There is an old joke regarding the structure of the universe. A student asked a wise person what carried the world through space. The answer was that the world rested on the back of an enormous tortoise. When the student asked, “What does the tortoise walk upon?” the answer was, “Another tortoise.” When the student inquired further as to what that tortoise walked upon the reply came, “Silly boy, it is tortoises all the way down.”
We really don’t know when people began to notice the night sky at all, let alone observe it systematically. By the time prehistory was transitioning into the historical era (that is, with the invention of writing), a great deal of observation was being passed down orally regarding the movement of the heavenly bodies including the sun and the moon and the fixed points of light and the wandering points of light (there were five of them, apart from the moon and the Sun), that all had their separate cycles during the day and the night, during the months and the year, and that they coincided with the seasons of the year somehow.
Humans were able to increase the systematic study of these phenomena with the advent of writing about 6,000 years ago, although it is the case that some astronomical records are preserved from stone-age, preliterate cultures in the form of pictures on cliff faces. There is even a stone observatory at Fajada Butte in the Four Corners area of the US, date unknown. Like the much larger Stonehenge (c. 2500 BC), it was accomplished without writing, and erected in stone. It throws spears of light from the sun and moon upon spirals pecked into a cliff face behind it. The glints of light mark the solstices and equinoxes, and the 19-year lunar cycle. This stone observatory is an even more sophisticated bit of stonework than Stonehenge, by the way, for it measures the solstices and equinoxes not at sunrise and sunset but at high noon which is a much more difficult engineering problem. So it is not absolutely necessary for a culture to have writing in order to advance a long way in the study and measurement of the cycles of the heavens. But writing does give an advantage.
Mesopotamian cuneiform writing was used to record astronomical observations—there is a clay disk with writing along angled lines and constellations scribed into it, for example, which is an obvious star chart.
Observers in China and India were not far behind them. The Babylonians were the first to be able to predict eclipses, which if you think about it is a pretty sophisticated bit of observation, but it requires only observation of the two largest bodies in the sky—the sun and the moon—a great deal of continuous observation, record keeping, and patience.
The Babylonians were also able to use their observations to create a calendar much like the one we use today.
Primarily, however, the sky and the movements of the lights in it were objects of reverence, and people’s observations were largely translated into a set of religious beliefs, practices and customs, as well as the planting and harvesting of crops. All through this long period from prehistory through the advent of writing and record keeping, the main assumption was that the Earth remained stationary and all the heavens including the Sun revolved around the Earth at the center. This is the Earth-centered, or geocentric model of the universe. This was the obvious interpretation of all the observable cycles; it held throughout millennia. But that was about to change.
About 2000 years ago, at about 300 BC during the Greek Seleucid Empire, Babylonian astronomers achieved the pinnacle of their studies. (Let us stop here a moment. The implication—the reality—is that after this pinnacle, there was a decline. Progress is not the norm in the world. Flows and ebbs are the norm. You and I have been fortunate to live in a period of flow, of increase in knowledge, capacity and wealth, but if the lessons of history apply to us as they have to our ancestors, our own period will (not may, but will) be followed by a period of decline. Much will be lost. Your future progress is not assured. Enjoy.)
During this apex of Seleucid astronomy, someone dug up an old record from an earlier Greek period, an old guy named Philolaus from around 400 BC. This Greek thinker had suggested that the Earth was not at the center of the universe, but he could not figure out what was and so his theory languished. During the Seleucid era, a guy named Aristarchus, who was from the Greek island of Samos, started with Philolaus’ idea and, combined with his own astronomical observations, made the suggestion that the Sun was at the center of the universe, and the Earth circled around it. This is the heliocentric theory. Not only that, but Aristarchus also was able to figure out that the wandering points of light—the planets—also circled the sun just like the Earth did. What’s more, he was able to put the five visible planets in their proper order. This was about 300 BC. The Babylonian Seleucus found Aristarchus’ model to make the most sense, but neither man could overcome the nearly universal resistance to this revolutionary idea, and it not only fell from favor, it fell into obscurity. More famous Greeks such as Plato and Aristotle who preceded Aristarchus, and the Egyptian Greek Ptolemy who succeeded him, assumed the geocentric model and for two millennia the world including the Roman Empire and the Roman Catholic Church continued in error.
