The top 5 astronomical discoveries of all time (so far)

We’ve managed to discover quite a lot about our Universe from our relatively limited vantage point here on Earth. Many of those discoveries have been worthy of nothing more than an updated entry in some catalog. But some have been deeply revolutionary, completely changing the way we view the cosmos and our relationship to it.

What follows is a list of what I, a theoretical cosmologist, believe to be the most impactful discoveries ever made in astronomy. To help winnow down the possibilities to a manageable top-five ranking, I had to concoct some criteria. First, we're looking at discoveries that are both broad and deep (in the scientific sense), findings that simultaneously reached further than any previous discovery and also enabled (or at least accelerated) a new paradigm or branch of astronomy.

Second, I want to emphasize discoveries that were not obvious and didn’t just need someone to build a big enough telescope or powerful enough computer. I want discoveries that needed radical leaps of intuition and science-minded daring—where an enterprising scientist went out on a limb and followed their curiosity wherever it led.

Lastly, these sorts of lists will always include bias, so let me put mine front and center. I am a theorist, so I'll naturally be more inclined to find theoretical insights more interesting, relevant, and horizon-expanding than purely observational exploits. That philosophy will help shape my list.

I’m sure you'll have your own picks, and you may or may not disagree with the rankings I’m about to present. That’s fine. In fact, I hope the following list provides a springboard for discussion and, because science is fun, celebration of our many accomplishments.

So without further ado, presented in chronological order because I couldn’t make myself rank them by order of importance, I present to you the greatest astronomical discoveries of all time.

So far. According to me.

1) We’re gonna need a bigger boat

This first discovery is so old that we don’t even have direct access to the work of the man who accomplished it, a certain Greek polymath by the name of Eratosthenes. Living around 250 BCE, Eratosthenes was the first to develop an accurate method for measuring the circumference of the Earth. And like all great theorists before and since, he didn’t even need to get out of his pajamas to do it.

We only know of the work of Eratosthenes from a summary provided centuries later by another Greek astronomer, Cleomedes, who is mostly famous for… telling us about Eratosthenes. According to his summary, Eratosthenes calculated the circumference of the Earth by waiting for the summer solstice. At the solstice, the Sun stood directly overhead at noon at the city of Syene (today Aswan) in southern Egypt. Eratosthenes lived in Alexandria, several hundred miles north, so at that precise moment, the Sun was a little off from directly overhead. By measuring the angle of the Sun’s position and combining that with the known distance to Syene (something calculated by professional walker-measurers), Eratosthenes could calculate Earth’s circumference. He arrived at a startlingly accurate measurement, within a few percent of the modern value.

Presumably by the time of Eratosthenes, anyone who was paying attention already knew the Earth was round—the point of this work wasn’t to disprove flatness but to measure the circumference of an already assumed globe. But Eratosthenes was perhaps the first person in history to make a measurement of something far beyond direct human perception. There was no way for anybody to send teams of walker-measurers out to travel the entire circumference; instead, Eratosthenes devised a clever trick that used our relation to the heavens to let the Sun do the measuring for us.

Eratosthenes wrangled celestial objects and made them do his bidding. This was no mere astrology, with its tortured attempts to use the stars and planets to divine the fortunes of us mortals on Earth. This was astronomy, using careful and clever measurements to discover something new about the world.

2) A call to order

At the dawn of the scientific revolution, many prominent thinkers, philosophers, and even theologians (to be clear, there wasn't a large difference between these groups at the time) helped us make radical leaps in our understanding of the Universe. We had Galileo with his fancy telescope, Copernicus with his wild heliocentric idea, and more.

And then we had Kepler. Johannes Kepler. The brightest pupil of Tycho Brahe (who was perhaps the greatest astronomer to ever live, at least according to one T. Brahe), Kepler would go on to provide the first clean, simple, and universal description of the motion of objects in the heavens. Among pages and pages of theoretical musings that mostly focused on the divine music of the heavens, Kepler described three laws of planetary motion: the planets moved in ellipses, their motion carved out equal areas in equal time, and there was a relationship between a planet’s distance from the Sun and the duration of its orbit.

Nothing like this had ever been deduced before. Working from an ungodly number of hand-written tabulations of planetary positions, Kepler’s work provided the theoretical springboard for the entire scientific revolution. Here, Kepler was able to succinctly and accurately describe a whole host of phenomena with three simple statements. Kepler’s laws were so simple and so easy to calculate that it drove the transition from a geocentric to a heliocentric conception of the Universe. Without ellipses, the heliocentric model of Copernicus, the one championed by Galileo, wasn’t that much simpler than previous models. With elliptical orbits, the Sun-centered Universe was a slam dunk.

But it was Kepler’s third law, where he discovered a universal formula relating a planet’s distance to its orbital speed, that really set the groundwork for the entirety of modern physics, a physics centered on unification and universalization. It’s all about trying to find simple statements that can explain as much as possible, and the first one to get us down this road was Kepler. He discovered a simple law that described the behavior of every planet, regardless of its composition or position or distance from the Sun, from any star.

Working nearly a century after Kepler, Isaac Newton would publish his theory of universal gravitation as an attempt to explain Kepler’s laws. From there, we can look to the work of Maxwell, Einstein, and other giants as they brought order to a chaotic cosmos—all building on Kepler.

3) Does my universe look big in these pants?

It wasn’t enough for Edwin Hubble to discover that a) galaxies exist and b) they’re really far away from us. These observations, made in the early 1920s, should have been more than adequate to earn him a Nobel prize and cement his place in scientific history. Hubble made our Universe very big in a way that nobody else ever had. He provided the first solid evidence that the Andromeda “nebula” was really an island of stars separated from us by millions of light-years, orders of magnitude greater than even the wildest conceptions of “extremely distant” were at the time.

