The Shape of the Earth #353 (P)

Ideas That Changed the World

Dear friends of Daily Philosophy,

Welcome in the new year, and I hope that the holidays have been kind to everyone! I’m obviously still among the living, so there’s no need to cancel your subscription just yet.

This year, this newsletter fell a bit short in the usual self-celebratory department that dominates the Christmas period. Everyone tells us what they did in the previous year and what they plan to do the next. Since I spent the best part of my holidays in a hospital bed, I could not yet annoy you with Daily Philosophy stats and plans, so let me briefly get it out of the way right now :)

In 2024, we published 56 articles, if I’m counting correctly. This means, we had at least one every week, and a couple to spare. 33 of these were written by guest authors, and I felt that this was a great mixture of different topics and voices! Towards the end of the year, we had our essay contest, which suffered a bit from my bad organisation, but that was to be expected in a first attempt, I guess. We will certainly repeat that in the future, and perhaps next time things will go more smoothly. I am now preparing the certificates and prizes for the winners, so please be patient if you are one of them. It will all come your way shortly.

This newsletter has 2,740 subscribers right now, which is wonderful, and I hope that we can double that this year! One year ago, in the beginning of January 2024, we had 1,733, almost exactly 1000 fewer subscribers, so that’s great growth for this year, and I thank you all for that: the authors who wrote these wonderful articles and you, the readers, who stayed with us and made this into a community worth writing for.

We covered a wide area of topics, from controversial politics (Israel’s Attack on Gaza. Some philosophical reflections) and the positive sides of human extinction (Human Extinction: An Even More Modest Proposal), to articles on veganism (Embracing Kindness — The Moral Argument for Veganism), Boltzmann Brains (Boltzmann Brains and Epistemology) and the beginning of my own series on ancient Greek thought.

All in all now, we have over 350 articles on the site, and if you get a premium subscription, you will be able to access all of them any time. You can do this very easily and painlessly here, and also support Daily Philosophy’s educational mission, which includes providing a forum to new writers in popular philosophy.

For the new year, I will continue to publish the series on ancient Greek philosophy, and I hope to cover most of it over the course of 2025. Of course, we will also have all sorts of other articles, as usual, to keep things fresh.

In terms of videos, I have just started a new series of video lectures on Early Modern philosophy, titled The Age of Reason. You can find the first few videos here, and more will come in the coming days:

And now, let’s go on to today’s topic! In our series on Ancient Greek Thought, we have until now discussed ancient Greece in general, its more mystical side, the beginnings of science, Diogenes Laertius, Thales of Miletus, Anaximander and Anaximenes, and a bit of Greek astronomy. Continuing where we left off, let’s today have a look at the first attempt to measure our planet.

The Shape of the Earth

This is a fascinating topic, and when I began researching it, it kept unravelling and expanding in all directions. Did the ancients really think the Earth was flat? How much geography did they know? Did they make maps and what did these maps look like? And when they travelled, how did they know which way to take? It’s not quite philosophy, but the ancients did not distinguish much between philosophy and science anyway. Thales, the first “philosopher” could be argued to be more of a first scientist, and most of the Presocratics of Miletus were more proto-physicists than philosophers in today’s sense of the word. So let’s have a look at these questions, and let’s file the whole thing under “history of ideas.”

It is commonly said that Pythagoras was the first to speak of a round Earth. But James Hannam [1] points out that we don’t have a primary source for that, and Pythagoras was notoriously attributed all sorts of beliefs throughout the ages, so it’s not really clear if he ever said that. The first mention of a spherical Earth we still have a source for is in Plato’s Phaedo. Socrates says:

“Well Simmias, I do not think I need the art of Glaucus to tell what it is. But to prove that it is true would, I think, be too hard for the art of Glaucus, and perhaps I should not be able to do it; besides, even if I had the skill, I think my life, Simmias, will end before the discussion could be finished. However, there is nothing to prevent my telling what I believe the form of the earth to be, and the regions in it.”

“Well,” said Simmias, “that will be enough.”

