#253 - Flat Earth Special
Chris Joins Matt to chat about the Earth and how we found out it was not Flat.
When people thought the Earth was flat, they were wrong. When people thought the Earth was spherical, they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together.”
― Isaac Asimov, The Relativity of Wrong
Human-like beings emerged around 300 thousand years ago, a blink of an eye in cosmic terms. For the past 100 thousand years or so it is most likely human behaviour took on the modern characteristics of abstract thinking, planning, symbolism, art, music and dance, tool technology, hunting etc.
Recent studies may show that a deep Human knowledge of the stars seems to go back to an incredible 40,000 years ago, with Cave paintings demonstrating an understanding of the precession of the equinoxes. This is an amazing piece of detective work, and show a deep connection of humans to the heavens. Ancient humans must have kept track of the position of certain stars and how they appeared at the same time each year in the night sky and noticed that they were slowly moving each year, this is a long term effect and best seen over many generations. In fact, the whole cycle takes 26,000 years. historically called the precession of the equinoxes because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic Think about a gyroscope
Why did they take soo much notice? Well, other clues in the paintings, depictions of hunting and gathering, keeping a track of the stars is very handy when it comes to animal migration and vegetation growth. Tantalising some suggest these same cave paintings may depict astronomical events such as comet strikes that affected their way of life and were in the traditions of people that remained unchanged for centuries.
It’s worth noting that the first person we know for certain knew this precession is Hipparchus (190–120 BC) of Rhodes, but as it was only Claudius Ptolemy(c. 100 – c. 170 AD) that used Hipparchus work and left a written record, all of Hipparchus work was lost. Both explained precession as the rotation of the celestial sphere around a motionless Earth. Did the ancients no better, we may never know. Many other civilisations around the world almost certainly figured this out as well, and it shows Humanities deep connection to the stars that such an obscure and subtle effect should demand our attention. I don’t see dolphins chatting about Precession.
So we know that Humans were fascinated with the stars and the way they moved but it would only be in modern times that we would or could understand their true nature. Many more jigsaw pieces were needed before then.
IT is highly likely that as modern humans first appeared all their creative energy was required just to stay alive, and to them, their Universe was just the local environment, where they hunted and roamed, but as they looked up there must have been a hominid that had that very first thought, “what is that” as they stared up at the moon.
About 12,000 years ago Humans discovered started to Farm, this not only gave them more free time but also more reason to know more about the Universe when your crops depend on it. Were the gods sending us signs.
First of all, we need to know more about home, what were we standing on, how far did it go, were there other worlds beyond the sea? what exactly were the sun and the moon, how far away were they. Why did they dictate everything we did? And more ominously what were the other strange events taking place in the sky, the comets, the shooting stars, the asteroid strikes of distant memories and stories.
With no concepts of many of the ideas of modern thought, it is very hard to imagine that Humans thought that the world was anything but flat. It looks flat in all directions, you walk up and down hills, but never does it feel easier in one direction than another on average.
Walk long enough and you eventually get to the sea, is Earth just a disc of land afloat in a vast ocean?
So why did the first humans question what seems perfectly obvious?
Travellers probably first noticed something odd, why was it that in different Greek settlements did the Pole star have a different altitude? Why could seafarers see land when higher on the rigging or on a wave, take this line from Homer’s (1200-800BC) Odyssey "As he rose on the swell he looked eagerly ahead and could see land quite near."
Also, how did Phoenician Sailors under the employ of a Pharaoh in 600BC, see the Sun shining in the north as they circumnavigated the coast of Africa? Were they mistaken?
Anaximander's c. 610 – c. 546 BC realization that the Earth floats free without falling and does not need to be resting on something has been indicated by many as the first cosmological revolution and the starting point of scientific thinking, "one of the boldest, most revolutionary, and most portentous ideas in the whole history of human thinking." Karl Popper
Such a model allowed the concept that celestial bodies could pass under the Earth, opening the way to further Greek astronomy.
