The Unfolding Universe

A subject can always be better understood if something is known about its history. Though we no longer worship our "honourable ancestors", it is a distinct help to look back through time in order to see how knowledge has been built up through the centuries. This is particularly true with astronomy, which is the oldest science in the world - so old, indeed, that we do not know when it began.

Most people of today have at least some knowledge of the universe in which we live. The Earth is a globe nearly 8000 miles in diameter, and is one of nine planets revolving round the Sun. The best way of summing up the difference between a planet and a star is to say that the Earth is a typical planet, while the Sun is a typical star.

Five planets - Mercury, Venus, Mars, Jupiter and Saturn - were known to the ancients, while three more have been discovered in modern times. Jupiter is the largest of them, and its vast globe could swallow up more than a thousand bodies the volume of the Earth, but even Jupiter is tiny compared with the Sun. The stars of the night sky are themselves suns, many of them far larger and more brilliant than our own, and appearing small and faint only because they are so far away. On the other hand, the Moon shines more brilliantly than any other object in the sky apart from the Sun. Appearances are deceptive; the Moon is a very junior member of the Solar System, and it has no light of its own. It has a diameter only about one-quarter that of the Earth, and it is much the closest natural body in the sky.

The whole celestial vault seems to revolve round the Earth once in 24 hours. This apparent motion is due, of course, to the fact that the Earth is spinning on its axis from west to east. Of all the celestial objects, only the Moon genuinely moves round the Earth. We are used to taking these facts for granted, but in early times it was (rather naturally) believed that the Earth was flat and stationary. The Sun and Moon were worshipped as gods, and the appearance of anything unusual, such as a comet, was taken to be a sign of divine displeasure.

It is usually said that the first astronomers were the Chaldaeans, the Egyptians and the Chinese. In a way this is true enough; these ancient civilizations made useful records, but they had no real understanding of the nature of the universe or even of the Earth itself.

The main story begins around 3000BC, when the 365-day year was first adopted in Egypt and in China. This, too, was the approximate date of the building of that remarkable structure which we know as the Great Pyramid of Cheops, still one of the world's main tourist attractions. Astronomically, it is of special interest because its main passage is aligned with what was then the north pole of the sky.

The Earth's axis of rotation is inclined at an angle of 231 degrees to the perpendicular to its orbit, and points northward to the celestial pole (Fig 2.1). Today the pole is marked approximately by a bright star known as Polaris, familiar to every navigator because it seems to remain almost stationary while the entire sky revolves round it. In Cheops' time, however, the polar point was in a different position, close to a much fainter star, Thuban in the constellation of the Dragon. The reason for this change is that the Earth is 'wobbling' slightly, in the manner of a gyroscope which is running down, so that the direction of the axis is describing a small circle in the sky. The effect is very slight, but the shift of the pole has become appreciable since the Pyramid was built.

The Egyptians divided up the stars into constellations, though their scheme was different from that which we follow today. They also made good measurements, and they regulated their calender by the 'heliacal rising' of the star Sirius - that is to say, the date when Sirius could first be seen in the dawn sky. Some of their other ideas were very wide of the mark. They believed the world to be rectangular, with Egypt in the middle, and that the sky was formed by the body of a goddess with the rather appropriate name of Nut.

The Chinese were equally good observers, and made careful records of comets and eclipses. Total eclipses of the Sun are particularly spectacular, and at this point I cannot resist re-telling a famous legend, even though experts assure me that it is certainly untrue! Here, then, is the story of Hsi and Ho:

The Moon revolves round the Earth once a month, while the Earth takes a year to complete one journey round the Sun. The Moon is much smaller than the Sun, but it is also much closer, so that - by pure chance - the two look almost exactly the same size. When the Sun, Moon and Earth move into an exact line, with the Moon in the mid-position, the result is a total solar eclipse. The Moon blots out the bright disk of the Sun, and for a few moments - never as long as eight minutes -we can see the glorious pearly corona and the 'red flames' or prominences; the sky becomes so dark that stars can be seen.

