Of course it was Sir Isaac Newton, the smartest person since himself, who figured out how Kepler's system works. For decades the question had been burning in the minds of academics: Ellipses certainly seemed the way to go—they were way too useful to be ignored—but seriously? Ellipses? How the heck do we explain that?
From Kepler's own work and additional Deep Thoughts, scientists (or, at least, protoscientists) realized the sun must exert some sort of cosmic influence on the planets. The concept of nested crystal spheres, so en vogue centuries earlier, was simply discarded, not so much due to any particular work or polemic—nobody stood up and said, “That's it, folks, crystals are out”—but through negligence. Elliptical spheres are kind of hard to nest, after all, and they simply weren't cool anymore.
Still, though, how does it all work? What is the connection between the sun and the planets, between the Earth and the moon, and among the moons of the giant worlds?
The question had been bugging the minds of England's Royal Society, the group partly devoted to serious discourse on scientific matters and partly devoted to drinking, for a few decades. Notable members such as Edmund Halley, Robert Hooke, and others took a ponder or two at the problem. According to Newton, it was his own flash of insight that made the tremendous leap in thought that connected the cosmos together. Of course, we only have his word for it, so make of it what you will.1
Outbreaks of plague make it hard for a Royal Society to be a society, and for a university like Cambridge to be a university, so in 1666, cultured life was suspended, and Newton was chilling at his mom's house in Lincolnshire, waiting for people to stop dying so he could get back to work. In the meantime, he walked around thinking all day.
By this time, he had already begun to develop his conceptions of the laws of motion: that it takes a force to make something change its velocity, that the change in velocity is proportional to the force applied and to the object's mass, and that if one object applies a force to another object, then that other object will simultaneously apply an equal force in the opposite direction on the first.
Everyone since there'd been an anyone knew that when you dropped something, it fell to the Earth. But when Newton happened to watch an apple detach from its tree and fall to the ground, he made a connection to his laws of motion—a connection nobody else in the history of anybody had made—and a mental puzzle piece slid into place.
The apple wasn't just falling to the Earth. The apple was accelerating toward the Earth. That meant that the Earth was exerting a force on the apple. That force was invisible, but the apple didn't seem to care: it fell. But only in a straight line. It didn't curve or zigzag. This “gravitational force” only connected objects in straight lines, from center of mass to center of mass.
What if the apple fell from a greater height? The force would be slightly weaker, since it would be farther from the Earth. What if the apple were moving sideways when it first started falling? Well, it would still be moving sideways, but it would still fall down.
Now for the big jump—are you ready? What if the apple were as far away as the moon? This “gravity” would be pulling it inward toward the Earth, but if it were fast enough, the apple would stay in orbit forever. What speed would that require?
Presto bingo, Newton was able to follow the logic train to derive the speed of the moon's orbit.
He didn't stop there. Once he realized that gravity might be universal, that the same force that pulls an apple from a tree might be the exact same force that keeps the moon in orbit around the Earth, he went nuts. In a good way. Example after example, he was able to show that all sorts of disconnected phenomena across the known universe were really the manifestation of a few simple laws.2
What is the source of universal gravitational attraction? Even Newton didn't attempt to go down that road. It works, he argued, so let's just go with it. And the big bow to put on the gravitational present: Newton was able to show that Kepler's laws—the elliptical orbits, the speeds, the harmonies, the whole lot—were a result of universal gravitation. One guess about how the universe works was enough to tie together Kepler's entire opus.
Sir Edmund Halley was a big fan of Newton's work, and he went about trying to put the universal in Newton's universal gravitation. Halley was also a huge history geek, and if you read any of his astronomy papers, you quickly find yourself being treated to summaries of entire ancient cities. Riveting stuff, if you're into that sort of thing.3
The twin passions of astronomical minutiae and historical minutiae led Halley to some seriously non-minute conclusions. You may already be familiar with him from his famous comet, whose reoccurrence he predicted by noting a pattern in the historical record and using universal gravitation to tie it together.
He also totally nailed the prediction of an eclipse to hit England in 1715, which gave him instant celebrity status around the country.4 Solar eclipses were notoriously hard to predict (as the ancient Chinese astronomers found, to their headless dismay) and the attempts of our ancestors to forecast them based on complicated and interweaving patterns, subpatterns, almost-repeating cycles, and exceptions to the rules is almost sad. They tried so hard, but they couldn't quite crack it because they simply didn't have the right tool.
