Many ancient thinkers' observations about nature - the origins of life, the size and scope and organization of the heavens, the causes of disease - were woefully inaccurate. But this is not because early theorists were dumb. In fact, they were often deeply observant, sophisticated, and rigorous in their thinking. They invented, or at least formalized, many of the concepts - from mathematics, to logic, to empiricism - that we take for granted as essential tools in understanding the world around us.
It’s easy to stand on a mountain of others’ achievements and look down on the poor rubes who believed the sun went around the Earth, or that disease was caused by poisonous "miasma" in the air or an imbalance of humors in the body. But it’s the height of arrogance to suppose that, in their shoes, modern thinkers would have done any better. It’s a lot easier to read about heliocentrism, or evolution, or germ theory, in a textbook than to figure it out for oneself.
Although this process of accumulating knowledge is a slow one, occasionally people from past ages did have tantalizing flashes of insight that, for whatever reason, didn’t become conventional wisdom for many centuries. Perhaps empirical data wasn't available to support these insights, or cultural currents were too strong to overcome. The flashes remain in the historical record and remind us of the truth of Einstein’s assertion:
Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
By the time Copernicus published his groundbreaking book On the Revolutions of the Heavenly Spheres in 1543, no educated person doubted that the Earth was round. This had been known since ancient times. The controversial part was Copernicus’s proposal that the sun, rather than the Earth, was at the center of the universe. (Moderns would say it’s at the center of the solar system - but one conceptual leap at a time.)
Copernicus was overturning a geocentric system that had been virtually unassailable for over 1,300 years. It had been codified by the Greco-Roman astronomer Ptolemy in his treatise the Almagest, which synthesized the findings of ancient astronomers up to that time. Ptolemy wrote that the Earth was the immobile center of the cosmos and the sun orbited it, its path resting between the planets Venus and Mars. In Ptolemy’s time, the idea that the Earth might orbit the sun was downright unthinkable.
Well, not quite.
Four hundred years before Ptolemy, Aristarchus of Samos had already proposed a heliocentric (sun-centered) universe. His writings have not survived, but his ideas are described in Archimedes’s The Sand Reckoner:
His hypotheses are that the fixed stars and the sun remain unmoved, that the Earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit...
The idea was given credence by other ancients, including Seleucus of Seleucia and Pliny the Elder. But Ptolemy’s geocentrism won the ancient conflict of ideas and held sway through the Middle Ages.
Copernicus had some awareness of Aristarchus’s work, and even mentioned him a few times in his own book, but without access to The Sand Reckoner (which was published in print the year after Copernicus’s passing), he may have been unaware of the full import of his predecessor’s ideas.
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Even today, it takes some pretty fancy tech to actually see an atom. So it’s safe to say that in the fifth century BCE, any evidence of their existence would have been purely conceptual, based not on observation but on logic. The ancient Greeks, however, liked to deeply think things through, and one of them, Democritus of Abdera, thought his way to the atom itself.
It’s important not to overstate the conceptual leap here. Democritus had no idea of electrons and neutrons, or the periodic table, or anything remotely like that. His concept of an atom was embedded in the word itself: The Greek atomon literally means "not divisible." It expresses the idea that with any substance, there must be a particle so small that it cannot be subdivided further.
Democritus imagined that these atoms were infinite in number and varied in size and shape, and that they were perfectly solid with no gaps inside (hence indivisible). Much of this is considered to be wrong now: The total number of atoms in the universe is believed to be vast but finite, and atoms are in fact mostly made of empty space, their various particles held together by powerful forces. Democritus might not have accepted that what we call atoms even deserve the name. After all, if an atom is by definition indivisible, how can you apply the term to a particle that is itself made up of smaller particles like quarks, gluons, and neutrinos?
Physics is more complex than what Democritus and his fellow philosophers from the fifth century BCE imagined. But considering what they had to work with, it was a pretty good start.
Eratosthenes lived in the third century BCE and was a director of the famous Library of Alexandria. Possessed of an unusually wide-ranging mind, he weighed in on some of the great questions of his day, including, “How big is the Earth?”
The roundness of our planet was well understood by the time of Aristotle, a century before Eratosthenes lived. But its size remained an open question. Eratosthenes was not the first person to attempt to calculate the Earth’s circumference, but he was the first whose methods have come down to us, and his answer set a new standard for accuracy.
Eratosthenes knew from previous surveying records - done with an ancient measuring stick called a gnomon - that at the same time of year and the same time of day, the sun would cast shadows of different lengths at locations in different latitudes. By comparing shadows cast at two latitudes (one was the town of Syene; the other may have been Alexandria or Meroë) on the day of the vernal equinox, Eratosthenes could compute the degrees of curvature between the locations. The angle came out to about one-fiftieth of a circle, so by multiplying the distance between the two towns by 50, Eratosthenes could obtain the Earth’s circumference.
Eratosthenes’s result was 250,000 stadia. Unfortunately, it’s hard to know precisely how that corresponds to modern miles and kilometers, because the precise size of the ancient Greek stadion is not known. Depending on the value chosen for the stadion, the circumference is somewhere between 40,000 km (uncannily accurate) and 48,000 km (pretty far off, but still respectable).
Eratosthenes, it should be noted, was not some solitary genius tinkering about with sticks and shadows. He was director of the greatest institution of learning in the Hellenistic world, and he had access to the most cutting-edge research from his colleagues and predecessors. Then as now, it’s amazing what someone can figure out with good data and sound calculations.
An early pre-Socratic philosopher who lived in the sixth century BCE, Xenophanes of Colophon was one of the first recorded individuals to deduce geologic change from physical evidence.
Xenophanes’s writings - if he made any - have not survived. Some of his philosophical ideas come down to us in the Refutation of All Heresies by the early Christian writer Hippolytus of Rome. This passage recounts Xenophanes’s observations on seashells and marine fossils:
Xenophanes is of the opinion that there had been a mixture of the earth with the sea, and that in process of time it was disengaged from the moisture, alleging that he could produce such proofs of the following: that in the midst of earth, and in mountains, shells are discovered; and also in Syracuse he affirms was found in the quarries the print of a fish and of seals, and in Paros an image of a laurel in the bottom of a stone, and in Melita parts of all sorts of marine animals. And he says that these were generated when all things originally were embedded in mud...
Xenophanes’s mud theory hasn’t aged well, of course. Hippolytus may have included this passage as evidence for a biblical flood, something modern geologists reject. What’s most important is that Xenophanes used observation of fossils to assert that great changes must have taken place in the Earth’s past. That’s a very logical, and strikingly modern, perspective - and one that must have been quite novel to some of his contemporaries.