The Heavens and the Earth
Rudolf Kippenhahn on how astronomy has altered our vision of the universe - from 10th-century Cairo to the Big Bang.
The astronomer Ibn Yunus had waited for over an hour until the first stars could be recognised in the evening twilight. The reddish Arcturus now shone over the city of Cairo. In the east, the moon had appeared over the horizon. In the west, Mercury and Venus were less than the breadth of a full moon apart in the twilight. Ibn Yunus had watched how they approached each other every evening for the past week. Now he seized the astrolabe to determine the separation between them.
By today's reckoning, Ibn Yunus made his observations on the evening of May 19th, in the year 1000 AD. They were intended for inclusion in a work which was dedicated to the caliph Al Hakim. Astronomical observations were important because believers had to know the movements of the sun in order to make their prayers at the right time. The morning prayer had to end by sunrise. The midday prayer was made when the sun passed the southern meridian, and the evening prayer when the length of the shadow thrown by a man's body reached the sum of his height plus the length of the shadow during the midday prayer. These times changed from day to day. They could only be determined using astronomical tables. To pray, the believer also had to know the direction of Mecca – another task for the caliph's astronomer.
The positions of the stars were not only important for prayers. At that time in the Arab world, astrology flourished. The earth stood in the centre of the universe, so the Greeks taught, and so it had reached the caliph's court in Baghdad via Syrian translations and Indian visitors. The sun, moon, Mercury, Venus, Mars and Saturn – known at that time – circle round the Earth. Human beings thus have a central position in that universe. Surely it was reasonable to suppose that the heavenly bodies which were subordinate to the Earth were closely linked with human fate?
Ibn Yunus used the best star maps of the period, probably including those of the Persian astronomer, Al Sufi. As well as the fixed stars, these show the constellations. Andromeda is shown as a girl, barefoot with extended arms, with two fish in front of her, the constellation of Pisces. In front of the mouth of one fish is a group of black dots. No one seems to have noticed this patch for the next 600 years. This is surprising, because the nebulous cloud which would later be called the Andromeda nebula can be seen with the naked eye. We have only known since 1924 that it consists of more than 100 billion stars. It is so far away in space that only twentieth-century telescopes have made it possible to see individual stars in the nebula.
The progress which astronomy has made since Ibn Yunus is largely thanks to optics, the science of light. For almost the whole of the millennium, knowledge about events out there came only through light. One of the first to practice this science was the Arab Ibn Al Haitam, who lived in Cairo at the start of the second millennium. He also constructed the first pinhole camera. In 1960, when the sun was first photographed using X-ray light, a rocket carried a pinhole camera over the earth's atmosphere, which is opaque to X-ray radiation. Probably no one at that moment thought of the scholar who had invented the principle of this photographic technique almost a thousand years before.
Neither the astronomer Ibn Yunus nor the optical scientist Ibn Haitam experienced the heavenly phenomenon which must have disturbed people in the year 1054, although we only have scanty records of it. On July 4th, 1054, in one part of the sky, a star began to shine so brightly that for two weeks it could even be seen in daylight. Chinese and Japanese sources report it. In Constantinople, the doctor Ibn Butlan believed that this event was responsible for an epidemic which swept away 14,000 people, and a large part of the population of Cairo. Almost a thousand years later, astronomers discovered that a nebula in the constellation of Taurus is a residue of that stellar explosion.
Arab scholars had preserved the astronomical insights of the Greek scientists through the 'Dark Ages' – the period of European migrations. Like other sciences, astronomy made little progress during the first half of the second millennium. A new development only began towards the end of the Renaissance, led by Nikolaus Copernicus. He realised that the earth is a planet like Mars, Venus or Jupiter. In his cosmology, all the planets circle round the sun at the centre, except the moon which circles round the earth. The Gottingen philosopher and physicist, Georg Christoph Lichtenberg, wrote in the eighteenth century: '... as long as the Earth stood still, all astronomy stood still, as it had to; but when the man who let the Sun stand still appeared, astronomy began to make progress’. For Copernicus, the planets still moved in circular orbits, or on circles with their centres on circles. Like the Greeks, he believed that circular movement was the most perfect, and was therefore the course which heavenly bodies would follow. But when the German astronomer Johannes Kepler, analysed many years of observations of the planet Mars, he realised that the planets do not move in circles, but in elliptical orbits. The sun is not at the centre, but at one of the two foci.