The Chinese knew how to predict eclipses - more or less - but they did not know that the Moon was involved; they thought that the Sun was in danger of being eaten by a hungry dragon, so that the only course was to scare the beast away by shouting, screaming, wailing, and beating gongs and drums. (It always worked!) The legend says that in 2136 BC, during the reign of the Emperor Chung K'ang, the Court Astronomers, Hsi and Ho, failed to give due warning that an eclipse was due, so that no preparations were made - and since Hsi and Ho had imperilled the whole world by their neglect of duty, they were summarily executed. I am sorry that the experts have demolished this tale. Had it been true, Hsi and Ho would have been the first known scientific martyrs in history. I have no idea where the story originated.

Astronomy in its true form began with the Greeks, who not only made observations but who also tried to explain them. The first of the great philosophers was Thales of Miletus, who was born in 624 BC; the last was Ptolemy of Alexandria, and with his death, in or about A.D. 180, the classical period of science comes to an end. During the intervening eight centuries, human thought made remarkable progress.

Thales himself may have been the first to realize that the Earth is a globe, but unfortunately all his original writings have been lost. The first definite arguments against the old idea of a flat Earth were given by Aristotle, who was born in 384 B.C.

Fig. 2.2. Eratosthenes' method of measuring the circumference of the Earth.

Fig. 2.2. Eratosthenes' method of measuring the circumference of the Earth.

and died in 322. Aristotle was one of the most brilliant men of the ancient world, and his reasoning shows the Greek mind at its best.

As Aristotle points out, the stars appear to alter in altitude above the horizon according to the latitude of the observer. Polaris appears to remain fairly high in the sky as seen from Greece, because Greece is well north of the terrestrial equator; from Egypt, Polaris is lower; from southern latitudes it cannot be seen at all, since it never rises above the horizon. On the other hand Canopus, a brilliant star in the southern part of the sky, can be seen from Egypt but not from Greece. This is just what would be expected on the theory of a round Earth, but such behaviour cannot possibly be explained if we suppose the Earth to be flat. Aristotle also noticed that during a lunar eclipse, when the Earth's shadow falls across the Moon, the edge of the shadow appears curved - indicating that the surface of the Earth must also be curved.

The next step was taken by Eratosthenes of Cyrene, who succeeded in measuring the length of the Earth's circumference (c. 240 BC). His method was most ingenious, and proved to be remarkably accurate. Eratosthenes was in charge of a great scientific library at Alexandria, in Egypt, and from one of the books available to him he learned that at the time of the summer solstice, the "longest day" in northern latitudes, the Sun was vertically overhead at noon as seen from the town of Syene (the modern Assouan), some distance up the Nile. At Alexandria, however, the Sun was at this moment 7 degrees away from the overhead point, as is shown in Fig 2.2. A full circle contains 360 degrees, and 7 is about 1/50 of 360, so that if the Earth is spherical its circumference must be 50 times the distance from Alexandria to Syene. Eratosthenes may have arrived at the final figure of 24,850 miles, which is only fifty miles too small.*

If the Greeks had taken one step more, and placed the Sun in the centre of the planetary system, the progress of astronomy would have been rapid. Some of the philosophers tried to do so; but unfortunately Aristotle held the Earth to be the centre of the universe, and Aristotle's authority was so great that few people dared to question it. Moreover, the decentralization of the Earth would have meant a change in the laws of "physics", since Aristotle's idea of "things seeking their natural place" would have been much disturbed.

Most of our knowledge of Greek astronomy is due to Claudius Ptolemaeus (Ptolemy), who around AD 150 wrote a great book known generally by its Arab title of the Almagest. In it, he sums up the ideas of the great philosophers who had lived before him; and the theory that the Earth lies at rest in the centre of the universe is therefore called the "Ptolemaic", though as a matter of fact Ptolemy himself was not directly responsible for it.

