300 Astronomical Objects: A Visual Reference to the Universe

0ne of the biggest challenges to humanity in understanding the universe is coming to terms with the immense scales involved. Our minds have developed to deal with local geography, such as the rivers and hills around a settlement. As we made the transition from a tribal to a global species, we explored Earth. About 3,000 years ago, at the time of the Greek poet Homer, maps of the world consisted only of the Mediterranean and its neighboring land. Earth itself was thought to be a flat body of land surrounded by unknown seas.

Scientific advancement pushed back the geographical boundaries. While investigating shadows cast by sticks at noon on a summer solstice at two places 500 miles (800 km) apart, Greek philosopher Eratosthenes (c.276-c.194 B.C.) determined that Earth's surface must be curved. On a flat Earth, the sticks' shadows would have been equal, but they were not. Furthermore, he was able to estimate Earth's circumference to be about 25,000 miles (40,000 km). This new understanding of the world prompted sailors and explorers to imagine circumnavigating the globe.

Of course humanity looked not only north, south, east, and west, but also up into the sky. The regularity of the passage of the Sun, the Moon, and the stars was understood well before people began keeping records. This led to an understanding of time and the passage of the seasons. Ancient monuments such as Stonehenge in England are aligned in a way that suggests they were used to calculate the annual calendar.

The motion of the skies was crucial to the survival of early societies and they attached great significance to them. Changes in the skies were thought to be omens.The practice of interpreting omens from the heavens is known as astrology. Throughout most of their recorded history, rulers in China paid great attention to their court astrologers. Huge effort was put into observing the heavens and recording the various phenomena, and China produced the earliest known records of events such as solar eclipses and comets. Texts describing every return of Halley's Comet for 2,000 years have helped modern astronomers refine their understanding of its orbit. References to sunspots provide us with information on potential long-term variations in the output of power from the Sun. The famous Crab Nebula is the remnant of a supernova explosion that was documented by Chinese court astronomers in 1054.

Astrology was by no means unique to the Chinese. Scientific study of the universe was typically driven by a desire to understand the future rather than the universe itself. The Ancient Greek philosophers used the stars to predict events. One of the greatest was Aristotle (384-322 BC), who studied almost every subject known at the time and produced many books in which he put forward his views. His concept of the universe was geocentric: Earth was stationary at the center, and the planets, the Moon, and the Sun were in orbit around it. He stated that the heavens (the sky) were perfect and incorruptible, unlike Earth. The path of the planets must therefore take the form of a circle, which was the most perfect shape known. Not all of Aristotle's pronouncements relied on real evidence, and in this case the observed motion of the planets was incompatible with his simple model. Viewed over several weeks, the path of a planet in Aristotle's system would simply trace a curved line across the sky. In reality, Mars, Jupiter, and Saturn all appear to slow down at times in their prescribed routes, and even go backward for a while (retrograde motion). To reconcile this apparent discontinuity, the concept of epicycles was proposed, whereby each planet moved in a circle (an epicycle), the center of which (known as the deferent) itself described a larger circle around Earth. This theory of the universe was codified by Ptolemy (c.100-c.1 70) in his treatise on astrology.

The line of Greek philosophers died out at this point, and Europe underwent a period known as the Dark Ages. The great library at Alexandria, the storehouse of ancient knowledge, was lost. Ptolemy's treatise survived, however, as an Arabic translation entitled the Almagest. Arabic civilizations drew on Ptolemy's work to refine their own brand of astrology and to achieve accurate timekeeping that was essential for their prayer times. Elaborate and accurate astrolabes followed, enabling followers of Islam to know the direction of Mecca.

Arabic astronomers soon improved on the Almagest, especially in the area of star catalogs. Most of the names we use for stars today come from the Arabic language, although often corrupted by the passage of time. (For example, the name Acubens, the Alpha star of Cancer, comes from the Arabic al zubanah, meaning "the claws.") While the precision of astronomical knowledge was greatly enhanced during this time, the geocentric cornerstone of the Ptolemaic system was not successfully challenged. After the chaos resulting from the fall of the Roman Empire had cleared, astronomical knowledge flooded back into Europe through texts translated from Arabic. The survival of Greek geometry alongside Arabic numerals and algebra was essential for what followed.

Although Polish astronomer Nicolaus Copernicus (1473-1543) was not the first person to conceive of a heliocentric (Sun-centered) universe, he is credited away from the beliefs of Ptolemy and Aristotle. In his controversial book De Revolutionibus he argued that the Earth moved, rotating on its own axis as well as orbiting the Sun. The other planets also orbited the Sun, and Copernicus correctly calculated their order, placing Earth between Venus and Mars. This simple alteration solved several of the issues surrounding the Ptolemaic system, which had remained the accepted model for 1,700 years. Unsurprisingly, it was not well received, and Copernicus was criticized in print by many who believed his theories to be an attack on religion itself.

Despite the advances made after Ptolemy's time, the charts and tables that were used to predict planetary motions were still inaccurate, partly because they used circular orbits and partly because they were based on imprecise observations. A Danish astronomer, Tycho Brahe (1546-1601), improved on previous attempts, in particular with measurements of the movement of Mars. His accuracy was all the more amazing since the telescope had not yet been invented. He was also a witness to the supernova of 1572, known thereafter as Tycho's Star. It was another nail in the Ptolemaic view of the world -- the Ancient Greeks had taught that the heavens were constant and unchanging (comets were seen as simply atmospheric phenomena).

