THE STORY OF ASTRONOMY
Peter Aughton
New York • London
© 2011 by Peter Aughton
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THE STORY OF ASTRONOMY
Peter Aughton is the author of ten acclaimed popular history titles including Endeavor, Resolution and Voyages that Changed the World, as well as biographies of Jeremiah Horrocks and Isaac Newton. Formerly a computer engineer in the aerospace industry, where he worked on the world’s first supersonic airliner, he went on to lecture at the University of the West of England for 25 years.
CONTENTS
INTRODUCTION
1 THE ORIGINS OF ASTRONOMY
2 FROM BABYLON TO ANCIENT GREECE
3 THE ALMAGEST
4 ASTRONOMY IN THE DARK AGES
5 THE COPERNICAN REVOLUTION
6 CHASING THE PATHS OF THE PLANETS
7 GALILEO
The Great Telescope Maker
8 JEREMIAH HORROCKS
Father of English Astronomy
9 THE CLOCKWORK UNIVERSE
10 ENGLISH AND FRENCH RIVALRY
11 FINDING LONGITUDE
12 WILLIAM HERSCHEL
Gazing Deeper into Space
13 UNDERSTANDING THE FORCES OF NATURE
14 ALBERT EINSTEIN
Relativity Redefines Astronomy
15 THE HUBBLE UNIVERSE
16 FROM MICROCOSM TO MACROCOSM
17 BEYOND THE VISIBLE SPECTRUM
18 BLACK HOLES, QUASARS AND THE UNIVERSE
19 STEPHEN HAWKING
Exploring the Boundaries of Space
20 ASTRONOMY IN THE SPACE AGE
21 THE BIG BANG AND THE CREATION OF THE UNIVERSE
22 DARK MATTER AND DARK ENERGY
23 PLANETS, MOONS AND THE SEARCH FOR LIFE
GLOSSARY
FURTHER READING
INDEX
INTRODUCTION
From the time of the very earliest civilizations people have wondered about the world they live in, about how it was created and about how it will end. Not the least they wanted to understand the mysterious world of the heavens above them. In this book we travel on a journey from those earliest peoples as they stared in fear and wonder at the night sky to the 21st century cosmologists as they peer into the deepest corners of space, often looking back in time.
Our journey follows an essentially chronological sequence. It pauses to look deeper at the significant discoveries and world of famous astronomers and scientists (such as Copernicus, Galileo, Newton and Hubble). The story is also punctuated by descriptions of significant advances in technology, such as that of the light telescope or space telescope, which have enabled astronomers to make huge leaps forward in their understanding of the universe. The role of the mathematicians and other theorists (from Aristarchus to Einstein and Hawking) are not forgotten—at many stages in this story they have propelled our understanding of the cosmos to new levels.
In the most distant times the Sun was seen to make its daily journey across the sky, followed each night by the arrival of the Moon. The Moon was seen to wax or wane a little each time it appeared, and on a few nights it did not appear at all. Tiny specks of light dotted the great black dome of the heavens; some of these appeared in different nightly positions against their neighbors and became known as planets. It took millennia for man to determine the true nature of these wandering stars and to evolve a model of the world to accommodate them and to predict their positions in the sky.
It was thought that every star had its own purpose and that the secrets of the universe could be discovered by making a study of them. The telescope was invented and the hundreds of stars became thousands and then tens of thousands. There was the telescope of Galileo, followed by observatories at Paris and Greenwich, the telescopes of William Herschel and later the giant telescopes of Mount Wilson and Mount Palomar. At the end of the second millennium came the Hubble Space Telescope. The number of stars in the catalogs grew to hundreds of thousands and the number of stars visible in the telescopes became millions, and then billions. The number of stars in the sky became countless. How could these tiny points of light in the sky explain the beginnings of the world?
The tiny dots of light came under more and more scrutiny. Many of them were not simple dots at all, but whole galaxies, each consisting of millions or billions of stars. It was discovered that there was much more to light than first appeared. A star made an image on a photographic plate, but the light from the star could be spread out into its different wavelengths by passing it through a prism. The result was amazing—not only could astronomers discover the very elements the stars were made of but they could also work out the speed at which a star was approaching us or rushing away from us. There was more: some stars did not have a constant brightness and their distance could be calculated as they went through a cycle of variable brightness.
Then man discovered other wavelengths of radiation beyond the visible spectrum. The stars emitted not only visible light but also infrared, ultraviolet, radio frequencies, microwaves, gamma rays and X-rays—much more was discovered about the stars by observing them in these wavelengths. The astronomer had to call on the physicist to explain many of his results; upon the mathematician to work out theories of gravitation and relativity; and upon the engineer to build more powerful telescopes, eventually sending his instruments into orbit to escape the filtering effects of the atmosphere.