On the Ptolemaic theory, all the celestial bodies move round the Earth. Closest to us is the Moon; then come Mercury, Venus, the Sun, Mars, Jupiter, Saturn and finally the stars. Ptolemy maintained that since the circle is the "perfect" form, and nothing short of perfection can be allowed in the heavens all these bodies must move in circular paths. Unfortunately, the planets have their own ways of behaving.

*There is some doubt whether Eratosthenes' estimate was accurate to within a few tens of miles, but at least his results were not wildly in error.

Ptolemy was an excellent mathematician, and he knew quite well that the planetary motions cannot be explained on the hypothesis of uniform circular motion round a central Earth. He therefore worked out a complex system according to which each planet moved in a small circle or "epicycle", the centre of which itself moved round the Earth in a perfect circle. As more and more irregularities came to light, more and more epicycles had to be introduced, until the whole system became hopelessly artificial and cumbersome.

Hipparchus, who had lived some two centuries before Ptolemy, had drawn up a detailed and accurate star catalogue. The original had been lost, but fortunately Ptolemy reproduced it in his Almagest, so that most of the work has come down to us. Hipparchus was also the inventor of an entirely new branch of mathematics, known to us as trigonometry.

When the power of Greece crumbled away, astronomical progress came to an abrupt halt. The great library at Alexandria was looted and burned in A.D. 640, by order of the Arab caliph Omar, though in fact most of the books may have been scattered earlier; in any case, the loss of the Library books was irreparable, and scholars have never ceased to regret it. For several centuries very little was done. When interest in the skies did return, it came - ironically enough - by way of astrology.

Even today, there are still some people who do not know the difference between astrology and astronomy. Actually, the two are utterly different. Astronomy is an exact science; astrology is a relic of the past, and there is no scientific basis for it, though in some countries (notably India) it still has a considerable following.

The best way to define astrology is to say that it is the superstition of the stars. Each celestial body is supposed to have a definite influence upon the character and destiny of each human being, and by casting a horoscope, which is basically a chart of the positions of the planets at the time of the subject's birth, an astrologer claims to be able to foretell the destiny of the person for whom the horoscope is cast. There may have been some excuse for this sort of thing in the Dark Ages, but there is none today. The best that can be said of astrology is that it is fairly harmless so long as it is confined to circus tents and the less serious columns of the Sunday newspapers.

However, mediaeval astrology did at least lead to a revival of true astronomy. The Arabs led the way, and presently interest spread to Europe. Star catalogues were improved, and the movements of the Moon and planets were re-examined. There were even observatories; very different from the domed buildings of today, but observatories nonetheless.

Astronomy was still crippled by the blind faith in Ptolemy's system. So long as men refused to believe that the Earth could be in motion, no real progress could be made. The situation was not improved by the attitude of the Church, which in those times was all-powerful. Any criticism of Aristotle was regarded as heresy. Since the usual fate of a heretic was to be burned at the stake, it was clearly unwise to be too candid.

The first serious signs of the approaching struggle came in 1546, with the publication of De Revolutionibus Orbium Caelestium (Concerning the Revolution of the Heavenly Bodies) by a Polish canon, Nicolas Copernicus. Copernicus was a clear thinker, as well as being a skilful mathematician, and at a fairly early stage in his career he saw so many weak links in the Ptolemaic system that he felt bound to abandon it. It seemed unreasonable to suppose that the stars could circle the Earth once a day. In his own words,"Why should we hesitate to grant the Earth a motion natural and corresponding to its spherical form? And why are we not willing to acknowledge that the appearance of a daily rotation belongs to the heavens, its actuality to the Earth? The relation is similar to that of which Virgil's Aeneas said, 'We sail out of the harbour, and the countries and cities recede.'"

Copernicus' next step was even bolder. He saw that the movements of the Sun, Moon and planets could not be explained by the old system even when all Ptolemy's circles and epicycles had been allowed for, and so he rejected the whole theory. He placed the Sun in the centre of the system, and reduced the status of the Earth to that of a perfectly ordinary planet.

