The Universe

A Historical Survey of Beliefs, Theories, and Laws

Britannica Educational Publishing, Erik Gregersen

• Publisher: Britannica Educational Publishing
• Year: 2009
• ISBN:
  9781615300556
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

The universe has been both a subject of study and supplier of fresh mysteries. This book tackles a topic that is infinitely broad with extreme precision and careful organization, bringing the far reaches of the universe squarely into the hands and minds of readers.

CHAPTER 1
IN THE BEGINNING

Humanity has traveled a long road since societies imagined Earth, the Sun, and the Moon as the main objects of creation, with the rest of the universe being formed almost as an afterthought. Today it is known that Earth is only a small ball of rock in a space of unimaginable vastness and that the birth of the solar system was probably only one event among many that occurred against the backdrop of an already mature universe. This humbling lesson has unveiled a remarkable fact, one that endows the minutest particle in the universe with a rich and noble heritage: events that occurred in the first few minutes of the creation of the universe turn out to have had a profound influence on the birth, life, and death of galaxies, stars, and planets. Indeed, a line can be drawn from the forging of the matter of the universe in a primal “big bang” to the gathering on Earth of atoms versatile enough to serve as the basis of life. The intrinsic harmony of such a worldview has great philosophical and aesthetic appeal, and it may explain why public interest in the universe has always endured.

EARLIEST CONCEPTIONS OF THE UNIVERSE

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All scientific thinking on the nature of the universe can be traced to the distinctive geometric patterns formed by the stars in the night sky. Even prehistoric people must have noticed that, apart from a daily rotation (which is now understood to arise from the spin of Earth), the stars did not seem to move with respect to one another: the stars appear “fixed.” Early nomads found that knowledge of the constellations could guide their travels, and they developed stories to help them remember the relative positions of the stars in the night sky. These stories became the mythical tales that are part of most cultures.

When nomads turned to farming, an intimate knowledge of the constellations served a new function—an aid in timekeeping, in particular for keeping track of the seasons. People had noticed very early that certain celestial objects did not remain stationary relative to the “fixed” stars. Instead, during the course of a year, they moved forward and backward in a narrow strip of the sky that contained 12 constellations constituting the signs of the zodiac. Seven such wanderers were known to the ancients: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Foremost among the wanderers was the Sun. Day and night came with its rising and setting, and its motion through the zodiac signaled the season to plant and the season to reap. Next in importance was the Moon. Its position correlated with the tides, and its shape changed intriguingly over the course of a month. The Sun and Moon had the power of gods; why not then the other wanderers? This probably led to the astrological belief that the positions of the planets (from the Greek word planetes, “wanderers”) in the zodiac could influence worldly events and even cause the rise and fall of kings. In homage to this belief, Babylonian priests devised the week of seven days, whose names even in various modern languages (for example, English, French, or Norwegian) can still easily be traced to their origins in the seven planet-gods.

Constellations were originally a navigational aide. Early travelers attributed shapes to certain star groupings, then gave these figures backstories in an effort to remember each constellation’s position in the night sky. © Corbis

ASTRONOMICAL THEORIES OF THE ANCIENT GREEKS

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The apex in the description of planetary motions during classical antiquity was reached with the Greeks, who were of course superb geometers. Like their predecessors, Greek astronomers adopted the natural picture, from the point of view of an observer on Earth, that Earth lay motionless at the centre of a rigidly rotating celestial sphere (to which the stars were “fixed”), and that the complex to-and-fro wanderings of the planets in the zodiac were to be described against this unchanging backdrop. They developed an epicyclic model that would reproduce the observed planetary motions with quite astonishing accuracy. The model invoked small circles on top of large circles, all rotating at individual uniform speeds, and it culminated about 140 CE with the work of Ptolemy, who introduced the ingenious artifact of displaced centres for the circles to improve the empirical fit. Although the model was purely kinematic and did not attempt to address the dynamical reasons for why the motions were as they were, it laid the groundwork for the paradigm that nature is not capricious but possesses a regularity and precision that can be discovered from experience and used to predict future events.

