The word ‘cosmos’ is synonymous with the word ‘universe’, but with an important overtone. Cosmos stands for the order and beauty of the universe. When we, humans, understood everything on the earth and the sky to be part of a single system, we created the word ‘universe’ to stand for that singular system. And when we saw the beauty and order of the universe, the word ‘cosmos’ came out which means ‘order’ and ‘beauty’ at the same time. The words ‘cosmos’ and ‘cosmetics’ come from the same root because both of these are about beauty and order. Here, we will see how the concept of the cosmos developed over the last three thousand years.

A large part of the region spanning from Bangladesh to Britain falls within the same linguistic zone because many of its languages are part of the Indo-European family. Here, ‘indo’ stands for India. Most, but not all, of the languages of Europe and India are part of this family. Many branches of this family are neither in India nor in Europe, but the name emphasizes these two regions because they define the eastern and western frontiers of this zone, respectively. The concept of cosmos arose in almost all cultures of the world, but we will start by focusing on the development of this concept in the Indo-European tradition.

And for that let us read aloud a poem from the tenth book of the Rigveda, a Hindu religious text and the oldest extant literature in any Indo-European language. Focus on the emphasized words and the related thoughts.

1. Neither non-existence nor existence there was, no realm of space, no sky beyond.
     What covered? Where? What gave shelter? Was water there, bottomless deep?
2. Neither death nor immortality was there, no sign for distinguishing night or day.
     The breathless One breathed by its own nature, other than it there was nothing.
3. Darkness there was, at first concealed in darkness all was indiscriminated chaos.
     Only formless void existed then; by the great power of warmth the universe was born.
4. Desire rose then in the beginning; desire, the primal seed and germ of spirit.
     Sages searching with heart’s thought discovered the kinship of existence with non-existence.
5. The dividing line was extended transversely, what was above it then, and what below?
     There were parents, there were powers, action below, energy above.
6. Who knows and who can proclaim it? Whence was it produced? Whence comes this creation?
     The Gods came later than the world’s creation. Who knows then whence it first came into being?
7. The first origin of this creation, maybe it formed all, or maybe it did not.
     Perhaps the eye controlling all from highest heaven knows, or perhaps knows not.

It’s a poem about creation, the creation of existence out of non-existence. A child is intimate with its parents because it is born from them, and existence is intimately connected with non-existence because it came out of non-existence.

Once there was neither existence nor non-existence, and then there was non-existence and existence like a twin. Non-existence is like death, night, darkness and chaos, and existence is like immortality, day, light and the ordered universe. In the first three verses, we encounter the idea of the birth of a single object called the universe that includes everything in existence. The fourth and fifth verses establish the kinship of this universe with its parent, the nothingness. The last two verses are about our fundamental ignorance about the whereabouts of the universe. This fundamental ignorance can never ever be mended no matter how advanced our schools of engineering, technology and sciences become.

The birth of a unique and unified order from a chaos-like nothingness has been described here. Along with the concept of order, do you see the birth of beauty from ugliness here? Think! And let me know.

Skipping many nations and generations, we come to the Greeks next because they did one of the most interesting synthesis of astronomical information gathered by different cultures of the ancient world.

The Greek synthesis was possible because of the records of systematic observations of stars and planets by the ancient Babylonians, the nation whose capital was at Babylon, a city very close to the present-day Baghdad, Iraq. These ancient Iraqis could track the path of Jupiter using sophisticated geometrical methods.

This tiny clay tablet displayed in the British Museum was made by Babylonian astronomer-mathematicians almost 2300 years ago. Thousands of such tablets have been discovered and they are small because scribes used to keep a tablet on the palm of one hand and write on it using the other hand. Recently, historians have uncovered the meaning of this tablet and found that it describes the method of calculating Jupiter’s position by measuring its speed at different times.

While the Babylonians were continuing their tradition of observation, a theoretical revolution was underway in Greece. Plato and his student Aristotle tried to explain the whole universe and all its motion using a single model.