Copernicus was wise enough to be cautious. He knew that he was certain to be accused of heresy, and though his book was probably complete by 1530 he refused to publish it until the year of his death. As he has foreseen, the Church was openly hostile. Bitter arguments raged throughout the next half-century, and one philosopher, Giordano Bruno, was burned in Rome because he insisted that Copernicus had been right.*

Tycho Brahe, born in Denmark only a few months after Copernicus died, was utterly unlike the gentle, learned Polish mathematician. Tycho was a firm believer in astrology, and an equally firm disbeliever in the Copernican system, so that it is ironical to realize that his own work did much to prove the truth of the new ideas. He built an observatory on the island of Hven, in the Baltic, and between 1576 and 1596 he made thousands of very accurate observations of the positions of the stars and planets, finally producing a catalogue that was far better than Ptolemy's. Of course, he had no telescopes; but his measuring instruments were the best of their time, and Tycho himself was a magnificent observer.

The story of his life would need a complete book to itself. Tycho is, indeed, one of the most fascinating characters in the history of astronomy. He was proud, imperious and grasping, with a wonderful sense of his own importance; he was also landlord of Hven, and the islanders had little cause to love him. His observatory was even equipped with a prison, while his retinue is said to have included a pet dwarf. Yet despite all his shortcomings, he must rank with the intellectual giants of his age. Nowadays, nothing remains of his great Uraniborg observatory.

When Tycho died, in 1601, he left his observations to his assistant, a young German mathematician named Johann Kepler. After years of careful study, Kepler saw that the movements of the planets could be explained neither by circular motion round the Earth, nor by circular motion round the Sun, so that there was something wrong with Copernicus' system as well as with that of Ptolemy. Finally, he found the answer. The planets do indeed revolve round the Sun, but not in perfect circles. Their paths, or "orbits", are elliptical.

One way to draw an ellipse is shown in Fig 2.3. Fix two pins in a board, and join them with a thread, leaving a certain amount of slack. Now loop a pencil to the thread, and draw it round the pins, keeping the thread tight. The result will be an

*This was not Bruno's only crime in the eye's of the Church, but it was certainly a serious one.

Fig. 2.3. Method of drawing an ellipse.

ellipse,* and the distance between the two pins or "foci" will be a measure of the eccentricity of the ellipse. If the foci are close together, the eccentricity will be small, and the ellipse very little different from a circle, if the foci are widely separated, the ellipse will be long and narrow.

The five planets known in Kepler's day proved to have paths which were almost circular, but not quite. The slight departure from perfect circularity made all the difference, and Tycho's observations fell beautifully into place, like the last pieces of a jig-saw puzzle. The age-old problem had been solved, though the Church authorities continued to oppose the truth for some time longer. Kepler's three Laws of Planetary Motion, the last of which was published in 1618, paved the way for the later work of Sir Isaac Newton.

Kepler's work was not the only important development to enrich the early part of the seventeenth century. In 1608 a spectacle-maker of Middelburg in Holland, Hans Lippershey, found that by arranging two lenses in a particular way he could obtain magnified views of distant objects. Spectacles had been in use for some time - according to some authorities, they were invented by Roger Bacon - but nobody had hit upon the principle of the telescope until Lippershey did so, more or less by accident.

A refracting telescope consists basically of two lenses. One, the larger, is the object-glass; its function is to collect the rays of light coming from a distant object, and bunch them together to form an image at the focus (Fig 2.4.). The image is then magnified by a smaller lens known as an eyepiece. This is more or less the principle used in the naval and hand telescopes of today, as well as in ordinary binoculars.

The news of the discovery spread across Europe, and came to the ears of Galileo Galilei, Professor of Mathematics at the University of Padua. Galileo, was quick to see that the telescope could be put to astronomical use, and "sparing neither trouble nor expense", as he himself wrote, he built an instrument of his own. It was a tiny thing, pitifully feeble compared with a modern pocket telescope, but it helped towards a complete revolution in scientific thought.