With universal gravitation, though, Halley was able to predict the next total solar eclipse to within four minutes. In the eighteenth century, that's practically atomic-clock-level accuracy. He even made handy-dandy maps detailing what you would see when and where. If you've paid any attention at all to modern-day maps of eclipse paths, you can thank Halley for setting the standard. He nailed that sucker.
Just as easily as Halley could turn his newfound superpowers to predicting the future, he could use them to understand the past (remember, he was a history dork). He was especially fascinated by records of eclipses and liked making maps of what ancient peoples would have experienced during those events.
The oldest one he could get his nerdy little hands on stretched back to about 900 BCE in the Middle East, after he interpreted (and corrected!) the translations and retranslations passed down through the centuries. And he spotted a slight, niggling issue.
Flexing his universal gravitational muscles, Halley could handily run the clock backward and compare predicted (postdicted?) eclipses to the actual historical records. At first everything was bang on, with each result of Newton's laws matching what folks wrote down so long ago. But far enough back, errors started to creep in, and the further he pushed into the past, the greater the divide between theory and experiment.
Understanding and confusion II: Left, Halley's amazing achievement in accurately predicting the 1715 total solar eclipse and giving an eager public a detailed map of the event. Right, more than 150 years later, Sir Norman Lockyer's sketches of various nebulae and clusters still defy explanation.
Halley didn't really know what to say about it. Newton's universal gravity was so gosh-darn universal that it was hard to discount it. But the historical record was the historical record. Assuming there wasn't some giant millennia-spanning conspiracy to fudge the eclipse records, he had to take them at face value.
Halley added a brief note as a closing remark to a long treatise on the long-dead city of Palmyra (you can try to visit the ruins in modern-day Syria), along the lines of “Hey, guys, I think the moon is doing something funny, but I haven't confirmed it yet, so hold on. Be right back.”5
And he never brought it up again.
But others did, and they confirmed Halley's suspicions: by carefully combining Newton's laws with the historical record, they could deduce that eclipses were slowly getting further apart.
After a bit of math (well, truthfully, a metric ton of math over the course of a few decades, not getting fully resolved until the mid-1800s), the answer was worked out. Indeed, the moon was slowly receding from the Earth, prolonging the duration between eclipses. That recession is caused by the same tides that the moon is responsible for.
When the moon is overhead, a lump of water rises up and tries to meet it: a tide is born. But the Earth is spinning, so it carries the tidal lumpy bit farther ahead of the moon's position. That leaves a giant blob of mass sitting “in front” of the moon from its perspective, and that lump, being massive, pulls on the moon, as gravity is wont to do. Like an invisible gravitational leash, the tide tugs on the moon, giving it energy and booting it to a higher orbit.
That means in a few hundred million years, the moon will be small enough in our sky that total solar eclipses will be impossible. So enjoy them while they last!
This is fine and dandy. Indeed, it was another spectacular result for Newton's brainchild. But what it meant about the past was a little more troubling. If the moon is moving farther away from the Earth, then simple kindergarten logic dictates that it used to be closer. And in the distant past it was so close it must have…touched…the Earth?
The universe was different in the past. And not just a little bit—wildly, fantastically different. So different it defies logic and common sense. It's a big pill to swallow, and that was a big reason for the objection to even working on the eclipse problem for a few decades. But eventually the math won out, as it usually does, and everybody had to accept that fact.
Their only solace was that you have to go waaaaay into the past for the moon to be anywhere near Public Displays of Affection distances to the Earth, like hundreds of millions of years into the past. And there's no way the Earth could be that old, right?
Right?
By 1800 William Herschel was already a superstar. There are only three people in all of human history who can lay claim to discovering a new planet in the solar system, and one of them (Clyde Tombaugh, who discovered Pluto in 1930) was later disqualified on a technicality. In 1781, Herschel was the first to grab that title,6 and had it been me, in all honesty the seventh planet of our home system would be called Sutter's Awesome Planet. But Herschel wasn't me, so after a few rounds of suggestions everyone settled on Uranus, the Greek god of the sky, thereby ensuring that generations of English-speaking school kids would have something to giggle about when memorizing the planets.
