Universe As a System

 Universe As a System 

 Historical Perspective

 You know that. the primitive human beings depended on food gathering and hunting for their survival. The availability of food depended on the seasons and the seasons depended on the movement of thc Sun and stars. Thus, the Sun and the stars controlled the Seasons, food and warmth. similarly, the Moon's motion controlled thc tides and the life cycles of many animals. Hence, it was natural that the primitive people noted the rising and setting of the Sun, the reappearance of the crescent moon after the new Moon and the waxing and waning of the Moon. Thc more accurately they knew the position and movements of the Sun, Moon and stars. the more reliably they could predict when to hunt, when to gather the tribe and when to move to warmer places. In other words, their survival depended, to a great extent, on their ability to read the 'calendars' in the sky (read the margin remark). The earliest such records are in the form of bone engravings depicting the phases of the Moon. These are estimated to be about 30,000 years old.

However, the primitive people's universe was restricted to only the small patch of land bounded, perhaps, by rivers, distant hills or by the blue line of the sea. Overhead was the sky across which rode the Sun, a god giving light and warmth, and the Moon, a lesser god shining with paler light. With the Moon at night rode innumerable brilliant stars. Outside this little universe lay unimagined mystery.

With the discovery of agriculture came the need for sowing and reaping of crops in the right seasons.  Fairly accurate calendars based on the regular movement of the Sun, Moon and stars were made in Babylon and Egypt long before 2500 B.C. With the passage of time, human thought grew and improved. Between 600 and 400 B.C., a great revolution in human thought began when philosophers in many societies all over the world tried to understand the universe without invoking the intervention of gods. They observed the world around them and looked for rational answers to questions like : Why did the Sun rise at different places? Why did the Moon change its shape? Why did a few stars, later called planets, move among the others? Did such things have any meaning for men? However, with their limited tools of observation and experimentation, their theories about the universe did not, for a long time, progress beyond an earth-centred system. Let us see what the ancient ideas about the universe as a system were.

 Geocentric Universe of the Ancients

The earliest ideas of the Egyptians and Sumerians about the universe may seem strange to us. The Earth appeared flat 2nd solid tg them. Similarly, the Sumerians visualised the universe as a flat Earth covered by the heaven made up of tin! Between them lay the glowing Sun, Moon planets and stars which were controlled by the gods. The Earth was obviously the principal thing in the universe. Indeed, they knew no reason to think otherwise. They accepted the Sun, Moon, planets and stars for what they looked like.

The Earth is Round

The idea of a flat Earth was discarded by the Greeks. As early as 600 B.C.; the philosopher Thales thought the Earth to be round. Pythagoras and his disciples also maintained that the Earth was spherical. They reasoned that it must be round because of the way ships seemed to sink below the horizon of he sea or because of the circular shadow it cast on the Moon during an eclipse.

Greek astronomers had also mapped the stars and constellations and had estimated the brightness of the stars. They had observed fie apparent motion of the planets which seemed to wander amidst the stars, with some, like Mars, even travelling backwards. The problem before them was to figure out the 'real' motion of the planets as seen from up in the sky, away from the Earth, in such a way that it explained their apparent motion as seen from the Earth. We will now describe the model of the universe figured out by the Greeks.

The Ptolemaic System 

The theoretical model of the universe given by the Greeks had a stationary Earth at its centre, around which the Sun, the Moon and the planets moved in circular orbits. I,n this model of the universe, stars merely acted as a bedrop, much like a painted screen hung by a photographer at a village fair! But, doesn't this seem to be the most natural idea in the world? The Earth seems steady, solid, unmoving, while we can see the heavenly bodies rising and setting each day. Most of the models constructed by the Greeks to explain the movement of planets consisted of perfect concentric spheres or circles. They held that each planet was attached to an invisible sphere or a circle that rotated around the Earth at a different speed from .the rest of the spheres. You may recall Eudoxus' model of 27 spheres.  The astronomical ideas of many earlier Greeks were gathered by Ptolemy who published them in his Almagest. This series of thirteen volumes contained the ideas of such men as Aristotle, Apollonius, Hipparchus, in addition to his own ideas. This combined picture of the r~niverse is called the Ptolemaic system. There were some exceptions to this model. Notable among these was the argument of Aristarchus of Samos, that the Earth was one of the several planets, which like them orbits the Sun which was at the centre of the universe. He also argued that the Sun was much bigger than the-Earth and stars were enormously far away. However, we do not know how he reached these conclusions, each of which is correct. These ideas were rejected under the overwhelming influence of Aristotelian ideas.