Galileo's first telescopic views of the heavens were obtained towards the end of 1609. At once, the universe began to unfold before his eyes. The Moon was covered

*The method is excellent in theory. In practice, what usually happens is that the pins fall down or the thread breaks. One day, I hope to carry out the whole manoeuvre successfully.

Rays of

Eyepiece (

Dbject glass

Fig. 2.4. Principle of the refractor.

Fig. 2.4. Principle of the refractor.

with dark plains, lofty mountains and giant craters; Venus, the Evening Star of the ancients, presented lunar-type phases, to that it was sometimes crescent, sometimes half and sometimes nearly full; Jupiter was attended by four moons of its own, while the Milky Way proved to be made up of innumerable faint stars.

Galileo had always believed in the new system of the universe, and his telescope work made him even more certain. Inevitably he found himself in trouble with the Church. It was hard for religious leaders to realize that the Earth is not the most important body in the universe, and Galileo seemed to them to be a dangerous heretic. He was arrested and imprisoned, after which he was brought to trial and forced to "curse and abjure and detest" the false theory that the Earth moves round the Sun.

Few people were deceived, and before the end of the century the Ptolemaic theory had been abandoned for ever. The publication of Isaac Newton's Principia, in 1687, led to real understanding of the way in which the planets move.

Most people have heard the story of Newton and the apple. It is interesting because unlike most stories of similar type, such as Canute and the waves, it is probably true. Apparently Newton was sitting in his garden one day when he saw an apple fall from its branch to the ground, and upon reflection he realized that the force pulling on the apple was the same force as that which keeps the Moon in its path round the Earth. From this he was led on to the idea of "gravitation", upon which the whole of later research has been based. It is fair to say that Kepler found out "how" the planets move; Newton discovered "why" they do so.

Newton also constructed an entirely new type of telescope. As has been shown, Galileo's instrument was a refractor, and used an object-glass to collect its light. Newton came to the conclusion that refractors would never be really satisfactory, and looked for some way out of the difficulty. Finally he decided to do away with object-glasses altogether, and to collect the light by means of a specially-shaped mirror.

When Newton rejected the refractor as unsatisfactory, he was making one of his rare mistakes. However the Newtonian "reflector" soon became popular, and has remained so. Mirrors are easier to build than lenses, and even today all the world's largest instruments are of the reflecting type.

Astronomy was growing up. So long as observations had to be made with the naked eye alone, little could be learned about the nature of the planets and stars; their movements could be studied, but that was all. As soon as telescopes became available, true observatories made their appearance. Copenhagen and Leyden took the lead; the Paris Observatory was completed in 1671, and Greenwich in 1675.

Greenwich was founded for a special reason. England has always been a seafaring nation, and before the development of reliable clocks the only way in which sailors could fix their position when far out in the ocean, out of sight of land, was to observe the position of the Moon among the stars. This involved the use of a good star catalogue, and the best one available, Tycho's, was still not accurate enough. Charles II therefore ordered that the star places must be "anew observed, examined and corrected for use of my seamen". A site was selected in the Royal Park at Greenwich, and Sir Christopher Wren, himself a former professor of astronomy, designed the first observatory building. The Rev John Flamsteed was appointed Astronomer Royal, and in due course the revised star catalogue was completed.

Telescopes continued to be improved. Some of the early instruments were curious indeed; one of them, used by the Dutch observer Christiaan Huygens, was over 200 feet long, so that the object-glass had to be fixed to a mast. But gradually the worst difficulties were overcome, and both refractors and reflectors gained in power and in convenience. Mathematical astronomy made equally rapid strides. The great obstacle had always been the Ptolemaic system, and once that had been swept away the path was clear. The distance between the Earth and the Sun was measured with fair accuracy, and in 1675 the Danish astronomer Ole R0mer even measured the speed of light, which proved to be 186,000 miles per second. R0mer did this, incidentally, by observing the movements of the four bright moons of Jupiter.

But though knowledge of the bodies of the Solar System had improved out of all recognition, little was known about the stars, which were still regarded as mere points of reference. The first serious attack on their problems was made by William Herschel, who is rightly termed the "father of stellar astronomy".