Just let that soak in for a moment. No, not the Uranus puns—the concept of a new planet. Planets are pretty easy to spot, if you're dedicated enough. They are the “wanderers”; they move, ever so subtly, across the background of the distant stars from night to night. Uranus itself is faintly visible to the naked eye on a clear, dark night (which the ancients had in abundance), but unless you're really looking for it, it's easy to miss.
Herschel wasn't exactly looking for it—he was hunting for ever-fainter stars—but he did notice a discrepancy between different observations. And almost overnight, our cozy little planetary family added a new member. I don't know how pre-Copernicus thinkers would have handled the discovery of a new planet. Just added another crystal sphere to ferry the new celestial denizen? Updated all the astrological charts with signs and portents and significance? “Oh, that's why we didn't predict you would get smallpox—we were missing the influence of Uranus!”
We'll never know, because Uranus was discovered in 1781 and not 1581, and everybody went crazy with the news (“news” was also now a thing) and Herschel was an instant astronomy legend.
Nineteen years later, he was playing around with light. A couple of generations earlier, Newton had already shown that white light was really a mixture of all the colors. A simple prism is enough to demonstrate the effect, but what Newton showed was that a prism wasn't creating the colors from white light but simply separating the colors already inherent in the beam.
Herschel got the bright (ha!) idea to measure the temperature of bits of light: Is red hotter than blue? Or vice versa? Or the same? Good old-fashioned science-type questions that only a science-type person would be bothered to (a) ask and (b) actually try to answer.
So he split a beam of sunlight using a prism and started sticking homemade thermometers on various colors and dutifully recording the results. Ever the careful observer, he put thermometers on either end of the rainbow as an experimental control.
But control it did not. Herschel noticed something funky going on: the thermometer sitting outside the red part of the spectrum was warmer than any other color! And it wasn't just a freak accident of experimental design: he started playing with these invisible “colorific rays” (a fancy term for “heat rays”) and discovered they did all the same stuff that normal light did. He could reflect them, refract them, absorb them with certain materials, transmit them through others, and on and on. These rays had all the same properties as light; they were just redder than the reddest thing we could possibly see with our eyes. Infra-red, if you will.
With a one-two punch, Herschel knocked our knowledge of the universe on its back: A new planet! And a new kind of light!
The cosmos was getting complicated, fast.
The telescope wasn't helping the situation at all, but at least folks like Charles Messier were taking the time to write things down. Take a moment to think about the sky that you see in your backyard with the naked eye versus what sky even a small telescope reveals. Galileo almost had his mind blown by his crude instrument's portrait of the heavens—shapes, textures, and depth that our lowly iris simply can't capture.
The fixed stars (though as we quickly learned, they're hardly “fixed”) weren't stuck to the outermost celestial sphere. Pick an empty patch of sky. It looks like nothing's there: pure, velvety, smooth blackness. Point a telescope there. What do you see? Stars. Loads of them. Pick an empty patch among them. Get an even bigger telescope and point it there. What do you see? No points awarded for guessing the correct answer.
The number and variety of creatures inhabiting our universe grew with every decade. A menagerie of comets, nebulae, multiple stars, other kinds of nebulae—it went on and on. It seemed endless and bountiful and utterly confusing.
Take just the nebulae, for example. Taken from the Latin word for “mist,” the name stuck for obvious reasons. If you see something in the sky that seems to be (a) far away and (b) not a star, it's a nebula. Some you can see with your eye, but most can only be viewed with an astronomical helper. And it's a sampler box out there: all manner of shapes and sizes and a dazzling array of colors.
Just check out the Messier catalog, a list of fuzzy objects that definitely aren't comets compiled by French astronomer Charles Messier in the later 1700s.7 Comet hunting was big business in those days, and so many excited astronomers were ecstatic to find something new in the sky but quickly disheartened to learn it was not a new comet but an already-identified fuzzy patch.
Messier wanted to fix that (probably mostly for himself, as he was a comet spotter extraordinaire, but it also turned out to be useful for other people), so he listed, in no particular order, a collection of fuzzy things. Some were really just clumps of stars. Some were mostly round and bland. Some had strange helical patterns and interwoven colors. Some were vast, with vague spiral-like appendages. They were all beautiful—there was no doubt about that—but they were downright mysterious.