In this model, the spherical earth is at the center of the cosmos. Aristotle gave the first recorded proof that the earth is spherical. He said, during a lunar eclipse, earth’s shadow moves over the surface of the moon and the moving front of this shadow is always circular; only a spherical object can cast a circular shadow, so earth is spherical.

In the model of Aristotle, earth is at the center of the Earth, surrounding earth there is water, then air and finally the fire. Fire is placed outside the other three elements (earth, water, air) because it always moves away from earth (upward from our perspective). As in China and India, ancient Greeks believed everything on the Earth is made of these four fundamental elements.

Outside the sphere of fire we have the spheres of the moon (LUNA), Mercury, Venus, the Sun (solis), Mars, Jupiter (iovis) and Saturn. Only five planets (out of our 7 planets, except the Earth) are visible with naked eye and they are all here. Outside the sphere of Saturn, you see the sphere of the fixed stars. The ancients imagined all the stars to be placed on the inner surface of a gigantic sphere encircling the earth and all the planets.

Now in this diagram, if you bring the sun at the center and take the earth-moon together at the position of the sun, then the order of the planets from the sun would be totally accurate. Mercury is indeed the planet closest to the sun and then we have Venus, Earth, Mars, Jupiter and Saturn, respectively. How did Aristotle and his predecessors come up with this order? How did they know the relative distances of the planets?

Answer: using Babylonian and their own observations of the positions of planets. If a planet moves slowly, it is farther away. If it moves fast, it is closer to us. The stars almost never change positions with respect to each other, they only rise and set due to the 24-hour motion. But the planets have a second annual motion. If I observe the position of Mars every night for two years, I will see that its position changes from every night and moves in a particular direction from one night to the next. After two years, it comes back to the position where it was two years ago. For Jupiter, this takes 12 years, and for Saturn, 29 years. Do not underestimate the observations! For finding the 29-year motion of Saturn, you have to observe Saturn regularly for 29 years and this is what the Babylonians did.

Using the time a planet (and the sun and the moon) takes to come back to its initial position, the Greeks fixed the order of the cosmos as depicted above.

The apparent motion of the sun around the earth was investigated by the Greek astronomer Hipparchus in more detail. He found the exact lengths of the different seasons. The seasons are not equal. Spring lasts for almost 94 days, but summer only around 92 days. This could only happen, as Hipparchus saw it, if the earth is not exactly at the center of the circle that describes the path of the sun. This eccentric circle haunted astronomers for more than a thousand years. But there was even a greater problem in Plato-Aristotle’s theory.

If the planets, the sun and the moon are rotating around the earth because their spheres are moving in a circular motion, then planets should always move forward with respect to the stars, they should never stop or move backward with respect to their original motion. But they actually do. If you keep observing Mars every night you will see that once in 2 years, that is once in its annual rotation, it stops and moves backward with respect to the stars. Jupiter shows backward motion 11 times during its 12-year journey and Saturn 28 times during its 29-year motion.

This motion was explained ingeniously by Ptolemy of Egypt around 100 years after the birth of Jesus. Ptolemy used to work in the largest library in the world at the time, and his mission was to solve all problems of the Greek model of the cosmos.

For solving the backward motion, Ptolemy imagined two circles for each planet: a deferent and an epicycle. A planet actually moves in an epicycle and the center of the epicycle moves in a deferent. The Earth is located near the center of all the deferents of all the planets. As the planet moves in the epicycle, it seems to move backward from an observer on earth.

For a more clear understanding, use the video above.

After the ancient Greeks, most advancements in the theory of cosmos were done by the Indians and the people of the Islamic world. Note that the ‘Islamic world’ denotes the regions ruled by one or more Muslim rulers until the 19th century and it comprised of various regions ranging from Spain in the west to Indonesia in the east at various periods in time.

India had its own tradition of cosmology since the most ancient times as evident from the poem above. The greatest of all ancient Indian astronomers lived in the sixth century. His name was Aryabhata and he wrote a famous book called Aryabhatiya which was later translated into Arabic in the Islamic world.