The great Greek Philosophers, Pythagoras, Parmenides and Empedocles started to formulate the ideas of a spherical earth, looking at the terminator of the moon they also concluded the moon was a sphere.
After that, sometime between 500 B.C. and 430 B.C., a fellow called Anaxagoras determined the true cause of solar and lunar eclipses - so now the shape of the Earth's shadow on the Moon during a lunar eclipse was also used as evidence that the Earth was round.
By the time of Plato, the Greek philosophers all talked of a spherical earth, but there is little evidence of any further explanation.
Aristotle (384–322 BC) noticed that the stars in Egypt could not be seen in Europe, he already assumed the stars were a long way away, so how could this be? Aristotle also noticed that when the earth moved between the sun and the Moon the shadow it cast was always round, and also it was obvious that the moon itself was a sphere, from its phases?. Aristotle reasoned if the Earth was also a sphere, that would explain these results?
Aristotle decided that a sphere was formed because everything was pushing to central point the centre of the earth was the centre of the universe, and all things moved in spherical shells around it including the sun moon and stars.
Archimedes would state that "The surface of any fluid at rest is the surface of a sphere whose centre is the same as that of the Earth"
How big is this sphere?
Eratosthenes, (276–194 BC) estimated Earth's circumference using the shadow cast by poles at noon in different parts of Egypt, Syene and Alexandria, Eratosthenes could only measure the circumference of the Earth by assuming that the distance to the Sun is so great that the rays of sunlight are practically parallel. The sun and the moon go around the observer once a day, Eratosthenes knew that the apparent size of the moon doesn't change. This must mean that Alexandria is near the centre of the moon's orbit. But the apparent size also doesn't change when viewed from anywhere. So everywhere is close to the centre of the moon's orbit. Thus the moon must be much further than the radius of the Earth. If the moon were 6000 miles from the Earth, then it would seem to grow and shrink in size as it passed by (such an effect can be seen on Mars, where the moon really does orbit close to the planet) And the Sun must be further still the moon blocks the sun during an eclipse, (one of the biggest cosmic coincidences granted that they are the same angular size)
Of course, you can measure the relative distance to the sun, if you accept all the planets are in orbit around the sun then using a bit of trigonometry you can measure the distance in terms of how much further the sun is away compared to the body at greatest elongation. To understand this is to look at the motion of Venus in the sky relative to the Sun: as Venus orbits the Sun, it gets further away from the Sun in the sky, reaches a maximum apparent separation from the Sun (corresponding to the greatest elongation), and then starts going towards the Sun again. The greatest elongation of Venus is about 46 degrees, so by this reasoning, the Sun-Venus distance is about 72% of the Sun-Earth distance.
In conclusion, the distance to the sun must be very very large in comparison to the radius of the Earth, and we can assume that that the rays of light from the sun are parallel.
Posidonius calculated the Earth's circumference by reference to the position of the star Canopus. he observed the star on the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7½ degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes). Since he thought Rhodes was 5,000 stadia due north of Alexandria, and the difference in the star's elevation indicated the distance between the two locales was 1/48 of the circle, he multiplied 5,000 by 48 to arrive at a figure of 240,000 stadia for the circumference of the Earth
The spherical earth was now the normal belief amongst the Romans too with the works of Cicero and Pliny referring very casually to a spherical earth. Claudius Ptolemy (90–168 AD) was making maps of the spherical earth knowing that he only knew a quarter of the surface from the Canaries to China, the arctic to southern Africa.
The Indians, with their great skill in astronomy also heard of the greek spherical earth and adopted the finding.
The idea spread with Christianity, although with pockets of literal resistance, that took many more centuries to fall. The Muslim world fully embraced the spherical world as people need to know what way to pray to mecca, Ibn Hazm also stated the quite remarkable sentence. "is that the Sun is always vertical to a particular spot on Earth" proof of circular earth.
Since then other observations just keep stacking up, many jigsaw pieces fit snuggly with the spherical earth model but are utterly incompatible with the flat-earth