Just to give you some context, Columbus, when he sailed, believed this model.
It wasn’t until the first quarter of the 16th century, in 1514 to be exact, that the heliocentric model re-emerged into the thoughts of astronomers. The first was Nicolaus Copernicus from Poland. He had come across the writings of Aristarchus and those, combined with his own observations, led him to propose a clearly demonstrated sun-centered model. He wrote this out for himself, and shared it with only a few friends. He knew that were he to challenge the Church-sponsored Earth-centered model, he would be accused of heresy and forced to recant, or be executed if he did not. His paper languished in obscurity for 30 years until, close to death and more or less no longer subject to the tender mercies of the Vicar of Christ, he published the work, and so started what we now refer to as the Copernican Revolution, probably the most important contribution of the era.
The Church was slow to react. Copernicus passed on, and his paper, On the Revolutions of the Celestial Spheres, circulated narrowly among the few who could comprehend his science. Sixty years later, however, the Church was forced to respond to this fundamental challenge. By this time, new advances had extended to the science of optics, and in about 1608 the fabulous contraption known as the telescope was invented (nearly a century after Copernicus). At first, this instrument was used by traders and speculators keeping watch from the hillsides and towers overlooking the Italian port cities, watching for the arrival of cargo ships. The telescopes gave them advance notice of impending arrivals, and through coded flags the amount and nature of the cargoes, and thus an advantage over those who were watching with eyes only. We probably should not be either surprised nor offended that this important instrument was used at first to make money in commodity markets rather than to advance science, but so it was.
In any case, the Italian astronomer Galileo realized the potential of a magnifying instrument for his own studies, and promptly made one of his own, and turned it upon those curious things in the heavens which had captured the imagination of mankind since before history began. And for the first time, mankind could begin to see the true nature of these things. The moon revealed its mountains and plains, and became an object of mere matter rather than a heavenly orb. So with the planets, particularly Mars with its curious markings. Jupiter ceased to be merely a point or disk of light, and clearly was seen to be a sphere, just like the moon. Venus, similarly revealed as a sphere, also went through phases just like the moon. And since the only thing that could illuminate it was the Sun, it was abundantly obvious to any schoolboy that it must orbit the sun, just as the moon orbited the Earth. Moreover, Jupiter was seen to have its own set of little points of light attendant upon it, soon to be calculated as multiple moons, orbiting the planet. The Copernican model, up to now safely hypothetical, was now proven by the sheer weight of visible observation. Galileo published a short treatise of his observations and conclusions, The Starry Messenger, followed by other papers which even more pointedly confirmed the Copernican model.
The Church was thus challenged in one of its central assumptions—that the Earth was the center of God’s Universe, and Man the center of His attention. To have the Earth relegated to a secondary position replaced by the Sun was heresy. The Church of Rome had to respond. Finally in the early 1630s, after about 20 years of studied silence, it called Galileo to account. He was allowed to explain and his explanation was rejected; he was convicted of heresy. The Church was gracious—it offered him a choice: He could be stubborn in his adherence to a belief contrary to Church doctrine and be tortured for the good of his immortal soul and executed for the good of the Church, or he could admit his error, recant his studies, and be allowed to live albeit under house arrest. Can we blame him for not wanting to suffer pain by the thugs of the Vicar of Christ? He was nearly 70 years old. He gave in. The Church maintained the error of millennia through threats of force and violence, in the name of the Prince of Peace.
But the Church could not suppress the grinding of glass. The science of optics grew apace; glass became more pure, grinding techniques became more precise, telescopes became larger and sharper. And math became more supple in its predictive capacity. It took more than another hundred years, but eventually the planet Uranus was discovered with a telescope, and seen obviously to circle the Sun. Sixty years later, the planet Neptune was discovered through the means of mathematical calculations based on observations of gravitational fluctuations in Uranus’ orbit.
It was about this time that even the heliocentric model fell into question, with the realization that the sun itself was not the center of the larger universe but rather it, too, along with its system of planets moved through space in relation to the rest of a swirling spiral of stars called even by the ancient Greeks the Milky Way. They called it that because in the dark night sky of ancient times, with no light pollution, the heavens in the deep of night pop out to an observer in a breathtaking river of undifferentiated light—it seemed to stream with spilled milk. They could not grasp it for what it was, but they named it nonetheless.