Herschel was born in Hanover in 1738, eleven years after the death of Newton. He came to England, and became organist at the Octagon Chapel in Bath; but his main interest was astronomy, and he built reflecting telescopes which were the best of their age. The largest of Herschel's telescopes, completed in 1789, had a mirror 48 inches in diameter and a focal length of 40 feet. The mirror still exists, and now hangs on the wall of Flamsteed House in Greenwich, though it has not been used since Herschel's time.

Herschel had his living to earn, and for some years he could not afford to spend all his time in studying astronomy. Then, in 1781, he made a discovery which altered his whole life. One night he was examining some faint stars in the constellation of the Twins, when he came across an object which was certainly not a star. At first he took it for a comet, but as soon as its path was worked out there could no longer be any doubt as to its nature. It was not a comet, but a planet - the world we now call Uranus.

The discovery was quite unexpected. There were five known planets, and these, together with the Sun and Moon, made a grand total of seven. Seven was the magical number of the ancients, and it had therefore been thought that the Solar System must be complete. Herschel became world-famous; he was appointed Court Astronomer to King George III, and henceforth he was able to give up his musical career altogether.

Herschel set himself a tremendous programme. He decided to explore the whole heavens, so that he could form some idea of the way in which stars were arranged. Until the end of his long life, in 1822, he worked patiently at his task, and his final conclusions have been proved to be reasonably accurate.

Naturally, Herschel made numerous discoveries during his sky-sweeps. Many apparently single stars proved to be double, and there were also clusters of stars, as well as faint luminous patches known as "nebulae", from the Latin word meaning "clouds". Herschel was a most painstaking observer. He catalogued all his discoveries, and when we examine his published papers we can only marvel at the amount of work he managed to do. Since he lived in England for most of his life, he was unable to examine the stars of the far south, which never rise in northern latitudes, and it was fitting that the completion of his sky-sweeps should be accomplished later by his son, Sir John Herschel, who travelled to the Cape of Good Hope specially for this purpose, and remained there for several years.

Another famous observer of this period was Johann Schröter, chief magistrate of the little German town of Lilienthal. Unlike Herschel, Schröter concentrated mainly upon the Moon and planets, and he is the real founder of "selenography", the physical study of the lunar surface. Unfortunately Schröter's observatory, together with all his unpublished work, was destroyed by the invading French armies in 1814, and Schröter himself died two years later.

In the early years of the nineteenth century a German optician, Fraunhofer, began to experiment with glass prisms. Newton had already found that ordinary "white" light is not white at all, but is a blend of all the colours of the rainbow. Fraunhofer realized that this discovery could be turned to good account, and his work led to the development of a new instrument, the astronomical spectroscope.

Just as a telescope collects light, so a spectroscope analyses it. By studying the "spectra" produced, it is possible to find out a great deal about the matter present in the material which is emitting the light. For instance, the spectrum of the Sun shows two dark lines which can be due only to the element sodium, so that we can prove that sodium exists in the Sun.

Today we can examine the spectra of stars and star-systems so far away that their light takes thousands of millions of years to reach us - and we find the same familiar elements.

In 1838, Friedrich Bessel, Director of the Observatory of Köningsberg, returned to the problem of the distances of the stars. By studying the apparent movements of 61 Cygni, a faint object in the constellation of the Swan, he was able to show that it lay at a distance of about 60 million million miles. About the same time a British astronomer, Henderson, measured the distance of the bright southern star Alpha Centauri, and arrived at the reasonably accurate value of twenty million million miles; the real value is about 24 million million miles, so that Henderson underestimated somewhat. Alpha Centauri is a triple star, and the faintest member of the trio remains the nearest known body outside our own Solar System.

Twenty four million million miles! Our brains are not built to understand such vast distances, and it is clear that the mile is too short to be a convenient unit of length. One might as well try to measure the distance between London and Melbourne in centimetres. Fortunately there is a much better unit available, based upon the speed of light.