In this depiction by an unknown artist, Ptolemy demonstrates his conception of the universe on a three-dimensional sphere. The Ptolemaic model was a refinement of previous attempts to explain the nature of the universe. Hulton Archive/Getty Images

The application of the methods of Euclidean geometry to planetary astronomy by the Greeks led to other schools of thought as well. Pythagoras (c. 570–? BCE), for example, argued that the world could be understood on rational principles (“all things are numbers”); that it was made of four elements—earth, water, air, and fire; that Earth was a sphere; and that the Moon shone by reflected light. In the 4th century BCE Heracleides Ponticus, a follower of Pythagoras, taught that the spherical Earth rotated freely in space and that Mercury and Venus revolved about the Sun. From the different lengths of shadows cast in Syene and Alexandria at noon on the first day of summer, Eratosthenes (c. 276–194 BCE) computed the radius of Earth to an accuracy within 20 percent of the modern value. Starting with the size of Earth’s shadow cast on the Moon during a lunar eclipse, Aristarchus of Samos (c. 310–230 BCE) calculated the linear size of the Moon relative to Earth. From its measured angular size, he then obtained the distance to the Moon. He also proposed a clever scheme to measure the size and distance of the Sun. Although flawed, the method did enable him to deduce that the Sun is much larger than Earth. This deduction led Aristarchus to speculate that Earth revolves about the Sun rather than the other way around.

THE PTOLEMAIC SYSTEM

The Ptolemaic system is a mathematical model of the universe formulated by the Alexandrian astronomer and mathematician Ptolemy in about 150 CE and recorded by him in his Almagest and Planetary Hypotheses. It is a geocentric cosmology. That is, it starts by assuming that Earth is stationary and at the centre of the universe. The “natural” expectation for ancient societies was that the heavenly bodies (Sun, Moon, planets, and stars) must travel in uniform motion along the most “perfect” path possible, a circle. However, the paths of the Sun, Moon, and planets as observed from Earth are not circular. Ptolemy’s model explained this “imperfection” by postulating that the apparently irregular movements were a combination of several regular circular motions seen in perspective from a stationary Earth. The principles of this model were known to earlier Greek scientists, including the mathematician Hipparchus (c. 150 BCE), but they culminated in an accurate predictive model with Ptolemy. The resulting Ptolemaic system persisted, with minor adjustments, until Earth was displaced from the centre of the universe in the 16th and 17th centuries by the Copernican system and by Kepler’s laws of planetary motion.

The first principle of the Ptolemaic model is eccentric motion. A body traveling at uniform speed on a circular path with Earth at its centre will sweep out equal angles in equal times from a terrestrial perspective. However, if the path’s centre is displaced from Earth, the body will sweep out equal angles in unequal times (again, from a terrestrial perspective), moving slowest when farthest from Earth (apogee) and fastest when nearest Earth (perigee). With this simple eccentric model Ptolemy explained the Sun’s varying motion through the zodiac. Another version of the model, suitable for the Moon, had the direction of the line from apogee to perigee gradually shift.

In order to explain the motion of the planets, Ptolemy combined eccentricity with an epicyclic model. In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent). Because one half of an epicycle runs counter to the general motion of the deferent path, the combined motion will sometimes appear to slow down or even reverse direction (retrograde). By carefully coordinating these two cycles, the epicyclic model explained the observed phenomenon of planets retrograding when at perigee. Ptolemy enhanced the effect of eccentricity by making the epicycle’s centre sweep out equal angles along the deferent in equal times as seen from a point that he called the equant. The centre of the deferent was located midway between the equant and Earth.

Although the Ptolemaic system successfully accounted for planetary motion, Ptolemy’s equant point was controversial. Some Islamic astronomers objected to such an imaginary point, and later Nicolaus Copernicus (1473–1543) objected for philosophical reasons to the notion that an elementary rotation in the heavens could have a varying speed—and added further circles to the models to achieve the same effect. Nevertheless, the equant would eventually lead Johannes Kepler (1571–1630) to the correct elliptical model as expressed by his laws of planetary motion.

Ptolemy believed that the heavenly bodies’ circular motions were caused by their being attached to unseen revolving solid spheres. For example, an epicycle would be the “equator” of a spinning sphere lodged in the space between two spherical shells surrounding Earth. He discovered that if he represented the motions of the Sun, the Moon, and the five known planets with spheres, he could nest them inside one another with no empty space left over and in such a manner that the solar and lunar distances agreed with his calculations. (His estimate of the Moon’s distance was roughly correct, but his figure for the solar distance was only about a twentieth of the correct value.) The largest sphere, known as the celestial sphere, contained the stars and, at a distance of 20,000 times Earth’s radius, formed the limit of Ptolemy’s universe.

Through Islamic astronomers, Ptolemy’s nested spheres became a standard feature of medieval cosmology. When Copernicus proposed a heliocentric model—with Earth and planets all orbiting the Sun—he was compelled to abandon the notion that there is no empty space between the spheres. After Tycho Brahe (1546–1601) demonstrated that the comet of 1577 would have had to pass through several of these invisible spheres, the hypothesis of solid spheres also became untenable.

