Stargazing Through the Ages: The Telescope Turns 400
By Marshall Perrin
13 April 2009
Die itaque septima Ianuarii, instantis anni millesimi sexcentesimi decimi, hora sequentis noctis prima, cum clestia sidera per Perspicillum spectarem, Iuppiter sese obviam fecit; cumque admodum excellens mihi parassem instrumentum (quod antea ob alterius organi debilitatem minime contigerat), tres illi adstare Stellulas, exiguas quidem, veruntamen clarissimas, cognovi.
de Sidereus Nuncius, Galileo Galilei, 1610
On the seventh day of January in this present year 1610, at the first hour of night, when I was viewing the heavenly bodies with a telescope, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself, I perceived (as I had not before, on account of the weakness of my previous instrument) that beside the planet there were three starlets, small indeed, but very bright.
from The Starry Messenger, Galileo Galilei, 1610
Some four hundred years ago, the news spread through Europe like wildfire: a strange device had been invented which made distant objects appear miraculously close. Sailors, scholars, soldiers and noblemen all eagerly sought out this high-tech wonder. The gossip reached a middle-aged math professor at the University of Padua, who immediately began trying to reverse-engineer the gadget. He soon heard that a foreign merchant had come to Venice hoping to sell one of the new far-seers to the city's ruler, the Doge, and hurried there to learn more—but when he reached the city, the professor found the salesman long gone. The hoped-for transaction had been thwarted by one of the prof's close friends, who promised the Doge that his scholarly pal could surely hack together an even better device, if only he got a bit of venture capital first.. . .
That long-ago math prof was, of course, Galileo Galilei, and his friend Paolo Sarpi's confidence was not misplaced: Galileo soon presented a telescope of his own construction to the Doge as a gift, earning himself an instant promotion to tenure on the University faculty and a doubling of his salary. Within weeks, that scope was soon surpassed by finer instruments boasting ever higher magnification. A humble scholar turned king of night vision, lifted his eyes upward and started writing down what he saw there, and the rest is history.
In fact, it's scientific history which is being celebrated all this year: The International Year of Astronomy. 2009 is a momentous year in science far beyond just being the 400th anniversary of Galileo's first gaze at the heavens. It's also the 400th anniversary of his contemporary Kepler's declaration that the planets move in elliptical orbits around the sun, the 200th anniversary of the births of both Charles Darwin and Abe Lincoln (a little-known politician most famous today for founding the National Academy of Sciences, of course), the 150th anniversary of a certain landmark science book, and the 40th anniversary of mankind's first footsteps on another celestial body. Whew! With a set of birthdays to celebrate like that, the only fitting thing to do is to throw a year-long party, and you're all invited.
The International Year of Astronomy (IYA09) was conceived as a worldwide, year-long celebration and presentation of science. Chances are there's probably a museum or planetarium near you with a packed schedule of presentations, and no matter where you live, there's a thundering torrent of online content: podcasts and blogs and virtual science fairs in Second Life, oh my! Meanwhile we professional science geeks are awaiting some fabulous presents of our own: the launch of powerful new spacecraft like the Kepler planet-hunter, Herschel far-infrared observatory and WISE mid-infrared all-sky mapper, the commissioning of new facilities like the mighty Atacama Large Millimeter Array (if only I had a dollar for every research talk these past few years that included the words "but of course ALMA will do all this incredibly vastly better once it's built [. . .]") and by far the largest telescope ever, the cubic-kilometer-sized Ice Cube neutrino detector in Antarctica. Meanwhile our old friend Hubble will hopefully get a desperately-needed makeover in early summer when astronauts visit one last time with a shuttleful of upgraded cameras and replacement parts.
The saga of the Hubble Telescope alone, from first conception to design and launch to its tragically aberrated primary mirror to its triumphant repair and scientific glory, has filled scores of books. And yet saga that is merely one chapter in the much longer history of the telescope, a history now four hundred years old. In honor of IYA09, in this column I've decided to look back at a few of the most notable scopes of the past four centuries, and how they've opened our eyes to the cosmos. The more keenly we can look outwards, the more clearly we see our own place within.