Shortly before he died, Tycho was joined by an assistant, the German mathematician Johannes Kepler (1571-1630). Armed with Tycho's observational data, Kepler was able to construct a new model of the universe. Kepler's breakthrough was the realization that the orbits of the planets are not perfect circles but are slightly elliptical. He formulated his ideas as a set of mathematical laws that also related each planet's orbital period to its distance from the Sun.

Overlapping the careers of Tycho and Kepler was the life of Italian scientist Galileo Galilei (1564-1642). His use of experimentation and observation was in direct contrast to the Aristotelian methods of arguing the nature of reality from pure reason. Sometime in the first decade of the 17th century the telescope was invented in Holland. Galileo heard about this new invention and proceeded to construct telescopes of his own, improving the quality and power in each one he built.

The increased vision from the telescopes showed Galileo that there were many more stars in the sky than any unaided observer would ever have guessed. The Milky Way was resolved into a dense region of stars rather than the cloudlike object it was previously believed to be. Galileo saw changing spots on the surface of the Sun and craters on the Moon. He could see the closer planets as disks, suggesting that they were much closer to Earth than the stars that remained point sources of light. He also saw the rings of Saturn, although his telescopes were not powerful enough to fully reveal their nature. Most importantly for the events that followed, he saw four tiny satellites flanking the planet Jupiter. Through repeated observations Galileo became convinced that these objects were in orbit about Jupiter, proving beyond any doubt that the Earth was not the only center of motion in the universe.

Living in Italy, home of the influential Catholic church, Galileo was bound to come into conflict with the prevailing Ptolemaic philosophy. He made no secret of his theories, which were technically heretical. He was, however, friends with many important members of the Church. At first, these friendships gave him protection, but pressure to reassert the dominance of the geocentric model prevailed. Galileo was ordered by his friend Cardinal Bellarmine to cease teaching the theory of a moving Earth. He kept quiet for a while, but the promotion to the papacy of another of his friends (Cardinal Barberini, who became Pope Urban VIII) gave Galileo fresh encouragement to push the boundaries. He wrote a dialog pitting a proponent of the Ptolemaic system against a believer in Copernicanism. It was a popularization of Galileo's theories and beliefs and was construed as an effort to present both sides of the argument. This enabled the dialog to be printed and distributed, but the Papacy turned against it and forced Galileo to renounce his ideas under threat of the Inquisition. But with the book in circulation, the damage to the geocentric model of the universe had been done.

In his masterwork Principia Mathematica, English mathematician and physicist Isaac Newton (1642-1727) completed the shift away from an Earth-centered to a Sun-centered universe. His universal theory of gravity gave the heliocentric model mathematical foundations, surpassing the laws of Kepler. Newton's theory of the force of gravity explained phenomena such as the fall of an apple, the changing of the tides, and the paths of the planets. Newton's contemporary Edmund Halley (1656-1742) used the theory to predict correctly the periodic return of a particular comet that both he and Kepler had observed -- now known as Halley's comet. Newton's other major advances in the theory of light would lead to spectrographic analysis, an invaluable tool for astrophysics right up to the present day.

It was not until about the 18th century that our knowledge of the universe extended beyond Saturn's orbit. German born English astronomer William Herschel (1738-1822) discovered the next farthest planet --Uranus -- in 1781. His study of the motion of stars in the sky independent of Earth's movement also led him to realize that the solar system itself was moving through space. His discovery of binary stars was the first evidence of "centers of motion" outside the solar system. With each increase in knowledge, Earth's place in the universe became less and less "special." Spectroscopy revealed that the light from the distant stars was very similar to that of the Sun, suggesting that it was not a unique object as the ancients had assumed.

In the same way that improvements in the engineering discipline of boat building enabled the early explorers to expand our geographical knowledge, the advances in telescope manufacture were revealing more and more clues about the nature of the universe. French astronomer Charles Messier (1730-1817) discovered a number of fuzzy "nebulae" with his telescope; later observers with better equipment began to determine individual stars in some of these clouds and spiral shapes in others. Eventually, in 1924, these spiral shapes were shown to be vastly more distant from Earth than ordinary stars -- they were galaxies. The size of humanity's known universe took another enormous leap forward. The man who determined the nature of the "spiral nebulae" was American astronomer Edwin Hubble (1889-1953). He was able to show that not only were the galaxies located at a tremendous distance from our own, but they were also moving away from us. Hubble's observations, combined with the newly developed general theory of relativity devised by German-born American physicist Albert Einstein (1879-1955), produced the concept of an expanding universe. Extrapolating these theories into the past resulted in the big bang theory, which is now well established as the dominant scientific theory of the origin of the universe.

In just 6,000 years or so our view of the universe has developed beyond recognition. Our forebears perceived a world that was small and flat with lights in the night sky above. After numerous revisions and theoretical blind alleys we have come to acknowledge the existence of a mind-bogglingly huge universe that is 13.7 billion years old, filled with galaxies and stars and planets. In spite of all our advances, there is still a vast amount about this universe that is unknown. We have not yet reached the end of our revelations.