This was the time when a new idea came to the fore. Whereas until then heavenly phenomena had been God-given, requiring no explanation or reason, because circular movements were seen as something absolutely natural, the less perfect movement in ellipses required an explanation. Was it possible that a kind of magnetic power was working through empty space and determining the orbits of heavenly bodies?
About seventy years after the first copy of the book which was Copernicus' life-work had been laid on his death-bed, the telescope, which was invented in Holland, began its victorious progress round the world. Through it, the Italian Galileo Galilei could see mountains on the moon in the sunlight, throwing their shadows over broad plains. The faintly shimmering band of the Milky Way dissolved into innumerable individual stars, each of them probably a sun like ours, unimaginably far away in space. But Galileo not only studied the sky, he also discovered the laws by which bodies fall to the ground or balls roll down boards placed diagonally. It was the time of the Thirty Years' War. The armies of Wallenstein and Gustavus Adolphus marched through Germany. The new thinking came into conflict with the Roman Catholic Church, which still insisted strictly on the exact words of the Old and New Testaments. The famous trial of Galileo was the result.
Kepler and Galileo were precursors of one of the greatest scientists of the millennium. The Englishman Isaac Newton, born in 1643, one year after Galileo's death, realised that the force which makes a body fall to the ground on earth is the same as the force which ties the moon to the earth and the earth to the sun. He gave the law of gravity its mathematical form, and formulated the equations by which bodies, whether they are cannon-balls or planets, move under the influence of gravity. Heavenly bodies were now no longer godlike phenomena, but obeyed the same laws which control movements on the earth. Since the time of the Babylonians, human beings had believed that the movements of the stars determined life on earth, and that the fate of each individual was shaped by the positions of the planets at the moment of his or her birth. Now the planets, with their divine names like Venus, Mars and Jupiter, were shown to be bodies which obey the laws of mechanics.
At this moment, astronomy and astrology parted company. Kepler had calculated Wallenstein's horoscope, but from now on astronomy was part of physics. The movements of the stars inspired Newton to set down the laws of mechanics, and thus laid a foundation on which all areas of physics would later be based, not least quantum mechanics in our own century. Now it was not only possible to calculate the movements of planets in advance, but even comets, which approach from some- where in space, make an arc around the sun and disappear into the vastness of the universe, were no longer mysterious messengers of misfortune but bodies whose orbits and velocities were defined by Newton's laws.
The crowning glory of the science of celestial mechanics was the discovery, in 1845, of the planet Neptune, exactly where it was predicted to be. The Frenchman Jean Urbain Leverrier and the Englishman John Couch Adams, working independently, had predicted where the new planet would be found from the perturbations which the gravity of Neptune caused to the orbit of Uranus. They themselves never saw it.
Not only are the laws of movement in the universe the same as those on earth, the whole of physics and chemistry, as they were developed in the middle third of the millennium, prove themselves in explaining the phenomena in the universe, which are observed using ever-improving telescopes and the measuring instruments connected to them. If light from the sun or from stars is arranged in a spectrum of colours, from red to violet, by a spectroscope, it shows spectral lines from which astronomers recognise that the material in the universe consists of the same chemical elements as we have on earth.
We now believe, differently from all previous periods, that we are inside a vast system of stars, which are arranged in a disc-shaped space. Indeed the English scholar, Thomas Wright of Durham, showed in 1750 that the appearance of the Milky Way can be explained in this way. If we look along the central plane of this disc, we see a particularly large number of stars, which form the band of the Milky Way, In this disc, there are not only stars like the sun, but also pairs of stars, which revolve round each other, star clusters and stars which become brighter over a few days and then become dim again. In places where there was previously nothing remarkable, stars suddenly light up, only to disappear again after a few days. A diffuse light comes from some regions of the sky.
Two such nebulae are particularly striking. One is in Orion, the other is the one in Andromeda which has been known since Al Sufi. In the second half of the eighteenth century, the Frenchman Charles Messier made a list of over a hundred such nebulae. He actually only wanted to search for comets, but he catalogued the nebulae, which looked similar, so that he would not be confused by them. Some of these patches had a very regular circular or elliptical structure. The Briton, Lord Rosse, realised in 1845 in Ireland that one of these objects, in the constellation of the Hunting Dogs, has a spiral structure within it. When the young Immanuel Kant heard of Wright's idea that we, together with the sun, might be inside a disc which is filled with stars, he conjectured that the elliptical nebulae far out in space were similar discs full of stars. That was in the eighteenth century. Alexander von Humboldt, who wrote the astronomical volume of his six-volume Cosmos in 1850, called the elliptical nebulae 'world islands'. But the nineteenth- century astronomers never managed to prove that these were collections of far distant stars and not relatively insignificant patches of cloud in our immediate neighbourhood.