This theme—“Let's explore the heavens with no clue what we're looking at”—resonates throughout the nineteenth century. The instruments of astronomy had advanced way beyond the capabilities of astronomers to understand their own observations. Problems mounted and intensified. Ever get hungry but not know what you're hungry for, and your indecision only makes the hunger grow? The 1800s were like that, but for science.
Here's another example: the rings of Saturn. First spotted by Galileo himself with his homespun optics, they appeared as two lumps on either side of the great planet. Over time they would flatten and disappear, only to return later and fatten up again. The very next generation of astronomers after the Italian realized that they were looking at a disk and Galileo's frustrated observations were caused by alignment: sometimes he would see them face-on, and other times edge-on, depending on our position relative to Saturn in the solar system.
But to him, it was just question marks all the way across the page. “Has Saturn swallowed his children!?” he wrote in a letter, perhaps only half-jokingly referring to the Greek myth.8 By the late 1800s the mystery was still unresolved (astronomy joke, sorry). We knew that it was rings, and that there were gaps, and that it wasn't a solid disk but made up of smaller particles. That last bit was proven by the great James Clerk Maxwell, the genius who united the forces of electricity and magnetism into a single unified description—electromagnetism—and also basically discovered light. Smart dude, right? As to his explanation for the cause and composition of the rings? Got nothin’.9
For those of you keeping track: no, we still don't fully understand the rings of Saturn today, despite having Hubbles and spacecraft.
I'm confident that Joseph von Fraunhofer wasn't planning on completely revolutionizing the field of astronomy when he got too caught up in staring at the sun in the early 1800s, but he totally managed to do that, so here we are.
We already talked about how Newton demonstrated that white sunlight was really a mixture of all the colors of a rainbow, which leads to a very natural question: how in blazes does the sun produce all the colors of the rainbow? If you hold a candle up to a prism, you also get a rainbow effect. So now you know that the sun, like a candle, is both hot and glowy. A somewhat mild accomplishment, but an accomplishment nonetheless.
Working in relative ignorance as the why of rainbow, Fraunhofer (and others before him) decided to tackle the what in more detail. By passing the prismed sunlight though even more prisms, he could spread the light out farther than anyone had before, and it was in the enhanced sunlight spectrum (because the word “rainbow” doesn't sound sciencey enough, I guess) that Fraunhofer found something fishy.
Specifically, he saw something missing. Embedded in the spectrum of sunlight were hundreds of distinct dark lines, no wider than a hair, at seemingly random places within the colors. So our sun isn't giving us, for example, 100 percent of the color yellow—we're only getting 99.9 percent of that color, with very specific wavelengths nibbled out.
But these wavelengths are not missing from the spectrum of a simple candle flame. Aha: the sun isn't quite what it seems to be.
It wasn't for a few more decades that a puzzle piece clicked into place when Robert Bunsen (of “the burner” fame) and Gustav Kirchhoff (of “who?” fame) figured out that when specific elements were tossed into a flame, bright lines would pop out of the flame's spectrum. It's as if every element has a fingerprint—a pattern of lines in an otherwise featureless spectrum that is unique to that element.
Perhaps—work with me here, guys—when an element adds its light, we get bright lines appearing, but when an element blocks a background light, the same lines appear, but dark. Like a cosmic crime scene, the spectral fingerprints point the way to the suspect elements.
And now we can figure out what the sun is made of. And the stars. And nebulae. And planet atmospheres. And…everything. Granted, at this time nobody understood why the elements produced these strange lines (and even the concept of “element” was still gaining ground in chemistry circles), but what mattered was that they did, and we could test that in a laboratory in the back of the office.
That's kind of a big deal. This technique, known as spectroscopy (because, again, “rainbowscopy” doesn't sound sciencey enough), is the ultimate key that unlocks the farthest reaches of the universe. We can taste the surface of the sun without having to visit it, simply by comparing sunlight to various heated gases. We can discover brand-new elements, as Jules Janssen and Joseph Lockyer did in 1868 when they identified in sunlight a never-before-understood series of lines as helium, which they named in honor of our own sun.