The Ptolemaic model of the geocentric universe held sway for over 1,000 years. With the Renaissance in Europe, scientists took to the path of observation and experiment.

The fifteenth century European astronomers built observatories, improved Ptolemy's instruments and devised novel ones. As was bound to happen, their observations began to clash with Ptolemaic theory. As the observations about the paths of the planets became more accurate, Ptolemy's model was increasingly strained to fit the facts. The Renaissance had opened the vast storehouse of ancient Greek knowledge to European astronomers. A Polish astronomer, Nicholas Copernicus, re-examined the long neglected sun-centred theories of the universe. In 1543, he published quite a different hypothesis, from the prevailing Ptolemaic model, to explain the apparent motion of planets. Its most daring feature was the proposition that the Sun. not the Earth, was at the centre of the universe.

The Copernican Revolution

The Copernican model consisted of the Sun at the centre with the six planets, Mercury, Venus, the Earth with the Moon-round it, Mars, Jupiter and Saturn going round it in circular orbits. In this model too the stars formed a fixed sphere in the background. Copernicus also believed all planets to be of the same size. His model worked as well as Ptolemy's spheres in explaining the apparent motion of the planets. But it led to a confrontation with the adherents of geocentric model. It was not generally accepted until much later when Galilee's and Kepler's works proved that the heliocentric model was valid.
The sun-centred model of Copernicus was established by the astronomical observations of Galileo Galilei when in 1609, he turned his small, imperfect telescope towards the sky. In the first few nights of observation of the heavens, Galileo saw enough to shatter the ancient picture of the serene, perfect, harmonious world.

For, the Moon, instead of being a perfect, smooth sphere, was found to be uneven, covered by mountains and deep depressions; the planet Saturn seemed divided into three. He also saw four Moons circling around the planet Jupiter. Hence in the heavens was a small scale model of the Copernican system. The planet Venus showed phases like the Moon. The fact that the Venus showed a fully lighted phase when it was-near the 'Sun could not be explained by the Ptolemaic system. Only the Copernican model which allowed Venus to circle around the far side of the Sun from the Earth, could explain it.

In spite of the prevailing opposition to Copernican model, these observations were eventually accepted and they led to the final overthrow of the geocentric model. Interestingly, the earth-centred ideas remain with us in our everyday lives even now. It is almost 2,200 years since Aristarchus and almost 400 years since Galileo, but our language still 'pretends' that the Earth does not rotate. For instance, we still talk about the Sun 'rising' and the Sun 'setting'!

Kepler's Laws of Planetary Motion

Further support to the heliocentric model came from the work of Johannes Kepler at around the same time as Galileo's observations. Kepler, a German astronomer, was trying to work out a theoretical model which explained all observations of planetary motion. The most accurate observations of apparent planetary positions had been made by Tycho Brahe.

Brahe invited Kepler to work with him. He recommended that Kepler study the planet. Mars because its motion seemed most anomalous, most difficult to reconcile with an orbit made of circles. Further, planets in circular orbits ought to move with constant speed. But Kepler found that their speeds changed with their distance from the Sun. After years of trial and error, he found that the only explanation of the observed movement of Mars was that its orbit was an ellipse with the Sun at one of its foci . Thus, the idea of circular orbits was abandoned. Kepler eventually succeeded in explaining Brahe's observations which could all be expressed simply, in the form of three laws of planetary motion.
Kepler's laws removed the main objection of the Copernican model, that this model could not give an accurate description of the observed path of the planets. These laws also led to the rejection of Pythagorean-Platonic view of the heavens showing only perfect circular motions, which even Copernicus had retained. By.the end of the seventeenth century the heliocentric model of the universe came to be accepted starlight generally. Interestingly, the physical proof of the movement of the Earth came when it was no longer necessary, because by then everybody had already accepted that the  Earth moved around the Sun. Let us see what it was.