But he just couldn’t help himself. Working with the 100-inch Hooker telescope at Mount Wilson Observatory—at the time, the largest telescope in the world—he discovered that our Universe wasn’t just big. It was getting bigger.

In a series of careful, precise, painstaking observations, Hubble revealed that galaxies were on the move. By then, astronomers had gotten used to the idea of planets moving, stars moving, and giant gas clouds moving. But we were just getting our heads around the concept of galaxies when Hubble forced us to contend with their motion. And in a surprise twist that almost nobody expected, the motion of those galaxies wasn’t random.

Hubble discovered that, on average, every galaxy is receding away from us and that the speed of their recession is proportional to their distance from us. At the end of his short, readable paper on the subject, Hubble wondered aloud if there was already a theory available to explain the evidence: the work of dear old Albert and his general theory of relativity.

Some other folks, like Alexander Friedmann and Georges Lemaitre, had already used relativity to guess that we might just live in an expanding Universe. But those were just guesses. This was evidence. Hard and hard-to-get evidence. Evidence that would give birth to an entire field: physical cosmology. In less than a generation, we would be forced to revamp the picture of the Universe that we all carry in our heads. Prior to Hubble’s work, astronomers assumed that the Universe was static, forever unchanging throughout eternity. Hubble’s result clearly demonstrated that we live in an evolving, dynamic, living Universe—one that was different in the past and continues to change with every passing day.

One that has a finite age and a finite extent. One that has a birth—the Big Bang—and one that will someday die.

4) Of course black holes are on the list

This entry has no single figure or unique observation that changed our view of the Universe in a short period of time. Instead, it took decades to convince the astronomical community that black holes were real. We are still grappling with that realization today.

It began with Einstein, although he didn’t know it. Einstein reformulated our understanding of gravity in terms of the bending and warping of spacetime (he did so, by the way, to unite Newton’s gravity with his own special theory of relativity, continuing the program of unification started by Kepler). Mere months after Einstein published his general theory of relativity, the German Karl Schwarzschild discovered how to write down the solution for the gravitational environment around a spherically symmetric ball of mass like, handily enough, the Sun.

In Schwarzschild's equations, there was a special radius, a distance from the center of that ball of mass, where all hell broke loose and the math suggested that something catastrophic happened. Unfortunately, Schwarzschild then promptly, and quite tragically, fell ill in the trenches of World War I and died. That horror meant he never got to witness the horror that he unleashed on the world of physics.

Physicists and astronomers spent decades debating what would happen if you could compress a bunch of matter below this “Schwarzschild radius,” as it came to be known. Some argued that gravity would always win and just keep compressing the matter until it collapsed into an infinitely dense point. Most argued that this was ridiculous because nothing is infinitely dense, and some other force or mechanism would avert complete disaster.

Before it was even discovered, the madness inside a Schwarzschild radius got a name: black hole, an object of intense theoretical scrutiny and observational skepticism. Slowly, ever so slowly, the evidence for their existence continued to grow. Nowadays, we take them as a given, but that’s after nearly a century of effort put into theory to flesh them out and observations to look for their influence in the cosmos.

But we still don’t understand them. We know that the singularities, those points of infinite density, don’t really exist, but we don’t know what to replace them with because we don’t have a better understanding of gravity at those scales. We don’t know what happens at the event horizons, the cooler name we have now for the Schwarzschild radius. And everything gets worse when we try to incorporate quantum mechanics into the picture. So black holes are almost a field of astronomy in their own right. They sit there, taunting us with their existence, refusing to yield their secrets.

Jerks.

5) How to make an apple pie

When Carl Sagan offered viewers the recipe for an apple pie in his famous Cosmos segment, he had to start with the Big Bang. In the first few minutes of the existence of the Universe, our cosmos manufactured hydrogen, helium, and a little bit of lithium. Fast-forward to the present day and we have an entire periodic table full of elements. But how do we get from a primordial ooze of light elements to the ingredients needed to make an apple pie?

The answer is in the stars. In 1920, the great astronomer Sir Arthur Eddington took the latest developments in quantum mechanics, special relativity, and subatomic physics to guess that nuclear fusion played a role in the generation of a star’s heat. But it would take until 1957 for the full picture to come into focus thanks to a quartet of researchers: Margaret Burbidge, her husband Geoffrey Burbidge, William (“Willy”) Fowler, and Sir Fred Hoyle.

No Nobel prizes. No tickertape parades. No headlines racing across the globe. Just good, careful, methodical, comprehensive science. In their work, they created a detailed and complete accounting of how stars produce elements through nuclear fusion. How primordial hydrogen is burned in the heart of every Main Sequence star to produce helium. How, once near the end of their lives, those stars fuse helium into carbon and oxygen. How heavier stars can go on to produce silicon, oxygen, iron, and more. And how their cataclysmic deaths via supernovae can flood the rest of the periodic table.

This is the story of us. Of everything. Of the grand symphony of stars that have lived and died and recycled their elements, generation after generation over billions of years. With this insight into stellar nucleosynthesis, the circle is complete. The heavens are not some abstract realm, accessible to human perception but otherwise disconnected from us. We are made of the ashes of dead stars, and someday in the distant future, our molecules will be recycled into the creation of a new system. There are chains, light-years long and millions of years deep, that connect the stars to us and us to the stars. It's in this work that we finally understand our place in the Universe, our part in the great cosmic drama.

And it turns out that the Universe can make a decent apple pie.