It wasn't all roses and unicorns, though. Every new discovery in science leads to a thousand more questions. Hey, there's oxygen in that nebula way over there, awesome! Wait—how did oxygen get into that nebula? Way over there?
It gets even better/worse. Once folks started to come to terms with the concept of light as waves (waves of what would have to wait until 1865, when Maxwell realized that they're waves of electricity and magnetism), there was another trick that spectroscopy could play. If you've ever heard something loud passing by, you're familiar with the Doppler effect: on the approach, sound waves get squished, pushing them into higher pitches. On the way out, sound waves get stretched, pulling them to lower tones.
It happens with sound waves, and it happens with light waves. It's a very subtle effect, though, since most things don't move very fast compared to the speed of light itself, so it's not like we see entire colors shifting redder or bluer. But the fingerprint pattern of spectral lines can shift. It's a wonderful tool: perhaps you recognize a particular arrangement in the spectrum of a star—oh, there's some iron!—but the whole pattern is shifted to the left or right by a few wavelengths. Well, if the spectrum from that star is shifted toward the blue end compared to something stationary, like a light source in the room you're sitting in, not only can you conclude that the star is moving, but you can measure very precisely its speed.
Not its entire speed—sideways movements won't change the spectrum from our perspective—but the in-out speed is fair game for measurement. And measuring the speed of star after star revealed that we don't live in a fixed cosmos. We live in a beehive.
There's one other piece of technology that opened a window of confusion to our universe: photography. Where the telescope acts as a super-eye that creates a bigger bucket to collect light and magnifies separations to make them more distinct, adding a photographic plate to the back end of that device amps it up to a hundred. No matter how good your telescope is, if you only look with your eyes, you'll be fundamentally limited by what you can see.
But a photographic plate can collect, collect, collect. Restless and unblinking, it continually absorbs light, adding it to the pile, revealing fainter and dimmer objects. And it records! No longer do you have to alternate between staring and sketching to record what you're seeing—the photograph does it all in one handy-dandy, convenient device.
Astrophotography is the ultimate extension of the human sense of sight into the cosmos. It's everything a human eye does, just way better. Combined with spectroscopy—the study of spectra—it opened up the heavens like never before. Information poured in from observatories across the globe. Expeditions were launched; telescopes were fashioned by professionals and enthusiasts alike. Never before had so much interest been focused onto the night sky. The number and variety of phenomena in the universe around us were almost overwhelming. Breathlessly, astronomers recorded and published their findings in between sessions of staring dumbstruck at their celestial revelations.
Over the course of the eighteenth and nineteenth centuries, we discovered and cataloged new kinds of nebulae. We confirmed that the distant stars were like our sun but also different. Some were smaller, some larger. Some hotter, some cooler. Comets came and went on repeatable cycles. Our solar system was belted with a ring of asteroidal debris. Dozens of moons danced around the outer worlds of our solar system. Dust glinted in the pale sunlight and was flung out between the stars themselves. The universe itself was beginning to open up before us.
I'm always hesitant to pull random quotes out of history just to mock them, because it's kind of challenging to predict the future, but this one is too juicy to pass up. In 1835, the philosopher Auguste Comte wrote, “I regard any notion concerning the true mean temperature of the various stars as forever denied to us” due to their extreme distance.10 He wrote many other things that turned out to be useful and respected, but in this one instance, the scientific community, after decades of labor, analysis, and careful study, responded with a resounding “Bite me.”
But nature has a habit of biting back, and I'm sure Heinrich Olbers thought he was making a really great point in 1823. Turns out he was far from the first person to have this thought, but he was pretty much the last, so his name got stuck to an apparent paradox.11 The paradox goes back to the old sun-centered versus Earth-centered arguments of generations past. In the solipsistic view, where we here on Earth are the literal center of the universe, the fixed stars are simply that: fixed. There are a few thousand of them, and they're all attached to the outermost celestial sphere, wheeling away through cosmic time. No big deal.
But in a sun-centered universe, you have to grapple with the distinct possibility that the fixed stars are really distant suns. And as soon you point your telescope into the deep void between them, you find other, fainter—and possibly more distant—fireballs.
So how far back in space does it go? How deep could we possibly perceive? What lies in between the in-between? Do stars stop? Is there a limit to their light? If that's the case, does that kind of universe even make sense? To have a cosmos filled with void, except a little cluster of lights in one corner?
