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 r
ises 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.
Your Place in the Universe Page 5