Nowadays, everyone knows that the planet we live in is, to a first approximation, shaped like a sphere. However, from the earliest known writings, we see that most cultures saw Earth as being a flat and finite area. That is relatively easy to understand: in ancient times, very few people ever travelled more than a few kilometres from their place of birth; the distance any given person might travel was limited by how much he or she could walk, and that is not very much — especially when you don't really have a reason to move that much. And, from the point of view of a person moving very little, the world is indeed flat; the curvature of the Earth has no visible effect in a small scale.
Therefore, we have the representations of the world made by the early Mediterranean peoples showing that sea at the centre, the known European, African and Asian lands around it, and a very large ocean all around that. The South American Incas had a similar view, with themselves at the centre and land extending a certain distance in all directions, with a more or less rectangular shape and four corners. Egyptians also saw Earth as flat and surrounded by mountains on which the large tent that is the sky is grounded, and even the Bible offers a similar view. Other cultures had more metaphorical views, such as that of the disc of the Earth lying on the back of four elephants, who are on top of a giant turtle swimming on an infinite sea; it's unlikely that these representation were ever intended to be taken as literally true, however (unless you happen to live in Discworld).
The path that leads to our current understanding of the Earth starts, as so much else, with the ancient Greeks. Having pioneered the study of geometry, the Greeks had a love for symmetry and perfection and tended to try to impose this love on the natural world. Pythagoras, in the 6th century BC, was one of the earliest proponents of the idea that the Earth must be a sphere, as that is the most perfect shape for an object, and the Earth, being the place where man and the gods live, must adhere to this ideal of perfection. Of course, he had no evidence for that; this is purely an aesthetic argument. Plato, a century later, defended the same ideas for the same reason, but he was taken much more seriously than Pythagoras, and his ideas had a large influence on people of the time.
A few decades later, Aristotle defended once more the idea of an spherical Earth, but differently from his predecessors he had evidences supporting his idea. In his book "On the Heavens", he came up with three basic arguments:
- one has a different view of the stars from places at the south than from places further north; some stars appear higher in the sky the further south you go, as if from places in the north your view of the sky is obscured by the curvature of the Earth
- the shadow of the Earth projected on the Moon during a lunar eclipse is always round; if the Earth were anything but a sphere, you'd see different shapes for the shadow depending on the angle of the Sun at the time of the eclipse
- and, since objects tend to fall towards the centre of the Earth, the force of attraction towards the centre would tend to cause the Earth to assume a spherical shape; this argument is incredibly advanced for its time, as it basically stands on the idea of gravity overcoming the tensile strength of the material forming the Earth to shape it like a sphere
By the 3rd century BC, then, everyone in the western world knew that the Earth was an sphere, fixed in the centre of the Universe, with several other spheres (planets, Moon, stars) rotating around it. This allowed people to ask the next question: how big is this sphere? Aristotle guessed that it was very large (because the effects of travel on the position of stars are very small), and this reinforced the idea the Earth was fixed; but how large, exactly, is it? We'll see about that next week.
Leave a comment