“I am convinced, then,” said he [Socrates], “that in the first place, if the earth is round and in the middle of the heavens, it needs neither the air nor any other similar force to keep it from falling, but its own equipoise and the homogeneous nature of the heavens on all sides suffice to hold it in place; for a body which is in equipoise and is placed in the center of something which is homogeneous cannot change its inclination in any direction, but will remain always in the same position. This, then, is the first thing of which I am convinced.”

“And rightly,” said Simmias.

“Secondly,” said he, “I believe that the earth is very large and that we who dwell between the pillars of Hercules and the river Phasis live in a small part of it about the sea, like ants or frogs about a pond, and that many other people live in many other such regions. ...”

Plato’s argument makes intuitive sense: if everything is homogenous, both the Earth and the medium that surrounds it, and if there are no other forces at play, why would the Earth go anywhere? For Pythagoras, the reasons to postulate a spherical Earth might have been more mystical in nature, perhaps because of the perfect symmetry of a sphere. We don’t know. But it is interesting that both were not entirely wrong. Not only is the Earth really a sphere (albeit an imperfect, squashed one, as we know today), but it is a sphere for similar reasons as the two philosophers suspected: when you have a force, gravity, that applies to a body equally from all directions, and the body grows large enough so that gravity is strong enough to shape it, it will automatically assume the most symmetrical form — because why would this not be so if the same force is being applied to it from all sides? This is how planets form and why all planets, after they reach a critical size, are spherical. I find it interesting that the two ancients came to roughly the right conclusion using roughly correct arguments for their case, without actually knowing anything about modern physics or gravity.

The librarian of Alexandria

But the most astonishing result of ancient astronomy, and probably one of the most spectacular measurements ever done, must be Eratosthenes’ estimation of the circumference of the Earth.

Eratosthenes of Cyrene (~280–194 BC) was a mathematician, astronomer, poet, music theorist and geographer; he was also the third librarian of the Great Library of Alexandria, one of the most important places of scholarship in the ancient world. During Eratosthenes’ time, the library would still have been at the height of its flourishing and influence — its decline began only around 145 BC when some intellectuals left Alexandria for political reasons, and continued until part of it was destroyed by Julius Caesar in 48 BC. But this would be 150 years after Eratosthenes’ time. When Eratosthenes came to the library, it would have contained between 40,000 and 400,000 scrolls [2], the biggest collection available anywhere in the West at that time.

It also helped that the time around 200 BC was a relatively quiet period for Egypt. We always tend to romanticise the Golden Age of ancient Greece, but if you look closer, it does not seem quite as golden: the first third of Plato’s life was overshadowed by the Peloponnesian War, in which Socrates is said to have fought at least three times. In 395 Athens was involved in the Corinthian war, which lasted until 387 BC. In 430 BC, Athens was decimated by the plague, and whatever was still standing after the Persians destroyed the city in 480 BC, was then burned down by the Spartans in 404 BC. And those wars were not, like today’s, largely television and media events, perceived from a distance. Every single citizen would be involved, either as a soldier or as a victim of enemy violence and destruction.

But Alexandria was far away from the turmoil of the Greek city-states and within the walls of its “Museum,” as its main research institution was called, scientists from all over the ancient world would come to exchange theories and ideas, and to consult its famous library: Callimachus, who created the first alphabetically sorted library catalogue; Hero of Alexandria, who developed the first steam engine; and Archimedes visited the library and developed the Archimedes Screw, an early water pump, there.

Eratosthenes came up with a new method of calculating prime numbers, which is still in use today, the so-called Sieve of Eratosthenes, a popular beginners’ computer programming exercise. And he also calculated the size of the Earth. We don’t have his own book, Geographika, any more, but we know that he discussed the whole Earth in it, dividing it into climate zones, parallels and meridians, and also coining the term geography itself. In old age, he lost his eyesight, rendering him unable to read the library’s books — and this depressed him so much that he committed suicide by starving himself to death. His own works were lost when his beloved library was destroyed some time in the 3rd century AD.

The size of the Earth

Eratosthenes’ measurement is one of those ingenious feats of geometrical thinking that the ancients were capable of, and that still surprise us with their precision.