Unfortunately, except for the conception that Earth is a sphere (inferred from Earth’s shadow on the Moon always being circular during a lunar eclipse), these ideas failed to gain general acceptance. The precise reasons remain unclear, but the growing separation between the empirical and aesthetic branches of learning must have played a major role. The unparalleled numerical accuracy achieved by the theory of epicyclic motions for planetary movement lent great empirical validity to the Ptolemaic system. Henceforth, such computational matters could be left to practical astronomers without the necessity of having to ascertain the physical reality of the model. Instead, absolute truth was to be sought through the Platonic ideal of pure thought. Even the Pythagoreans fell into this trap. The depths to which they eventually sank may be judged from the story that they discovered and then tried to conceal the fact that the square root of 2 is an irrational number (i.e., cannot be expressed as a ratio of two integers).

THE SYSTEM OF ARISTOTLE AND ITS IMPACT ON MEDIEVAL THOUGHT

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The systematic application of pure reason to the explanation of natural phenomena reached its extreme development with Aristotle (384–322 BCE), whose great system of the world later came to be regarded as the synthesis of all worthwhile knowledge. Aristotle argued that humans could not inhabit a moving and rotating Earth without violating common sense perceptions. Moreover, in his theory of impetus, all terrestrial motion—presumably including that of Earth itself—would grind to a halt without the continued application of force. He took for granted the action of friction because he would not allow the seminal idealization of a body moving through a void (“nature abhors a vacuum”). Thus, Aristotle was misled into equating force with velocity rather than, as Sir Isaac Newton was to show much later, with (mass times) acceleration. Celestial objects were exempt from dynamical decay because they moved in a higher stratum whereby a perfect sphere was the natural shape of heavenly bodies and uniform rotation in circles was the natural state of their motion. Indeed, primary motion was derived from the outermost sphere, the seat of the unchangeable stars and of divine power. No further explanation was needed beyond the aesthetic one. In this scheme, the imperfect motion of comets had to be postulated as meteorological phenomena that took place within the imperfect atmosphere of Earth.

The great merit of Aristotle’s system was its internal logic, a grand attempt to unify all branches of human knowledge within the scope of a single self-consistent and comprehensive theory. Its great weakness was that its rigid arguments rested almost entirely on aesthetic grounds. It lacked a mechanism by which empirical knowledge gained from experimentation or observation could be used to test, modify, or reject the fundamental principles underlying the theory. Aristotle’s system had the fundamental philosophical drive of modern science without its flexible procedure of self-correction that allows the truth to be approached in a series of successive approximations.

With the fall of the Roman Empire in 476 CE, much of what was known to the Greeks was lost or forgotten—at least to Western civilizations. (Hindu astronomers still taught that Earth was a sphere and that it rotated once daily.) The Aristotelian system, however, resonated with the teachings of the Roman Catholic Church during the Middle Ages, especially in the writings of St. Thomas Aquinas in the 13th century. Later, during the period of the Counter-Reformation in the 16th and early 17th century, it ascended to the status of religious dogma. Thus the notion of an Earth-centred universe become gradually enmeshed in the politics of religion. Also welcome in an age that insisted on a literal interpretation of the Scriptures was Aristotle’s view that the living species of Earth were fixed for all time. What was not accepted was Aristotle’s argument on logical grounds that the world was eternal, extending infinitely into the past and the future even though it had finite spatial extent. For the church, there was definitely a creation event, and infinity was reserved for God, not space or time.

CHAPTER 2
THE COPERNICAN REVOLUTION

The Renaissance brought a fresh spirit of inquiry to the arts and sciences. Explorers and travelers brought home the vestiges of classical knowledge that had been preserved in the Muslim world and the East, and in the 15th century Aristarchus’s heliocentric hypothesis again came to be debated in certain educated circles. The boldest step was taken by the Polish astronomer Nicolaus Copernicus, who hesitated for so long in publication that he did not see a printed copy of his own work until he lay on his deathbed in 1543. Copernicus recognized more profoundly than anyone else the advantages of a Sun-centred planetary system. By adopting the view that Earth circled the Sun, he could qualitatively explain the to-and-fro wanderings of the planets much more simply than Ptolemy. For example, at certain times in the motions of Earth and Mars around the Sun, Earth would catch up with Mars’s projected motion, and then that planet would appear to go backward through the zodiac. Unfortunately in his Sun-centred system, Copernicus continued to adhere to the established tradition of using uniform circular motion, and if he adopted only one large circle for the orbit of each planet, his calculated planetary positions would in fact be quantitatively poorer in comparison with the observed positions of the planets than tables based on the Ptolemaic system. This defect could be partially corrected by providing additional smaller circle...