Praise for Parallax “An accessible and charming history of how we know what we know about our place among the stars.” —Providence Journal “Alan Hirshfeld’s engaging account moves to a satisfying climax as the exciting race to find stellar parallax heats up. It’s a thrilling detective story!” —Owen Gingerich, Research Professor of Astronomy and the History of Science, Harvard-Smithsonian Center for Astrophysics “An enthralling, behind-the-scenes glimpse into the lives of . .. heroic stargazers.” —Science News “Writing in the genre of Dava Sobels Longitude, Hirshfeld sheds light on the important problem of finding our cosmic place.” —David Levy, science editor, Parade “[Parallax shows] historic figures as people, full of false hopes, fear of failure, jealousy, victory, and joy. This comprehensive work... will appeal to astronomy buffs and curious readers alike.” —Astronomy “Alan Hirshfeld’s authoritative and gripping tale of the search for stellar parallax makes me proud to be a part of such a relentlessly curious and persistent species.” —Chet Raymo, columnist, The Boston Globe, author of 365 Starry Nights, and professor of physics and astronomy at Stonehill College in North Easton, Massachusetts “This book is not just a keeper; it will also become a gift.” —Sky & Telescope “In this thrilling history of the search for parallax, Hirshfeld urges us to ‘fly together.’ The human failures and frustrations and the crucial discoveries in the quest for parallax are as thrilling as the story of the determination of longitude, and just as important.” —Jane Langton, author of two astronomical mystery novels, Dark Nantucket Moon and The Shortest Day “I thoroughly enjoyed reading this excellent book. It is an admirable account, and I’m sure it will be of great value to many readers.” —Sir Patrick Moore, host of the BBC’s The Sky at Night “Hirshfeld breathlessly annexes familiar astronomical legends ... and his social history ... engages.” —Publishers Weekly “Parallax is a fascinating celestial detective story, written in a beautifully lucid, engaging style.” —Dorrit Hoffleit, senior research astronomer, Yale University “Parallax, like Dava Sobels Longitude, is a wonderfully told story of the challenge of measuring our place in the universe. It reaches the stars and the ride is exciting and irresistible.” —Margaret J. Geller, Smithsonian Astrophysical Observatory “Fun, readable, humorous, gripping, fascinating, sophisticated, informative, and suspenseful all at the same time. And it’s about astronomy. Wow.” —Metrowest Daily News “With this highly readable and cosmically accessible book, Alan Hirshfeld has done for the measurement of the cosmos what Dava Sobel did for the measurement of longitude. Readers will never again look into the night sky the same way.” —Michael Shermer, publisher, Skeptic magazine, author of Why People Believe Weird Things and How We Believe: The Search for God in an Age of Science “A delightful history of a crucial advance in knowledge.” —Booklist “Alan Hirshfeld has done a magnificent job of telling the story of the race to measure the cosmos. His perspective, in keeping with the best tradition of astronomical history, provides rich insight into the progress and personalities of those who push technology along its cutting edge.” —Leif J. Robinson, editor emeritus, Sky & Telescope PARALLAX Aian Hirshfeld, Professor of Physics at the University of Massachusetts Dartmouth and an Associate of the Harvard College Observatory, received his undergraduate degree in astrophysics from Princeton and his Ph.D. in astronomy from Yale. He is author of Parallax: The Race to Measure the Cosmos, The Electric Life of Michael Faraday, and Eureka Man: The Life and Legacy of Archimedes, as well as a forthcoming title about the birth of modern observational astronomy. A regular reviewer of science books for the Wall Street Journal, he has lectured at educational institutions nationwide about scientific history and discovery. ALAN. W. HIRSHFELD PARALLAX The Race to Measure the Cosmos Dover Publications, inc. Mineola, New York Copyright Copyright © 2001, 2013 by Alan W. Hirshfeld All rights reserved. Bibliographical Note This Dover edition, first published in 2013, is a revised and updated republication of the paperback version of Parallax: The Race to Measure the Cosmos, first published in 2002 by Henry Holt and Company, New York. It was originally published in hardcover in 2001 by W. H. Freeman and Company, San Francisco. The author has provided a new Postscript for this Dover edition. Library of Congress Cataloging-in-Publication Data Hirshfeld, Alan Parallax : the race to measure the cosmos / Alan W. Hirshfeld. — Dover ed. p. cm. “This Dover edition, first published in 2013, is a revised and updated republication of the paperback version of Parallax: the race to measure the cosmos, first published in 2002 by Henry Holt and Company, New York. It was originally published in hardcover in 2001 by W. H. Freeman