Isaac Newton provided one answer. If the universe were finite in space, then eventually gravitational interactions would, over the course of uncountable eons, cause all the stars to accumulate into a single pitiful lump.
The easier thought to swallow is that the universe is infinite. It simply goes and goes, with countless stars backed by ever-larger multitudes. Thus any spot in space is perfectly gravitationally balanced by the infinity on either side.
But how far back in time does it go? Various religious traditions teach about the (re-)creation of the physical universe at distinct points in the past, but if there's one thing the Copernican revolution taught us, it's that maybe we should give the scientific method a chance at answering some of those questions.
The Earth isn't going anywhere soon, and neither is the sun. And neither are the stars, or comets, or nebulae. They're just there. Sure, a new star will occasionally appear, or a comet will break apart upon encountering the inner solar system, or the moon might be slowly circling away from us, but for the most part, the universe today looks like the universe yesterday. And the day before, and the day before. Maybe the universe is simply infinite both in space and time. Maybe there is no beginning, no primordial ooze, no “Let there be.” The universe is.
But that doesn't work, and that's where Olbers’ paradox comes in. In a universe with infinite extent in both time and space, there shouldn't be any dark. If you look in any random direction in the sky in an infinite universe, then you must be looking in the direction of a star, somewhere, at some distance. “But maybe the light hasn't reached me yet,” you retort. Good point, except that in a universe that has existed for eternity, there's been way more than enough time for that light to reach out.
So night shouldn't be night at all; instead it should be aglow with the fire of literally an infinite number of suns. But it's pretty dang dark, which means the universe isn't infinite in time and/or space. But all the lines of thinking and evidence point toward infinity. What gives?
It will take me a lot of words to fully deconstruct the apparent paradox in detail—and don't worry, I certainly will in later chapters—but the short version is that the universe is definitely not infinite in time (at least, into the past) and most likely not infinite in space. But our dear nineteenth-century friends didn't know that, so they had to grapple with the central conundrum.
They attempted to tackle it one step at a time, and the first step is getting a distance to a star: any star at all will do. Just give us one hook into the extrasolar system, and we can start putting together a map of the cosmos and figure out the flaw in Olbers's reasoning.
I'm never one to call Newton naïve, but he did advocate a naïve method for measuring distances. If you assume that all the stars are the same true brightness—in essence, that they're all identical copies of the sun—then if you can measure their perceived brightness with incredible precision, you can do some math and figure out a distance. Astronomers over the following decades confirmed that most stars are totally unlike our sun in color and temperature and elemental composition, so that's a nonstarter. The method isn't totally without merit, and later generations will use the same principles to great effect, but that's a story for a later chapter.
Instead they had to give parallax a try. Parallax is the simple geometric measurement where you pick a star, record its absolute position in the sky, then wait six months until the Earth is on the opposite side of the solar system. Repeat your measurement, and if you're very good and even luckier, you'll have recorded a small shift in its position. That gives you an angle, and since you (hopefully) know the distance to the sun, you can construct a long skinny triangle toward the star, do some basic trigonometry, and compute a distance.
Simple, but not easy. We first encountered Tycho Brahe himself attempting a parallax measurement to put the nail in the coffin of all this sun-centered universe nonsense. He succeeded: according to the very best measurements the world had ever produced (ahem, his own), there was no observed parallax, and hence if the sun were the center of the cosmos with the Earth flinging itself about it, that would mean the celestial sphere had to be…let's see here…seven hundred times farther away than the planet Saturn. Preposterous that the universe should be so large! Earth-centered it is, chaps.12
Even though Kepler and then Newton won the sun-centered day based on other arguments, the problem stuck. Surely somebody would eventually measure a reliable parallax, get a fix on a star, and start to put the nomy in astronomy. But decades, and even centuries, churned by without a measurement. Telescopes got bigger and better. Catalogs of the heavens grew thick with entries. Innovative techniques were developed and deployed. The heavyweights I've already introduced in this chapter all took a crack at it.
Nothing. Not a single distance. With every failed attempt, we had to stretch the yardstick of space out farther. With every false report, the universe grew larger: the greater the distance to the stars, the smaller the seasonal wobble, and the better our instrumentation had to be. It was getting kind of scary, honestly.