The Granddaddy of Glass
We begin, of course, with Galileo's telescope itself. Or rather telescopes, plural, as he went through three or four instruments in rapid succession as his lens-making techniques improved. Galileo's most famous discoveries were made with his third telescope, which boasted a factor of about 20 in magnification (twice what a decent modern pair of binoculars provides). Galileo's telescope was about a meter long, built using a pair of two-inch lenses, a wooden tube, and some copper wire to hold it all together. Though by modern standards its performance was rather limited, especially by the low quality of glass available then (the state of the art was "greenish and full of little bubbles," unfortunately), Galileo's skill in polishing and assembling lenses placed his instrument head and shoulders above anything else of the time. When he turned that wood-and-copper tube to the sky, he made in rapid succession more major discoveries than probably any astronomer ever since: In the space of one year, from December 1609 to December 1610, he learned that the Milky Way is in fact composed of countless millions of faint stars; saw that the Moon is mountainous, made of rock and soil like the Earth instead of being some sort of ideal perfect celestial sphere; discovered four moons orbiting Jupiter like a little mini solar system; observed strange extensions on either side of Saturn (which we know today to be the famous rings, of course), and found that Venus goes through phases just like the Moon. His observations of Jupiter's moons and Venus provided unequivocal evidence that the Earth is not the center of the cosmos, overturning at one glance the Ptolemaic model of the universe which had stood for thousands of years. (Well, "unequivocal" maybe to everybody except the Inquisition, unfortunately for Galileo . . . But the Church officially forgave him in 1992, so that makes everything better, right?)
Galileo's speed wasn't limited just to telescope-making or observing, either: By March 1610, barely half a year after he built his first scope, Galileo had already written and published his famous pamphlet Sidereus Nuncius, the Starry Messenger. He wrote letters, published books, and gave public lectures whenever possible. While his new discoveries rapidly earned the disapproval of the Church, by that point they were unstoppably widespread, carried across Europe by letter and mail coach. Were Galileo alive today, I'm sure he would be jumping on the web publishing bandwagon and posting pre-prints to the Arxiv. Galileo's passion for public communication of science played an equal role to his skill as an optician and observer in bringing about revolutionary change in our conception of our place in the universe. In that sense, modern scientists who devote time and energy to popularizing knowledge are not just following in the footsteps of the great Carl Sagan, they're acting in accordance with an unbroken line of tradition to Galileo and beyond. (So go thank your local teachers and science educators!)
Not only did Galileo's telescope revolutionize our understanding of the cosmos, it also kicked off a telescopic arms race that continues to this day. Over the course of his lifetime, Galileo built and sold dozens of telescopes (some of which still can be seen today), and his contemporaries and followers soon far surpassed his achievements. The polishing techniques of those days only worked well for lenses with low curvature, so bigger lenses inevitably meant longer telescopes—much, much longer. (As a result, early telescopes are typically described by their focal length, not by the diameter like today's telescopes are.) By the 1650s, the great Dutch astronomer Christian Huygens had a 23-foot-long telescope using lenses several inches across. Giovanni Cassini studied Saturn through a telescope some 35 feet long. A few years later Johannes Hevelius assembled in Germany a 60-foot telescope, and then in 1673 using 8-inch lenses built a monstrous 150-foot-long beast which required entire teams of workmen to point and aim. Perhaps unsurprisingly, it proved far too unwieldy and impractical to ever produce any good discoveries. Despite further efforts in implausibly-long-telescope technology (170 feet! 210 feet! even 300 feet!) refracting telescopes had reached a limit well beyond usefulness, and it seemed like no more progress could be made.
Mirror Mirror on the wall . . .
Yet just a few years earlier, across the English channel a second revolution in telescope-making had began, courtesy of the great Isaac Newton himself. Say what you will about Newton—he surely was a misanthrope, spiteful, superstitious and vain—but he was also very, very good at what he did. The idea of reflecting telescopes was not a new one—in fact, Galileo had contemplated it by 1620, and the British astronomer James Gregory shortly thereafter invented a design for reflecting telescopes which is still in use today. Yet until Newton, all those designs existed on paper only. A subtle truth of optics is that it is much harder to make a good curved mirror than to make a good lens, due to the index of refraction of glass and the double-pass nature of reflective optics. As a result, all attempts to make curved mirrors for telescopes had met with failure, resulting in at best uselessly blurry and distorted images. Newton was both sufficiently motivated and sufficiently skilled to at last develop the necessary techniques for polishing curved mirrors. By the time Hevelius was assembling his 150-foot-long refracting telescope, Newton was already on to his second reflecting telescope, which had nearly the same collecting area in a much more practical package.