The Church in the meantime has had to make its accommodations with the undeniable realities exposed by science. It has done so quietly, abandoning its usual threats of force and violence. Those two means of convincing people are not gone, however—they are still used frequently by various States, but at least the Church may now be counted upon to behave more in keeping with the admonitions of Christ, at least in respect of its relationship with science.
In the 1920s there came to a head one of those differences of opinion among the astronomical community (some might call it a petty squabble, given the behavior of some of the participants) regarding the nature of the Milky Way. Clearly it had limits, an edge, boundaries within which its vast swirling arms kept. This, we knew beyond doubt, was The Universe. That was It. The End. We knew it All. This massive spiral of stars, with its splotches of nebulae and its swirls of dust and gas and its aching fields of endless stars, unimaginably huge, was finally known to be That Which Is. And beyond that? Nothing? But wait! What’s this little dab of gauzy light, this little oval of bright mist? Is it just one more unremarkable nebula with which the Milky Way is strewn from one end to the other, each one more dramatic and fantastic than the next, extending in their various convolutions within the spiral arms across many degrees of arc in the sky (the full moon only covers a half a degree of arc). Or could it be… something else? Something… outside the Milky Way? This question generated furious debate among otherwise nerdy guys who could not get a date with a girl if their lives depended on it.
The Church had no opinion.
Within 5 years the debate was settled and Science had done it without
burning anybody at the stake although a lot of egos were smashed and
careers ended. That little wisp of mist, as delicate and
insubstantial as a patch of glitter from the wing of a moth on black
velvet was resolved into a vast, second, insanely distant spiral
galaxy just like the Milky Way. It had arms, it had its own nebulae,
it even had its own satellite dwarf galaxies just like the Milky Way
has the Large and Small Magellanic Clouds. Today we know this as
the Andromeda Galaxy, our nearest large galactic neighbor. 
Subsequent observations revealed a larger neighborhood of similar
bodies, vastly distant, yet gravitationally locked with one another
in their own orbiting spiral dance. This illustration (not to scale)
is now what we call the Local Group: 
And this is but a part of a larger structure, a grouping of
galaxies,
of which this is but a part of an even larger conglomeration of
galaxies called the Virgo Cluster, which is itself but one of many
galactic superclusters as seen here. 
And this is not the end. It is just what we can see.
As to that, all these masses of galaxies, all this star stuff, all this matter, all those atoms which seem to be endless, countless, impossibly vast now are considered to comprise but 4% of the true measure of the Universe. The rest? Well, uh, we can’t see it. It is inferred. It is as if you were to stand on one hill and observe another distant hill. You can see the trees on that hill bending and waving, so even though there is no breeze where you stand, you can infer that there is a brisk wind at work over there; you cannot see the wind, but you can see its effects. The same holds true with the universe. We cannot see this other matter—or more properly this other non-luminous part of the universe—but we must infer it is there because we can observe its effects, largely gravitational. But for now, it is a mystery.
You’d think, therefore, that we would label this unseen stuff “Mystery Matter”, but no. These days we must not allow the universe to be mysterious. We call it “Dark Matter”, dark because we cannot see it, not because it has anything to do with Darth Vader. By the way, this matter is accompanied by Dark Energy as well. The comparative amounts are as follows: The visible (luminous) universe, 4%, comprised of 3½% intergalactic gas, and 0.5% stars; Dark Matter, 23%; Dark Energy, 73%. How are we to think of the above picture of the giant superclusters of galaxies as only one half of one percent of the apparent universe? The mind boggles.
And so here we are. Welcome to a rather larger world than our ancestors knew. I’m 65 years old now as I write this in 2012. When I was a 10 year old boy, we knew about the nearest galaxies. When my father was a 10 year old boy in 1920, the argument about other galaxies was just starting but was not resolved. When his father was a ten year old boy in 1883, even the nature of the Milky Way galaxy was not entirely known. So we are looking at a mere three generations during which advances in understanding about the universe grew exponentially, in the face of millennia of error.
Don’t assume such progress must continue.
And oh by the way, you will note that at the end of this enormous expansion of our understanding of the universe, we suddenly are in a place where we are absolutely certain that we know absolutely nothing of fully 96% of the universe. For all we know, it could be tortoises, all the way down.
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