Since the beginning of the twentieth century, the kind of star system in which we live has become increasingly clear. It is a flat, lens-shaped space in which thousands of millions of stars orbit around the centre, taking hundreds of millions of years for each orbit. Light takes about 80,000 years to cross the disc from edge to edge. The disc is surrounded by a larger, spherical volume, in which more thinly distributed stars and star clusters move. We and the sun are not at the centre.
When large telescopes were built, particularly the 2.5 metre diameter reflecting telescope on Mount Wilson near Los Angeles, they made tremendous new understanding possible. With its help, the astronomer Edwin Hubble could show that the Andromeda nebula really is a 'world island', just like our own star system, but 2,000,000 light-years away. It was thus established that the elliptical, often spiral nebulae, are star systems like our own. In 1929, Hubble succeeded in proving that these distant star systems are moving away from us, in fact that the universe is expanding. It can be deduced from distances and velocities that this movement must have begun a few thousand million years ago. The universe apparently began a finite time ago with a 'big bang’.
The next advances were not made by astronomical observations but by physics. Albert Einstein improved on Newtonian mechanics with his theory of relativity. In the period after the First World War, a group of young physicists extended mechanics so that it applied even to the smallest particles, atoms and parts of atoms. Quantum mechanics thus appeared.
An unknown radio engineer, who wanted to investigate radio interference caused by thunderstorms, discovered in 1992 that radio radiation reaches the earth from the centre of the Milky Way system. This was the birth of radio-astronomy. The technical development which was later encouraged, above all, by the Second World War, made it possible to investigate not only radiation from space in the wave-length ranges of light and radio radiation, but also in the wave-length ranges to which the earth's atmosphere is opaque. High-flying balloons, rockets and artificial satellites surveyed almost all the planets and their moons. Improved radio-telescopes discovered heavenly bodies which either cannot be seen in visible light or had not yet been noticed, the point-like quasars, which were later found to be the bright cores of distant star systems. Radio-astronomers discovered pulsars, from which radio pulses and light flashes reach us in rhythms measured in seconds or milliseconds. The astronomers then became aware of the radio-galaxies, which are significant in visible light, but rule the sky with their radio radiation.
Finally, men visited the moon, and unmanned probes flew to other planets and landed measuring instruments on the hot surface of Venus and the sandy deserts of Mars. Within the past twelve months, a probe has been thrown into the atmosphere of Jupiter which transmits measurements of the atmosphere for fifty-seven minutes.
Human beings have learned a great deal about the Universe and its laws in the millennium now approaching its end, particularly in the last century. However, our thinking is not so far from the old ideas about natural processes. At the start of the second millennium, it was assumed that all bodies, including the planets, moved in circular orbits – that, in the most perfect and symmetrical movement. It was believed that Nature always chose the most beautiful, the most symmetrical movement possible. Physicists who research elementary particles today, and are probably on the track of the most profound secrets of nature, test new mathematical formalisms in their theories – and they use those which satisfy certain aesthetic principles. Expressed in a more scholarly way, their formalisms have symmetric properties, and the more perfect the symmetry the better. These thought structures seem to have lasted for a thousand years.
The explosive development of astronomy in the second half of the last century has not only answered questions which could never be answered before, but also posed new questions. Has life developed on planets which circle around other stars? Where are the invisible masses of material which we only know about from their gravity, and which must be ten times as much as the stars we see? What is the origin of the energy which is radiated from the centres of many galaxies'? Is it true, as is assumed, that there concentrations of material in the form of 'black holes' exert so much gravitational force that even the light which they emit falls back into them'? Is the Universe infinitely large? Will it go on expanding forever?
For the answers to these questions we must wait for the researchers of the next millennium.
Dr Rudolf Kippenhahn was Professor of Astronomy and Astrophysics at the University of Gottingen, 1965-75, and Director of the Max Planck Institute of Astrophysics in Garching, 1975-91.
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