Finally, after centuries of previous attempts and years of his own hard labor, Friedrich Bessel nailed it: 61 Cygni, a star in the constellation Cygnus that's unremarkable except that it's close to Earth. Bessel didn't know it was close, but he guessed based on its larger proper motion over the decades and centuries. (“Proper” here is a bit of astronomical jargon to mean motion that belongs to the star itself, not due to any “fake” motion that we might observe from the rotating vantage point on the surface of the Earth.)
It was already realized that stars move of their own accord, even before later spectroscopic measurements would confirm it. Since the stars are very far away (as has been established), it takes time for their motion to be noticed, but noticed and measured they can be, and it was (correctly) argued that if a star is closer to the Earth, it should have a bigger proper motion, because that's how geometry works: cars moving across the intersection right in front of you will have greater proper motion than ones a few blocks away, even if the cars are all moving at the same speeds.
61 Cygni is one of the fastest stars, so Bessel figured it would give him a shot of winning the big prize, and he was right. In 1838, after a few years of observations, he came up with a distance of ninety-six trillion kilometers. That's right, “trillion” with a terrible t.
Remember, just a couple of centuries earlier, Tycho Brahe was nauseated by the thought of the stars sitting seven hundred times farther from the sun than Saturn. Bessel's measurement, which is only 10 percent off from the current best measurement, placed 61 Cygni about sixty thousand times farther than Saturn.
In one clean measurement, Bessel (who, I feel compelled to note, received no higher formal education and also managed to develop suites of mathematical functions that bear his name today) finally put to rest the ultimate question of the heliocentric debate. It was already on firm theoretical grounds thanks to Kepler, Newton, and others, but this was a key piece of data that had been missing from the arguments.
Other parallax measurements quickly followed (Bessel wasn't the only one interested in the problem). The universe was getting larger and more complex with every new telescope and every new catalog. It was a heyday for the experimentalist and a nightmare for the theorist. None of it made any sense: just how big is our home, and what is it made of? These stupidly simple questions were getting frustratingly hard to answer.
At about this same time, astronomy was finally splitting off from astrology. For millennia, the two had been intertwined, and the words were roughly synonyms. Measurements of the stars went hand in hand with their forecast effects on our daily lives. In a complex, chaotic world where nothing made sense and everything was changing all the time, it was no wonder our ancestors looked to the steady, regular patterns overhead and drew solace from them. The clockwork regularity of the stars and planets must hold the keys to the underlying order of life on Earth.
But by the close of the nineteenth century, we knew that the universe at large was as frightful and complex as anything here on Earth—and even more so. There were forces and motions at play too great to comprehend. 61 Cygni itself was computed to have a proper motion of hundreds of thousands of miles per hour. How does a star, a massive burning ball of gas, achieve such incredible feats? How can new stars appear and familiar ones go silent? How can Newton's laws account for all this?
The telescope, the spectrometer, and the photograph opened up the cosmos before us, but it was a cosmic Pandora's box. We struggled and grasped to connect the physics we were learning on the Earth—electricity and magnetism, heat and energy, chemistry and the element, and other hot topics of the day—to the scales of the heavens, and we failed, terribly.
It was becoming painfully obvious that the cosmos was not connected to us, did not care about us, and did not care for us. We were an ant climbing on a branch of a vast tree that was incomprehensibly larger and more complex than we ever thought. We were reaching out with our enhanced senses and the powerful tool of the scientific method, and we were not liking what we were seeing.
While astrologers clung—as some still do today—to the notion that the motions of the planets govern and predict our lives, astronomers were left in a much more befuddled state. They could record, measure, and study, but they could not understand.
By the early twentieth century, astronomers were especially concerned by the nature of the spiral nebulae—just one branch of that fuzzy family tree, but one that seemed to be different from the others. How far away were they? Were they part of our universe—an unbroken field of stars stretching from one end of the cosmos to the other—or somehow isolated from us in their own “island universe”? Data and argument swung either way depending on who was more persuasive and whose data you believed to be more reliable.
Our understanding of the universe was at a breaking point. A hurricane of raw data was slamming into previously held notions. We couldn't crack the code; we couldn't navigate the storm of conflicting ideas and theories.
Our perception of the cosmos was ready, if you will, for a phase transition.