Starlight provides all the information we need to put together the story of how we come to be here, on what may well prove to be a unique planet in the universe. We still do not know the answers to all of the questions, but we can now build on the observations of hundreds of generations of astronomers. At the start of the third millennium, we can put together a credible account of the story of the creation of the universe.
Peter Aughton
2008
1
THE ORIGINS OF ASTRONOMY
To the earliest peoples the heavenly bodies were a source of fear and wonder. The rising and setting Sun and the waxing and waning Moon seemed to be things controlled by the gods. In time, civilizations began to link celestial sights with earthly events, noting that they often coincided with fertile floods, harvests and other important features of everyday life. Later, they began to plot the positions of the stars and planets, and to give them names and meanings. So began the first studies in astronomy and astrology. By observing the heavens, the first calendars could also be made.
On the appointed day, in broad daylight, the whole community gathered outside the temple. There was a hush of reverence as the astronomer priests conducted a solemn ceremony before the frightened and apprehensive crowd. The astronomer priest
s had calculated that the Moon was rapidly approaching the Sun and that a once-in-a-lifetime event was about to happen. The wise refrained from looking at the Sun, but the foolish stared and saw that a small dark bite had been taken from the shining golden disc. At first the bite was trivial in size, but as it grew steadily it became a large, dark shadow blocking out the sunlight. Soon it was obvious even to those who lowered their eyes that the Sun had become weaker. Daylight was fading. There was a fear amongst the crowd that the Sun was under attack from an unknown and powerful enemy. Not even the village elders could remember an event like this before. What was this unknown force that was draining the strength of the Sun? What if the Sun were to be extinguished altogether and the world doomed to live in a state of perpetual darkness?
Now the Sun was covered completely. It became dark—not totally black, but as if it were no longer daytime. There was a great hush as the community waited in fear. The birds had been chattering, but like the people present they, too, suddenly fell silent. Never had there been such a breathtaking silence when so many had been gathered together. Those who dared still gazed at the sky. They saw that the stars and planets, the familiar patterns of light in the night sky, had appeared in the middle of the day. These were sights that belonged with the night, not the daytime. It was a terrible omen from the gods. The total darkness lasted only a few minutes but it seemed much longer, and in the middle of the day it felt much stranger than any darkness they had known before.
Some believed the end of the world had come. They waited hushed in the darkness and they stared at the stars. Then, after a time, there came a small but brilliant flash of light from the edge of the dark disc. It became a little brighter and then it appeared like a string of golden beads as the craters and the mountains of the Moon broke the thin crescent of sunlight into an irregular arc of fire. There was a murmur of expectation, and the arc of light grew brighter until the slender crescent could be seen behind the black disc that many thought was an evil dragon consuming the Sun. But the astronomers knew that the dragon in the sky was the familiar Moon. The crescent of light grew slowly, and by degrees the Sun began to appear again. There was a great feeling of relief. Perhaps the Sun had not been destroyed after all. Then the silence was broken and everybody was chattering about the event. They had seen a spectacle that they would never forget. Many years later the children would tell the tale to their grandchildren, but those who were not present would never believe the details of their story.
Observing and Understanding
Of all the branches of knowledge known to us, astronomy has always held a special place. It has a history that spans every era of human development, and its origins can be traced back to the earliest civilizations. It holds a special place because it strives to explain the origins and the purpose of the universe. Every new attempt to study and measure the skies reveals new and surprising facts. As soon as one astronomical feature appears to be solved and understood, new and stranger phenomena are discovered.
The Sun and the Moon are the most prominent objects in the sky; it is pure coincidence that they appear almost the same size to us. But there is also a myriad of tiny dots of light in the night sky, and these, too, must be studied to discover the secrets of the nature and origins of the universe. The deeper we penetrate into the night sky and the more we study the objects we find, the deeper becomes the mystery of creation—a mystery that we will probably never solve to our complete satisfaction.
In the ancient world, as the first stable civilizations developed, there were always some who were fascinated by the night sky. The shepherd in his lonely vigil had plenty of time to study the sky, to learn the positions of the stars, to observe the phases of the Moon and to notice that the stars appeared in the same patterns every night. While the stars maintained these patterns, the Sun and the Moon had their own independent motion across them. The whole night sky appeared to revolve throughout the night about a point in the north; it was as though the sky were painted on the inside of a huge globe rotating about the Earth. There were also seasonal differences in the sky. The night sky in winter did not have the same appearance as the night sky in summer. Sometimes new constellations (patterns of stars) appeared near the horizon and old constellations disappeared. There was an annual cycle, and the missing constellations always returned as the seasons changed. There was also a small number of star-like objects that wandered across the night skies; they sometimes exhibited a backward (or retrograde) motion before progressing forward once more.