Light is known to travel at 186,000 miles per second. A ray from the Sun takes 83 minutes to reach us, but in the case of Alpha Centauri the time of travel is 43 years; we see the star not as it is now, but as it was 4j years ago. Alpha Centauri is therefore said to be 43 light-years away, while the distance of 61 Cygni is nearly 11 light-years.

Bessel's success gives us an added idea of the real importance of the Solar System. Rather than quote strings of figures, it will be more graphic to imagine a scale model. If we begin with making the Sun a 2-foot globe, and putting it on Westminster Bridge, the Earth will become a pea at a distance of 215 feet; Uranus, the outermost of the planets known in Bessel's time, will be represented by a plum 5 of a mile away from our 2-foot Sun. What of the nearest star? We shall not find it in London, or even in England; it will lie some 10,000 miles away, in the frozen wastes of Siberia. We have learned much since the days when the Earth was thought to be the hub of the universe.

A vital development in the early 19th century was the beginning of astronomical photography. The first "Daguerreotype" picture of the Sun was taken in 1845, followed in 1850 by a good photograph of the Moon. Progress was rapid, within fifty years magnificent photographs were being taken, not only at professional observatories but also by amateurs. Only since around 1960 has photography in its turn been superseded by electronic devices. Officially, sheer visual observation belongs to the past and it is seldom that a professional astronomer actually looks through a telescope. Times have indeed changed.

Herschel's 48-inch reflector was outmatched in 1845, when the third Earl of Rosse, at Birr Castle in Ireland, constructed a 72-inch. He built it himself - even grinding the mirror - and though it was awkward to use, and could examine only part of the sky, it was by far the most powerful telescope then in existence. Lord Rosse used it well. In particular, he paid close attention to the star-clusters and nebulae which had been pointed out by Herschel and catalogued by the French observer Charles Messier. Some of the nebulae proved to be made up of faint stars, but others could not be resolved. Many of the "starry nebulae" revealed a spiral structure, so that they looked like shining Catherine-wheels.

For some years the Rosse telescope was in a class of its own, but well before the end of the century the first large refractors were built, and were far more convenient and effective. In fact, the 72-inch fell into disuse after 1909, but it has now been restored, though its main interest is historical. Nothing like it has been built either before or since, but it was a great achievement, and will always be remembered for its discovery of the spirals.

Alone, the telescope could never decide upon the nature of the irresolvable nebulae; the spectroscope was able to do so. In 1864 Sir William Huggins examined a faint nebula in the Dragon, and found that it was made up not of stars, but of luminous gas.

It is now known that the nebular objects are of three types, Inside our own starsystem, known commonly as the Milky Way but more properly as the Galaxy, we find the normal star-clusters and the gaseous nebulae, most of them hundreds or thousands of light-years from us. Beyond the Galaxy there is a vast gulf, and then we come to the separate external systems, lying at immense distances. The most famous of them is the Great Spiral in Andromeda (Fig. 2.5), which can be seen with the naked eye as a faint misty patch, and which proves to be a galaxy in its own right, even larger than our own. Herschel had suspected something of the sort, and the work of Rosse and Huggins supported his view, though the question was not finally settled until 1923.

The latter part of the nineteenth century was the age of the great refractors. The largest, set up in 1897 at the Yerkes Observatory at Williams Bay in Wisconsin, has an object-glass 40 inches in diameter. It is unlikely to be surpassed, at least so far as Earth-based telescopes are concerned, because a lens has to be supported around its edge, and if it becomes too heavy it tends to distort, making it useless. Other big refractors still in use are those of the Lick Observatory in California (36 inches), Meudon in France (33) and the Lowell telescope at Flagstaff in Arizona (24). Reflectors then took over, largely due to the energy of one American astronomer, George Ellery Hale, who not only planned large reflectors but had the happy knack of persuading millionaires to finance them. He was responsible for the 60-inch reflector at Mount Wilson, California, and then in 1917, the 100-inch Hooker reflector, also at Mount Wilson.