By 1721, the Royal Society held a face-off between a 6-inch diameter, 5 foot long reflector made by John Hadley versus Huygen's 120-foot-long 7-inch diameter refracting scope. The far more convenient reflector was judged to have equally great magnifying power, though not quite the clarity of image as the gargantuan refractor. The next century or so of astronomical history continued this back-and-forth leapfrogging between reflecting and refracting telescopes of increasing sophistication. Reflectors generally had an edge in size (both in terms of greater diameter and increased practicality), but reflecting telescopes provided crisper images, especially once achromatic lenses were developed in around 1760. But for sheer light gathering power, nothing could beat a big reflector.
The biggest around were those carefully made in the 1780s by the great British astronomer William Herschel, discoverer of the planet Uranus. Herschel was a self-taught amateur who rose to become the world's foremost expert in mirror-polishing and telescope-making (abandoning a successful career as a musician and composer along the way). He supplemented his salary as Astronomer Royal by making and selling countless telescopes, but kept the best for himself: in particular his 18-inch diameter, 20-foot long reflector and its 40-foot big sibling, whose 50-inch diameter would hold the world record for 60 years. But the 20-foot telescope was his favorite, and with it Herschel made a slew of remarkable discoveries: He was the first to realize that binary stars were true gravitationally bound systems rather than just chance alignments, thereby proving that Newton's Laws of gravity and motion applied outside of our solar system. By painstaking observations of apparent motions of stars, he determined that the solar system is moving through space, and even measured the approximate direction. He discovered that our galaxy is shaped like a disk, studied the "diffuse nebulae" which we now know to be nearby galaxies, and invented the word "asteroid." As a fitting tribute for the master telescope maker and the discoverer of infrared radiation, the largest-ever infrared space telescope (due to launch in mid-April) will be named in his honor.
What makes Herschel's work so significant is not just his extraordinary skill as an instrument maker, it's the breadth of subjects he studied and the fundamental insights he obtained about the scope of the broader universe. Galileo's revolution was to show that the Earth is but one planet in our solar system, orbiting the sun like any other. Herschel's revolution was to show that our solar system itself, and all other nearby stars, orbit through our galaxy in a vast slow stately dance.
Refractors Strike Back
But despite the many improvements to reflecting telescope technology made by Herschel and those who came after him, such as Lord Rosse and Leon Foucault (also of pendulum fame), a hundred years later astronomers were still building refractors for their unparalleled image sharpness. The nearest and dearest of these to my heart is the Great Lick Refractor atop Mount Hamilton near San Jose, California. At 36 inches in diameter, it remains the second-largest refractor in the world today (and was always far more scientifically successful than its slightly larger cousin the 40-inch Yerkes refractor). Part of the secret to its success is that Lick Observatory was the world's first permanent mountaintop observatory when it opened in the 1880s. Forget about better telescopes for a moment, let's try putting them on vastly better sites! Lick Observatory also represented a new model for astronomy on a new continent: rather than being funded by a nobleman or a king, the financial backing came from a rich businessman, and rather than being the province of a single expert observer, the telescope was owned and operated by the fledgling University of California. The saga of Lick Observatory and how it came to be founded is a fascinating one (particularly for the eponymous philanthropist James Lick, whose story you really must read to believe. Let's just say they don't make crazy egotistical misanthropic robber baron billionaire piano-makers like they used to . . . ). The Great Lick Refractor is a gorgeous thing, all polished brass and steel and rich wood paneling like a dream from Jules Verne, with a tube 60 feet long weighing 14 tons yet balanced so perfectly you can swing it with a finger. I'll readily admit I'm biased here; Lick Observatory was where I came of age as a scientist. It's quite a thrill to gaze at Saturn or the Ring Nebula through the Lick Refractor and know the long history of great astronomers who have done likewise over the last century, and all the discoveries made there. (And to know that there's a dead guy buried underneath the telescope. See above comment about crazy robber barons . . . )
But it's worth remembering that the most significant telescopes are not always the largest or showiest. One of the first astronomers hired to work at the new Lick Observatory was a young man named Edward Emerson Barnard, another self-taught amateur who had risen to be one of the greatest comet hunters of the day (and even made a living at it, due to another rich benefactor who paid a $200 prize for every new comet found!). With the 36-inch refractor, Barnard made many significant discoveries, most notably finding the first new moon of Jupiter since Galileo's four satellites almost 300 years earlier. But his true passion was photography. Barnard had apprenticed as a photographer's assistant starting at age nine, and by the time he switched his focus to astronomy he was an expert at all sorts of photographic print making. So it's no surprise that he became one of the first great astrophotographers. He wasn't the first to try capturing the night sky on film, but he was the first to truly excel at it. While the great telescopes were suitable for taking detailed photographs, their limited field of view limited them to only small portions of the sky. Rather than a telephoto zoom, Barnard wanted a wide-angle lens to capture the full sweep of the Milky Way.