Every civilization wondered about the heavens. Priesthoods developed in which men were trained to make a special study of the stars. The Sun was very obviously the most influential object in the sky. It was so strong, bright and powerful that as soon as it rose in the morning all the stars faded from view. The purpose of the Sun was obvious. It gave light in the daytime and it helped the crops to grow. The purpose of the Moon was not so obvious; although it often gave a pale light by night, there were many nights when it only made a very late appearance and there were nights when it gave out no light at all. It exhibited phases, from the thin crescent of the new Moon to the bright circle of the full Moon, with every other possible phase in between. These phases were easily predicted, and it was not difficult to deduce that they depended on the relative position of the Sun. It did not take astronomers very long to realize that the Moon was actually a sphere, and the phases came about because it was illuminated by light from the Sun. It came as a revelation when it was found that the phases of the Moon were also related to the movement of the tides. The Moon obviously held a great sway over the seas, but it was hard to explain in physical terms how a sphere traveling across the sky in a monthly cycle could affect the tides. It was logical to deduce that every object in the skies had some small influence on the Earth. The learned people of the day worked hard to discover the nature of these influences.
Discovering the Planets
As we have noted, there were also other objects in the heavens that were visible to early astronomers apart from the Sun, the Moon and the stars. There were wandering “stars” that seemed to have their own motion. It was thought that these bodies, which became known as the planets, must also have some strange influence over the Earth. One of these planets was Venus, the “morning star” that sometimes heralded the dawn, but which at other times appeared in the evening when it was known as the “evening star.” It was the brightest planet in the sky. There was the red planet that became known in the Roman world as Mars. There were the slower-moving planets that the Romans called Jupiter and Saturn. There was a small planet they called Mercury that was difficult to see, for it stayed close to the Sun. It was assumed that the planets had all been put in the sky for a reason, and that every star had a message to tell. These reasons and messages were not easy to explain, however.
Signs from the Heavens
In Egypt, it was discovered that the rising above the horizon of Sirius, the Dog Star, heralded the flooding of the River Nile. It was a great triumph for the astronomer priests to be able to predict when the Nile was about to flood, for it enabled the farmers to prepare themselves for the event—fertile soil brought down by the river enriched the farmland and improved the harvest. The astronomer priests also hoped to discover the purpose of other stars in the sky. The occasional meteor flashed across the heavens. It never returned, but it was not difficult to make a link with the phenomenon and some earthly event—an important death or a birth, for example. Less frequently a comet appeared; this was so rare that it was frequently deemed to be a bad omen in many cultures. Surely, reasoned the astronomer priests, all these portents in the skies were signs from the gods trying to tell them something.
The development of astronomy and astrology were not confined to Egypt and the Mediterranean world. The Chinese, the Aztecs and the Incas all studied the stars and tried to predict eclipses and other events in the heavens. Some of the stars appeared to form distinct constellations, and the vivid imagination of the ancient astronomers s
aw all kinds of patterns formed by them—mainly creatures and ancient gods. There were dogs and bears, there were archers, and there were dragons, serpents and scorpions. Twelve of these constellations formed a wide path through which the Sun, Moon and planets appeared to move during the year. These were the constellations of the zodiac, and they held special significance. The stars were not visible in the daytime, but it was easy to calculate which sign of the zodiac “contained” the Sun at any given time. It was also possible to calculate in which part of the zodiac the Moon and each of the planets lay. Thus came the beginnings of astrology and the belief that the positions of the heavenly bodies in the sky affected our life on Earth. Chinese records relating to the study of the stars go back to the third millennium BC, and many of the dates in their chronology can be identified from their records of eclipses. Thus the Shang Dynasty, dating from about 1760 BC to 1122 BC, is well recorded astronomically and it contains much data from these early centuries.
Early Theories about Earth
It was only a question of time before the first theories were put forward about the origins of the world. The earliest ancient civilizations thought the Earth to be flat. They thought it was natural for everything to be drawn to the ground. There was no need to explain gravity; it was a familiar phenomenon. If voyagers traveled far enough in any direction they would find that the Earth had a limit, a place in the ocean where any ship would fall off the edge of the world or perhaps a place beyond the mountains where there was a sudden end to the land. To explain the night sky, a great sphere of clear crystal was envisaged that carried all the stars and the planets on their diurnal journey. The Sun and the Moon had their own spheres and their own independent motions. The extent of the Earth was not known, but it was obvious to all that it was the center of the universe; it seemed larger than the Moon or the Sun and far larger than any other celestial object.
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