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Fig. 2.5. The Andromeda Galaxy - like our Galaxy, a spiral system. (Photographed by Bill Patterson with his Takahasi FSQ106 telescope and SBIG ST10XME CCD camera. See www.laastro.com.)

For decades the 100-inch was supreme; only it was powerful enough to enable Edwin Hubble to prove that the spirals seen from Birr were independent galaxies, millions of light-years away, rather than minor features of our own Milky Way. It was finally surpassed in 1947 by the 200-inch reflector at Paloma, in California. Others have followed, some with huge mirrors made up of many smaller components fixed together to form the correct optical curve ("segmented mirrors"), while in many cases there are large telescopes working together. Such are the Keck twins, on Hawaii, each of which has a 387-inch (10-metre) mirror. At the moment pride of place goes to the Very Large Telescope in Northern Chile. There are four 8-metre mirrors which can be used either individually or collectively. In theory, the VLT could detect the headlights of a car, separately, over a range of more than 20,000 miles.

Originally, most major observatories were in the northern hemisphere, but it so happens that some of the most important stellar objects lie in the far south of the sky, and recent emphasis has been on places such as Chile, Australia and South Africa. It is also important to consider conditions of "seeing"; the less atmosphere you have above, the better the conditions are likely to be. This is why major telescopes have been located at, for instance, Mauna Kea in Hawaii, at an altitude of 14,000 feet, and at the lofty Atacama Desert of Chile. Britain's largest telescope, the 165-inch William Herschel reflector, has been set up atop an extinct volcano, the Roque de los Muchachos, in Las Palma, one of the Canary Islands.

Obviously there are tremendous advantages in going into space, above the atmosphere, and in 1990 the Hubble Space Telescope was launched into orbit. Moving round the Earth at an altitude of over 370 miles it can in some ways outmatch any telescope on the ground, even though it has a mirror "only" 94 inches in diameter.

There are many optical systems in use; for example a Schmidt telescope can photograph a comparatively wide area of the sky with a single exposure. However, it is fair to say that electronic devices are now taking over from photography, and many modern amateurs are extremely well equipped. With a CCD or ChargeCoupled Device, a 12-inch telescope can match the performance of, say, the Mount Wilson 60-inch used with conventional film. Nowadays it is seldom that a professional astronomer actually looks through a telescope; instead he sits in a warm, comfortable room studying the results as they come through on a television screen.

Radio astronomy began in the early 1930s, when an engineer named Karl Janksy, working for the Bell Telephone Company, was investigating problems of "static" and found that he was picking up radio waves from the sky. This was the beginning of radio astronomy, which has now come so very much to the fore.

Radio telescopes are not in the least like optical telescopes, and they do not produce visible pictures of the objects under study; one cannot look through them, as some earnest inquirers fondly believe! They are designed to collect the long-wavelength radiations coming from space, and they are of many different designs. The most famous radio telescope is probably the 250-foot steerable "dish" at Jodrell Bank, in England (Fig. 2.6), but each design is tailored to suit its own special needs. I am not a radio astronomer, but electronically-minded amateurs will certainly find plenty of scope. Grote Reber, who built a "dish" before the war and was probably the first true radio astronomer, was an amateur.

Fig. 2.6. Jodrell Bank Radio Telescope, Cheshire. (Aerial photograph by Jonathan C.K. Webb.)

The opening of the Space Age caused a complete change in outlook. On 4 October 1957 the Russians launched the first space satellite, Sputnik; since then men have been to the Moon, unmanned probes have explored all the planets except Pluto, and instruments have been landed not only on Mars and Venus but also upon the tiny asteroid Eros. Yet all this does not mean that the day of the amateur astronomer is over. His field of research is much more restricted than it used to be, but his role remains as important as ever.

I am writing these words in early 2005. Before they appear in print, much may have happened, but the basic problems will remain unaltered. And as one puzzle is solved, a host of others arises to take its place. This has been the case since ancient times; it is still the case today.

Telescopes Mastery

Telescopes Mastery

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