The History of the Spherical Earth Idea

When I was in elementary school, I learned that the Earth was round. I even remember a silly rhyme in a cartoon I watched in school about Christopher Columbus, who "proved" the Earth was round. To me, the fact that the Earth is round seems almost like an instinct. That being said, I never looked up the history of why and how it's known the Earth is round. Today I started to correct that.

The idea that the Earth is a sphere can be traced back as far as the ancient Greeks. Although the idea is attributed to Pythagoras, a Greek polymath most famous for his Pythagorean theorem of triangles, both he and his contemporary Parmenides of Elea are known to have taught the idea of a spherical Earth. Moreover, it is possible the discovery was made by another Greek and falsely associated with Pythagoras by historians. Aristarchus and Eratosthenes improved on this work by estimating the circumference of the Earth, and noting how the visible constellations changed as the observer traveled nearer to and farther from the equator.

It is also possible that other cultures (Chinese dynastic empires, African tribes, Egypt, and Native Peoples in the Americas) noted evidence that the Earth was round, but few, if any, records of these people survive. Future discoveries may change this. In ancient India, for example, a Greek ethnographer wrote in 300 B.C.E. that the Brahmans (the high scholarly caste in Hindu society) believed that the spherical Earth was the center of the universe. As Hellenistic ideas spread via Alexander the Great's conquest, the Greek ideas merged with those of the Brahmans, and survived well into the next millennium. Aryabhatta I, an classic Indian astronomer and mathematician who lived from 476–550 C.E., wrote in his work, the Aryabhattiya, that the Earth was spherical. He also made an incredibly accurate estimation of the Earth's circumference of 4,967 yojanas, which is equivalent to 24,835 miles. The equatorial value of Earth's circumference is today calculated at 24,901 miles.

In any case, the spherical Earth idea was a moderately popular one in the ancient world, as it persisted throughout Greek dominance and was later adopted by Roman astronomers as the standard. Later on, Ptolemy of Alexandria wrote one of his best-known books, Almagest, in which he defended and built on the arguments for a spherical Earth. He also argued that the Earth could curve in both north/south and east/west directions. The Almagest remained one of the standard astronomy texts for the next 1,400 years. As Europe experienced its Medieval period, the knowledge of a spherical Earth was retained by monks translating ancient Greek and Roman texts. It was also kept alive, and eventually reintroduced to Europe, by many Muslim astronomers, including Ibn Hazm, Al-farghānī (Alfraganus), and Abu Rayhan Biruni. These and other early scholars used this information to develop spherical geometry, an important tool still very much in use today.

A page from Almagest. U.S. Library of Congress.

So exactly how did they all know that the Earth is spherical? Much of it came from the knowledge of sailors. For example, as mountains appeared over the horizon from the top down. Had the Earth been flat, the mountains should have appeared in full as soon as the ship passed any obstructing object in front of it. Another piece of evidence is the shape of the terminator on the moon. A solar terminator is the line that divides the "day" from the "night" side of a planetary body. For us here on Earth, it's the line edge of the shadow that passes over the Moon when it's not full. Ancient astronomers recognized this line as a representation of the shape of the Earth as it passed between the Moon and the Sun. This line is curved, and so the shape of the Earth must too be curved.

https://www.farmersalmanac.com/terminator-22380

Finally, there was the sky itself. The sun is always directly above one point on Earth at solar noon, and this happens predictably for a single location on a yearly schedule. The north star, Polaris, never moves from its place in the night sky, but its declination, or height above the horizon, does change with an observer's latitude on Earth. There are also stars that are not visible from certain points on Earth. If the Earth were flat, then all stars should be visible at all times, as a view from a plane should only rotate in one direction. This logic, coupled with knowledge of geometry, was yet more evidence for a spherical Earth.

Of course, the fact that Ferdinand Magellan and his company sailed completely around the globe between 1519 and 1522 was perhaps the most solid piece of evidence for a spherical Earth before the space age. Now, satellites have orbited the Earth, Moon, Mars, Mercury, Jupiter, and Saturn, returning pictures of all of these that show they are spherical. It would not be possible to orbit a flat plane. Why planets are spherical is another discussion. For now, it is enough to discuss an abridged history of the spherical Earth idea and a few of many pieces of evidence that support it.

Sources:
Burkert, Walter (1 June 1972), Lore and Science in Ancient Pythagoreanism, Cambridge, Massachusetts: Harvard University Press, ISBN 0-674-53918-4, page 306.
https://en.wikipedia.org/wiki/Spherical_Earth
https://www.scientificamerican.com/article/why-are-planets-round/
https://www.livescience.com/60544-ways-to-prove-earth-is-round.html
Direct adoption by India: D. Pingree: "History of Mathematical Astronomy in India", Dictionary of Scientific Biography, Vol. 15 (1978), pp. 533–633 (554f.); Glick, Thomas F., Livesey, Steven John, Wallis, Faith (eds.): "Medieval Science, Technology, and Medicine: An Encyclopedia", Routledge, New York 2005, ISBN 0-415-96930-1, p. 463
Cormack, Lesley B. (2015), "That before Columbus, geographers and other educated people thought the Earth was flat", in Numbers, Ronald L.; Kampourakis, Kostas, Newton's Apple and Other Myths about Science, Harvard University Press, pp. 16–22, ISBN 9780674915473
Otto E. Neugebauer (1975). "A History of Ancient Mathematical Astronomy". Birkhäuser: 577. ISBN 3-540-06995-X.