In the Aryabhatiya, Aryabhata proposed a geocentric theory like Ptolemy but with significant differences. Most important difference was that Aryabhata believed in the daily rotation of the Earth: the Sun revolves around the Earth, but the Earth rotates around its own axis. The diurnal motion of the whole sky from east to west was explained by the daily rotation of the Earth from west to east.

In the Islamic world, we see large-scale observatories for the first time. In this picture you see an observatory built for the ‘head astronomer’ of Istanbul, Turkey, in around 1570 which lasted only ten years. The observatory was destroyed within a decade of its construction because of political reasons and Taqi’s failure to predict future events properly. Taqi wrote a book whose name began with Sidrat al-muntaha (the ultimate tree of knowledge) and in it he presented astronomical observations that were more precise than the observations of Copernicus and Brahe.

The most important Islamic astronomer of the 14th century, Ibn al-Shatir, lived in Syria and he was more interested in theory than observation, unlike Taqi. Shatir made quite a few interesting modifications to the ancient Greek model of Ptolemy and created a new model of the cosmos that was almost identical to model of Copernicus, the only difference being that Shatir’s model was still strictly earth-centric.

Here you see the model of the revolution of Mercury around the Earth made by Shatir. Copernicus used this model in his theory with one modification: the Earth at the center was replaced by the Sun. The figure clearly shows that Shatir introduced more than one epicycle to explain the motion of Mercury.

But Copernicus, a Polish Christian monk, was the first scientist of the middle ages who was bold enough to overthrow the Earth from the center of the Cosmos and replace it with the Sun. Aristarchus of ancient Greece also presented a heliocentric (sun-centric) theory almost two thousand years before Copernicus, but it never took hold of the imagination of the scientists. Copernicus was the first person who presented a sun-centric cosmos at the right time in the right place. The times were ripe for a revolution.

This figure with the Sun (‘Sol’ in Latin) at the center is from the book De revolutionibus of Copernicus published in 1543, the year he died. The book is called ‘on the revolutions’ and it truly started a ‘revolution’ in Europe, but maybe Copernicus could not appreciate the double meaning of the word ‘revolution’ (revolution around the sun, overthrowing the current political system) at the time.

This model of the Cosmos is still very much like the model of Ptolemy, only the sun and the earth have changed places with each other. There are still 8 spheres (compare with the 8 spheres of Ptolemy in Section 2), and all the stars are still placed in the outermost (first) sphere. The moon is moving around the Earth (Terra).

The most striking advantage of the Copernican model was the explanation of the backward motion of the planets. As mentioned above, the Greeks solved the backward motion using epicycles. But Copernicus solved it using ‘relative motion’ in a way similar to Aryabhata. Aryabhata said, we see the stars move from east to west because actually the Earth is rotating from west to east. This is a relative motion. And Copernicus applied the same concept to explain the motion of Mars for example, as shown in the video above.

As the Earth is close to the Sun than Mars, the Earth overtakes Mars once during one orbit of Mars, because Mars moves around the Sun in around 2 years. When the Earth overtakes Mars, we see Mars move backward, just like passengers in a bus see another bus on the road move backward when their bus overtakes it.

Thomas Digges, the leader of the English Copernicans, went one step farther than Copernicus and scattered the stars throughout an empty space. In the Copernican model, the stars are fixed on the surface of a sphere, but Digges’ model has the stars all over the place outside the spheres of the planets in our solar system which is very close to the current picture of the Milky Way, our Galaxy.

Copernicus and Digges were almost correct, but the observations of the positions of the planets still could not be predicted with certainty using the Copernican model. The figure above shows the Stjerneborg, the largest observatory of the time built by Tycho Brahe in Denmark in around 1580, the year when Taqi’s observatory was destroyed in Turkey. Brahe’s observation did not match perfectly with the model of Copernicus. Therefore, Brahe almost did not believe that the Sun is at the center.