and Company, San Franciso. The author has provided a new postscript for this Dover edition.” Includes bibliographical references and index. eISBN 13: 978-0-486-31591-1 1. Parallax—Stars. I. Title. QB813.H57 2013 523.8'1—dc231 2013003386 Manufactured in the United States by Courier Corporation 49093901 2013 www.doveipublications.com To Sasha, Josh, and Gabe, the stars of my universe Contents Introduction Part 1 1 Reinventing the Cosmos 2 The Circle Game 3 What if the Sun Be Center to the World 4 Crossed Eyes and Wobbling Stars Part 2 5 The Heavens Erupt 6 The Turbulent Lens 7 The Wrangler of Pisa 8 The Archimedean Engine 9 A Coal Cellar with a View 10 Double Vision Part 3 11 Dismal Swamp 12 The Twice-Built Telescope 13 Quest for Precision 14 So Many Grasshoppers 15 The Star in the Lyre 16 The Subtle Weave Epilogue A Drink from the Well Postscript Acknowledgments Notes and Further Reading Index Introduction Meet the photon. Not just any photon. Your photon. What is a photon? The fundamental unit of light: a submicroscopic jot of pure energy, an astronomical Hermes delivering the message of the stars. Nothing appears to distinguish your photon from the trillions of others that breach the surface of the distant star. All are descended from photons born in the star’s fiery core. All have survived the turbulent, outbound journey through the star’s gaseous envelope. In fact, only one thing sets your photon apart from the others streaking into outer space. This photon is destined to enter your eye. This photon belongs to you. Your photon never slows along its journey through interstellar space, never strays from its straight-line course. After centuries in the void, it plunges into the Earth’s atmosphere, fortuitously avoiding annihilation by air molecules and dusty pollutants. Meanwhile, darkness has fallen on your side of the Earth. Strolling underneath the night sky, you look up. The photon enters your eye, strikes your retina, and turns over its luminous energy to the biochemical process we call “sight.” Your photon, plus countless others that precede and trail it, paint in your consciousness the impression of a luminous speck in the heavens. On this clear, dark night, thousands of such specks are visible, altogether a twinkling stellar tapestry arching high above your head. This is the visual siren song that has beckoned observers like yourself since the dawn of humanity. And it is the sight that started me, now some forty years ago, on the road to becoming an astronomer. From the astronomer’s perspective, there is much more to starlight than the visual wallop of a star-studded sky. Bound up in these outer-space photons are clues to the nature of the stars themselves. In the pages that follow, you will read of the astronomers who struggled for centuries to wrest from starlight one fundamental stellar parameter: the distance to a star. There is no way to determine the distance to a star by a casual inspection of the night sky. With the notable exception of our Sun, stars appear as but luminous pinpoints. Any observable difference in their apparent size stems from the combined distortions of the Earth’s atmosphere and the optical instruments—including the eye —through which we observe the heavens. Thus, one might use a star’s apparent size to measure the unsteadiness of the air or the optical properties of a telescope, but not to gauge stellar distance. Likewise, a stars brilliance reveals nothing about its remoteness. A visually bright star might be a moderate light-emitter sitting on our solar system’s doorstep—or it might be a luminous “supergiant” star parked halfway across the Galaxy. To measure a star’s distance requires a pair of human attributes that astronomers have long nurtured: patience and cleverness. The pathway to the stars is rooted in an everyday phenomenon called parallax. Parallax is the apparent shift in an object’s position when viewed alternately from different vantage points. Parallax is a primary basis upon which our eyes gauge distance within our surroundings. Distance and parallax go hand in hand: The farther away an object, the smaller the perceived parallax shift. Ancient astronomers had hoped to apply this parallax principle to render distances of celestial objects, such as the Moon, Sun, planets, and stars. Except for the Moon, they were utterly defeated. The cosmos appeared to be far larger than had been supposed. This book is divided into three parts. The first part chronicles efforts to prove that stars might display a measurable annual “wobble” because of parallax. The linchpin of this assertion is a moving Earth; setting our planet in motion provides it with different vantage points upon the heavens. Thus, stellar-parallax proponents sought to overthrow the traditional model of the cosmos, in which the Earth is central and immobile, and replace it with the revolutionary model of Copernicus (and his predecessor Aristarchus), in which the Earth orbits the Sun.