museum The Moon has always been the most obvious feature in our night sky. It is our nearest celestial neighbour, at an average distance of 384,400 km, and with an equatorial diameter of 3,476 km it subtends an angle of approximately half a degree in the sky, large enough to display significant detail even to the unaided eye. More­ over, since it describes a regular orbit around the Earth (or, more accurately, around the common centre of gravity of the Earth–Moon system) in a period of only 27.3 days, it is more or less a constant presence in our skies, going through its familiar monthly phases of New Moon, Half­moon and Full Moon. This period of revolution is about two days shorter than the 29.5 days that the Moon takes to complete that cycle of phases (its synodic period, or the time between successive New Moons). The reason for this is that while the Moon is revolving around the Earth, the Earth is in turn continuing along its own orbit around the Sun. So the Moon has to travel that bit farther each month in order to produce the alignment of Sun, Moon and Earth that gives rise to New Moon (or indeed any other phase). Our Moon has drawn observers since the dawn of humankind, and all have tried to make sense in their own ways of the puzzles it poses and the questions it raises. What is it? How did it get there? What should we make of the appearances, or phases, it presents? Why does it move as it does? What does it mean to humankind, and what opportunities and threats might it pose? 9