But the main reason why theory and observation did not match was that there was one little problem in the theory of Copernicus. And this problem was solved by Kepler, an assistant of Brahe. Kepler realized that the planets actually do not move around the sun in a circular orbit as prescribed by Copernicus, but in an elliptical orbit. The shape of an orange is almost circular and the shape of an egg is elliptical. Moreover, Kepler realized that the Sun is actually not at the center of the elliptical orbits, but in one of the two focuses of the orbits as shown above.

Kepler also realized that the planets move faster when they are closer to the sun and slower when farther away. The three triangular areas shaded in blue are all equal according to Kepler. This means a planet takes the same time ($t$) to move from the initial to the final point of the arcs in all three cases. And this can only be true if the planet is faster when closer to the sun and vice versa.

But Kepler could not prove Copernicus definitively yet. Galileo provided some farther supports for the Copernican system even though he was not very enthusiastic about the discoveries of Kepler. In the diagram above, you see almost 80 new stars discovered by Galileo through his telescope for the first time. No one saw them before him because you cannot see them with the naked eye. Galileo was the first one to point a telescope toward the sky. But how does this diagram support Copernicus?

Copernicus knew that if the Earth moves around the Sun, then the position of a nearby star with respect to the more distant stars would change throughout a year, throughout one orbit of the Earth around the Sun. Why? Let me explain this way. Let us say I stand near the right side of the classroom and look at a student sitting at the central chair on the first row. When I look from the right side of the room and try to find the position of the student with respect to the back wall of the classroom, I will see that the student is close to the left corner of the back wall. Now when I change my position and go to the left side of the room and look at the same student, I will see that he is close to the right corner of the back wall. I am like the Earth, the student is like a nearby star, and the back wall is like the background of the more distant stars. The position of the nearby stars would change with respect to the background stars if the Earth moves around the Sun.

But Copernicus knew that in reality we do not see any such change of positions of the stars during real observations. His answer to the problem was, the stars are so far away that we cannot discern the change in positions, the change is too little. And the above diagram shows that the stars are indeed far away. How? Because if the stars were closer, they would appear bigger through a telescope. But all the stars in the figure still look like tiny dots, their size does not increase when seen with a telescope. That means they are indeed far away, and Copernicus was right in saying that we cannot discern their change of positions because of the distance.

Galileo provided one more support for the sun-centric system by discovering four new astronomical objects, four moons (natural satellites) of the planet Jupiter. His observation of the moons from January 17–31, 1610, is shown above. You can see four dots around Jupiter and the dots change their positions every day. Sometimes you see only two or three of them because the other moons are behind Jupiter on that night. This clearly proves the moons are moving around Jupiter and not the Sun. This supported the idea that it is possible to move around an object in the cosmos which is not the Earth. The Earth lost its special significance because of this discovery.

By the modern origin story I mean the the theory of the cosmos we believe in today. And this story was developed gradually over the last five hundred years. I have already mentioned the main problem in the Copernican theory of the cosmos: it is that the positions of the stars in our sky should change if the Earth is actually moving around the Sun, but we do not see any such change during real observations.

But the change was actually discovered in around 1830 in Europe again. Last time I explained the change of positions verbally, this time let us try to understand it more clearly using the figure above. The Earth is moving around the Sun. When the Earth is at point $E$, the star $S$ seems to be located at point $V$ on our sky. But when the Earth moves to point $E'$, the star $S$ seems to be located at point $V'$ and the position keeps changing throughout the year following the ellipse drawn between the points $V$ and $V'$.

This phenomenon has a technical name: parallax. And the change of positions of a star was indeed discovered by observing a star continuously for a few years before 1830. This definitively proved that the Earth indeed moves around the Sun. The theory of cosmos given by Copernicus was proved almost 300 years after its publication.

And astronomers never looked back. Their telescopes became more and more advanced until we were able to observe even the whole 14 billion years of history of the universe. But that story is for another time…