Stellar Parallax

If the Earth were stationary, a given line joining point A on the Earth, a nearby star C and any given distant star would never vary. However, if the Earth changed its position in space and moved from A to B, this alignment would also change. Thus, in the background of more distant stars, the nearby star would appear to shift from C, to C, as the Earth moved from A to B. This apparent shifting of nearby stars against the background of more distant ones has been observed, and the phenomenon called Stellar parallax. It is a periodic kind of a change. A given star first shifts one way and then the other, during the course of one year, hence it must be due to the fact that the Earth is moving around the Sun.
The change is small, less than a second of an arc. It was only in 1838 that Friedrich Bessel, a German astronomer, could measure the stellar parallax of a star. The nearest star, the Sun when viewed against distant stars appears to shift approximately 1" per day.

Stars in the Limelight

 Among Galilee's my discoveries with the telescope was his observation that the white nebulous band In the sky known as the Milky Way (Akash cranga) was in fact made up of very many stars. Ti this time, the model of the universe had consisted merely of the then known Solar System, with stars being nothing more than point sources of light. With'the availability of bigger and better telescopes in the post-Galilean era, the remaining planets of the Solar System, Uranus, Neptune and Pluto were discovered and the stars came to be examined in greater detail.

The first ever study of the stars was made by the English astronomer William Herschel (1738-1822), who had earlier discovered the planet Uranus. Herschel showed in 1785 that the stars were not the backdrop to 'the Solar System but were individual objects that extended to infinity.. He prepared the first ever map of the Milky Way Galaxy and showed that it was, in fact, a part of a flat disc of countless stars. In his model, the Solar System was situated within the milky Way Galaxy which constituted the whole Universe. At that time, the telescopes were not very powerful. One could see in the sky, point-like objects, the stars. One could also see white fuzzy clouds called nebulae.

When we entered the twentieth century, the model of the universe was still heliocentric. Our Sun was at the centre of the Milky Way Galaxy which with its stars and nebulae was the whole of the universe. However, it did not take long for the heliocentric model to be abandoned. We will now describe, in brief, the observations that led to the rejection of heliocentric model.

Rejection of Heliocentrism 

This happened in the year 1918 when the astronomer Harlow Shapley (1885-1972) first surveyed the size and shape of the Milky Way Galaxy. He showed, by his most original researches, that the Sun does not lie at the centre of the Milky Way Galaxy, but is located a great distance away from the centre. But the question whether the Milky Way Galaxy was the whole of the universe or not, still remained. The answer came in the year 1924, when Edwin Hubble (1889-1953), another great astronomer, showed that the fuzzy cloud called Andromeda nebula was not a member of the Milky Way Galaxy. In fact it was a separate galaxy. Soon, other galaxies were spotted, but the Milky Way Galaxy appeared to be the largest. This was some consolation to the human ego: if the centre of the universe was neither reserved for our Earth, nor for our Sun, at least we lived within the largest galaxy. This idea also did not last long. Walter Baade (1893-1959) turned the telescope on many cosmic details that Hubble had skipped over. He discovered that the other galaxies were farther away than we had supposed, and the Milky Way Galaxy was no bigger than the others. It was merely one galaxy among countless others.

The revolution that had begun with Copernicus was now complete. We had no special place in this universe! We were not at its centre. In fact, the universe was found to have no centre and not even a boundary. Indeed, the greatness of Copernican ideas lies not so much in what Copernicus did as what his work led to.

. For our prehistoric id Bronze Age ancestors, the Earth was at the centre of the universe. The dark heavens beyond were a mystery to them working under the control of some supernatural forces.
. The Greek philosophers tried to understand the universe on the basis of observations, logic and reasoning, and gave many models which were absorbed in the Ptolemaic geocentric model of the universe.
. Galileo's observations and later Kepler's formulation of the three laws of planetary motion based on the observations made by Tycho Brahe established the revolutionary heliocentric model.
. The discovery that the Sun was one of the millions of stars in the Milky Way and was located only at a large distance from the centre of the Galaxy led to the rejection of heliocentrism.
. We now know about the structure of the universe on a large scale. The planets, stars, galaxies, clusters, and superclusters form a dynamic universe which is always changing.