He found what he needed in a six-inch photographic portrait lens bought at surplus, cheap on the streets of San Francisco. In the shadow of the mighty 36-inch, using a jury-rigged setup with the portrait lens mounted on top of a smaller telescope for tracking, Barnard worked obsessively throughout the 1890s and early 1900s on obtaining the highest-quality photographs of the night sky. He struggled for years to produce sufficiently good prints that captured all the detail of his original negatives, eventually checking by hand tens of thousands of individual photographic prints. Barnard's Photographic Atlas of the Milky Way is a towering achievement, both stunningly gorgeous and also scientifically ground-breaking. With his surplus lens on a hacked-together retrofit telescope mount, Barnard's photographs of the Milky Way showed without a doubt that there were mysterious dark clouds between the stars. Space is not truly empty! In fact, it's filled with vast clouds and nebulae, such as the mysterious, gigantic Barnard's Loop which wraps around most of the constellation of Orion. It's the size of your outstretched hand across the sky, yet it's nearly impossible to see without long-exposure photography.
In time, Barnard's work paved the way for studies of star birth deep within these nebulae, proving that the creation and recycling of stars and planetary systems is ongoing all around us today. But more than that, it opened our eyes to the rest of the universe, metaphorically speaking, by demonstrating that our actual eyes are far from the best detectors:
Eyes differ so much, and astronomers, as a rule, are such very poor artists that we may never expect to get anything like a fair delineation of the Milky Way by the human hand along; and if we could, the human eye is too feeble to grasp the more important details.. . . The exceeding beauty of a glass positive from this plate is beyond description.
Galileo's telescope enabled new discoveries by increasing the light-gathering area above that of the human eye; Barnard's photographic setup expanded our observational capabilities in time: allowing hours-long exposures to bring out faint details, and providing accurate records of data which could last years or even centuries. As a result of Barnard and his colleagues, the 20th century became first and foremost an era of discovery driven by photography. In time, the drive for greater sensitivity would result in experimentation with first TV cameras, then other digital detectors, eventually helping give birth to the digital camera industry today. In the last thirty years or so, detector technology has exploded outwards in wavelength coverage, giving us views of the universe from X-ray to ultraviolet to infrared to radio wavelengths. Today we study phenomena that Galileo couldn't dream of, using forms of light he'd never heard of, and analyze it by the terabyte, night after night.
Yet the amazing thing is, for all our technological prowess, there's still something special about doing exactly what Galileo did, and gazing up at the heavens with our own eyes. You'd think that after a decade of studying the sky with telescopes the size of my house, I'd be jaded, that I'd have lost excitement at looking through a scope I can hold in my hands—and yet I haven't. I know my eyes are wretchedly inefficient detectors, capturing at best only a few percent of the photons that make their way into my tiny pupils, and yet they're my inefficient detectors, and I suspect gazing upwards and collecting million-year-old photons from Andromeda with them will never lose its charm. I'll bet you feel similarly (especially if you care enough about outer space to read to the end of this rambling discussion . . . ). And that's why I'm excited that one of the key projects for the International Year of Astronomy is the Galileoscope: a cheap ($15!) but high quality, easy-to-assemble and easy-to-use telescope modeled roughly on Galileo's. (Actually, it's far superior to Galileo's in almost every way optically. Well, except the plastic tube doesn't look quite as cool as the wood-and-copper original . . . ). The goal is to get literally millions of people out there looking through these telescopes this year, and in many years to come. Oh, don't worry, if you want high-tech science content (and/or are scared of getting off the computer . . . gasp!) there's ever more astronomy happening online, from pure fun to do-it-yourself big science. But there is nothing quite so evocative, so profound, as exploring the universe with your own eyes and contemplating its incredible vastness—and the equally great wonder that we can comprehend it so deeply without ever leaving home. Even after all these years, that's what backyard telescopes remain unmatched at. So get one for a kid you know (or a kid-at-heart), head out there somewhere, gaze up at the dark sky, and dream big dreams. Happy International Year of Astronomy!