Dominic Ford The Observer’s Guide to Planetary Motion Explaining the Cycles of the Night Sky The Patrick Moore The Patrick Moore Practical Astronomy Series For further volumes: http://www.springer.com/series/3192 The Observer’s Guide to Planetary Motion Explaining the Cycles of the Night Sky Dominic Ford Dominic Ford The Naked Scientists Cambridge , UK ISSN 1431-9756 ISSN 2197-6562 (electronic) ISBN 978-1-4939-0628-4 ISBN 978-1-4939-0629-1 (eBook) DOI 10.1007/978-1-4939-0629-1 Springer New York Heidelberg Dordrecht London Library of Congress Control Number: 2014937791 © Springer Science+Business Media, LLC 2014 T his work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifi cally the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfi lms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifi cally for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. T he use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specifi c statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) About the Author Dominic Ford is a science radio presenter and producer at the N aked Scientists , where he presents the monthly podcast N aked Astronomy. He is also a regular guest on BBC and ABC radio stations, talking about the latest astronomical news. In his spare time, he runs I n-The-Sky.org , a website which lists forthcoming astronomical events, and manages the website of the British Astronomical Association (BAA). D ominic previously worked as a professional astronomer at the University of Cambridge, where he completed a doctorate on the processes by which stars form. He went on to design components of the computer software needed to process observations made by the Square Kilometre Array (SKA). When it commences operations in around 2019 this will be the world’s largest radio telescope, split over two sites in Southern Africa and Australasia. Dominic also works on open-source software projects for the amateur community. In 2005, he wrote G repNova , an automated image-comparison tool for supernova hunters which has assisted in the discovery of over 50 events to date. Among its users is the British supernova hunter Tom Boles, who presently holds the world record for the largest number of supernova discoveries made by any single indi- vidual. More recently, Dominic has worked as the lead author of the vector graphics package P yxplot , which was used to produce most of the line diagrams in this book. v Pref ace A stronomy is among the oldest recorded human activities, and the planets have always held a special fascination. As seen by the unaided eye, their motion across the night sky is the only way in which the heavens appear to change noticeably from one century to the next. While the patterns of the constellations have remained largely unchanged for thousands of years, the Sun, Moon and five naked-eye plan- ets have always been the seven exceptions to the rule. Efforts to understand their motion date back to at least 1500 B C , when Babylonian court astronomers were already compiling systematic tables of observed planetary positions and trying to formulate mathematical procedures to predict their future paths. More recently, as we shall see, it was this same problem that motivated Isaac Newton to formulate his laws of motion and gravity. This book is about the ways in which the orbits of the planets produce complex patterns in their brightnesses, sizes and positions from one week to the next. It tells the story of how mankind has struggled to understand those orbits over the past centuries, and what they can tell us today about the Universe we live in. For active observers, it provides tabulated almanacs of significant events and alignments in the paths of all the planets over the period 2010–2050. In the cases of rarer events, I have been able to cover much longer time periods still, sometimes extending to 2200. These tables serve not only as catalogs of examples of each of the types of events that will be described, but also as a long-term handbook for active observers, which can be used to plan suitable dates for future star parties, for preparing ‘sky diary’ columns in advance, or simply for personal use. Since the computer revolution, the role of printed celestial almanacs has changed considerably. Nowadays, almost every amateur astronomer has access to a com- puter, and there is no need to look far to find out where the planets are on any given vii viii Preface night. There is a wide range of planetarium programs to choose from, ranging from freely available packages such as S tellarium and G oogle Sky to deluxe paid-for packages such as T he Sky and S tarry Night, to countless astronomy apps that are available for each and every type of smartphone or tablet computer. Few astronomers would now choose to pore over printed tables of planetary positions to find out where they might find Mars on any particular night. The only reason why they might is that such planetarium software remains, for the time being at least, accessible only on devices with backlit screens. Holding a planetarium program up to the night sky is a fun way to navigate when learning the constella- tions and bright stars, but at a dark site it can take several minutes to reacquire night vision after using one. Nonetheless, just as printed encyclopedias have had to evolve in response to the wealth of information that Wikipedia places at the finger- tips of anyone with access to a web browser, so too printed ephemerides have had to evolve. The drawback of planetarium software is that having so much information avail- able can make it difficult to see the wood for the trees. A recurring theme in this book is that there are long-term cycles in the positions and visibilities of the planets. Some, such as eclipses and the phases of the Moon, are very evident. Others, such as ocean tides, are no less evident, but it is not immediately obvious that they are triggered by astronomical mechanisms. The vast majority, however, are rather more subtle and can only be spotted by compiling detailed observations over long time periods. In light of this, it is rather remarkable that there is extensive evidence that ancient civilizations were very familiar with many of them. The Babylonians, for example, were fully aware by 1500 B C that days can vary in length by up to 30 s, depending on the time of year (see Chap. 4 ). I t remains interesting to study such patterns, not just to better understand when and where the planets will appear in the sky, but also to understand the gravitational and geometric mechanisms which lie behind them. Why does Saturn sometimes remain unfavorably placed for northern-hemisphere observers for several years at a time? Why does Mars appear much brighter at some oppositions than others? Why does Venus move from the evening sky to the morning sky in only 4–5 months, but take over a year to make the return journey? Such are the questions with which astronomers have had to grapple over the centuries in working out the three- dimensional structure of the solar system and the gravitational forces that act between the planets. As we shall see, in modern times, new questions have been added to the list. Why did the solar system form with all of its gas giants further from the Sun than its terrestrial planets? Are Saturn’s rings a permanent structure, or a temporary result of some recent event? Why do we sometimes have to insert leap seconds into our system of timekeeping? This book is free from equations, relying instead on diagrams of the physical geometry of the solar system, so as to be accessible to readers with little or no mathematical background. It is intended both for amateur astronomers who are Preface ix interested in the mechanics which underlie the motion of the solar system and also for science students who are interested in the historical and observational origin of the physical models of gravity and planetary motion that remain in use by astronomers. Cambridge, UK Dominic Ford March 2013