The sky is the most mysterious part of our everyday experience. Familiarity may make the amazing events going on at ground level seem almost ordinary. Plants and animals grow and die, rain falls, rivers flow. We feel we understand that.
But the sky is beyond comprehension. Two great objects travel through it, one hot and constant, the other cold and changeable. In the daytime it is moody; there may be blazing sun, or racing clouds, or darkness followed by thunder and lightning. And yet on a clear night the sky is the very opposite - predictable, if you look hard enough, with recognizable groups of stars moving in a slow but reliable manner.
Man's interest in the sky is at the heart of three separate stories - astronomy, astrology and the calendar.
Astronomy is the scientific study of sun, moon and stars. Astrology is a pseudo-science interpreting the supposed effect of the heavenly bodies on human existence. In early history the two are closely linked. The sky is the home of many of the gods, who influence life on earth. And the patterns in the sky must surely reflect that influence.
Mesopotamia and the Babylonians: from 3000 BC
Astronomical observation begins with the early civilizations of Mesopotamia, where prominent constellations (the patterns formed by stars in the galaxy) are recognized and named soon after 3000 BC. Similarly the sky-watchers of Mesopotamia identify the five wandering stars, which with the sun and moon form the seven original 'planets' (Greek for 'wanderers').
Within Mesopotamia the Babylonians, flourishing from the 18th century BC, are the first great astronomers. The minutes and seconds of modern astronomical measurement derive from their number system. And it is the Babylonians who introduce the useful concept of the zodiac.
The Babylonians realize that the zodiac - the sequence of constellations along which the sun and the planets appear to move in their passage through the heavens - can serve as a yardstick of celestial time if divided into recognizable and equal segments. They select twelve constellations to represent these segments, many of them identified by the names of animals. The Greeks later provide the term for the zodiac when they describe it as the 'animal circle' (zodiakos kyklos).
The zodiac links constellations with times of the year; and the constellations have their own links with the gods. So scientific observation of star positions merges with speculation about divine influence. The zodiac, as a concept, is of use to both astronomers and astrologers.
The Greeks: from the 6th century BC
The Greeks make significant advances in the fields of both astronomy and astrology. In astronomy their analytical approach to the heavens leads to early insights of great brilliance, even though they eventually blind European astronomers for more than a millennium with the elaborately observed but entirely false Ptolemaic system.
Meanwhile astrology benefits from the range and vitality of the Greek gods. Linked with the planets and constellations, these very human divinities make astrology dramatic and exciting. And Greek interest in the individual extends the astrologers' range. Evolved originally to help in affairs of state, the art finds its lasting role in casting the fortunes of ordinary men and women.
The Pythagoreans and astronomy: 5th century BC
Followers of Pythagoras, in the 5th century, are the first to produce an astronomical theory in which a circular earth revolves on its own axis as well as moving in an orbit. The theory derives in part from the need to locate the great fire which they believe fuels the universe.
The Pythagoreans place this fire at the hidden centre of things, with the earth revolving round it more closely than any of the other bodies visible in the sky. The reason why we never see or are scorched by the fire is that we live on only half the sphere of the earth, and the earth revolves so that our half is always turned away from the flames.
Moving outwards from the earth in the sequence of heavenly bodies, they place the moon next, then the sun, the planets and finally the stars, which are unlike the others in being fixed on an outer sphere.
Heavenly spheres: from the 5th century BC
This theory introduces the concentric circles which become the false orthodoxy of the next 2000 years, as eventually enshrined by Ptolemy. It also starts a wild goose chase which will exercise many brilliant minds: what mechanical model can explain the erratic motion of the planets? Eudoxus of Cnidus, in the 4th century, is the first to propose a series of transparent spheres in the heavens, carrying the heavenly bodies at different speeds in linked groups with slightly varying centres.
To make such machinery conform to what can be observed in the sky, ever more complex arrangements are needed. Later in the 4th century Aristotle believes he has solved it. He requires no fewer than fifty-five transparent spheres.
The Pythagoreans are too far ahead of their time in proposing their one central grain of truth - the revolving globe of the earth. But Copernicus, developing this idea, will acknowledge them as his earliest predecessors.
For most Greek astronomers there seems to be overwhelming evidence that the earth is stationary and the heavens move. This is true even of the greatest among them, Hipparchus. Like his predecessors, he believes that it must be possible to analyze the movement of the spheres. He finds the available data inadequate, so devotes himself not to cosmology but to the prime task of an astronomer - observation of individual stars.
The earth and the sun: a heresy of the 3rd century BC
A lone voice on the Greek island of Samos. In about 270 BC Aristarchus is busy trying to work out the size of the sun and the moon and their distance from the earth. His only surviving work is on this topic, and his calculations are inevitably wide of the mark.
But references in other authors make it clear that his studies have brought him to a startling conclusion.
Aristarchus believes that the earth is in orbit round the sun (quite contrary to what is plain for anyone to see). There is an attempt, which comes to nothing, to have the man prosecuted for impiety. His idea joins the many other dotty notions which enliven the history of human thought, until Copernicus mentions him, in an early draft of his great book, as someone who had the right idea first.
On reflection Copernicus drops the name of Aristarchus from later versions of the text.
Hipparchus, a scientific astronomer: 2nd century BC
An observatory is erected by Hipparchus on the island of Rhodes. Here, in 129 BC, he completes the first scientific star catalogue. He lists about 850 stars, placing each in terms of its celestial latitude and longitude and recording its relative brightness on a scale of six.
He measures the altitude of a star by means of an astrolabe, a revolving calibrated disc which will be used for this purpose for nearly two millennia. It is invented either by Hipparchus himself or by his 3rd-century predecessor, Apollonius of Perga. Hipparchus also imagines another use for his astronomical instruments, to create maps of the earth's surface. But this is a task even more demanding than his charting of the heavens.
Hipparchus is so accurate in his placing of the stars that he becomes the first scientist to observe an important phenomenon. Although almost fixed in relation to the sun, the stars move gradually over a long period. This means that at any repeated and identifiable moment in the sun's year, such as the equinox (when day and night are of equal length), the star positions will be seen to have shifted very slightly.
Hipparchus observes this effect in relation to the equinox, and calculates that there is a shift each year of about 45 seconds of arc. It is a phenomenon known now as precession, or the precession of the equinoxes.
Hipparchus has no way of explaining this phenomenon (which is due to a slow wobble of the earth's axis, completing one cycle every 26,000 years), but his accuracy is astonishing. Modern measurements give a figure close to 50 seconds of arc. His 45 seconds are only about 10% out.
The works of Hipparchus are lost. They are known only through the use made of them by Ptolemy, a much less scientific astronomer whose influence derives from the encyclopedic nature of his work. Ptolemy acknowledges the greatness of Hipparchus, and fails lamentably when he tries to improve on his predecessor. Attempting to make the figure for precession more accurate, he moves in the wrong direction - and comes up with 36 seconds of arc.
The influential errors of Ptolemy: 2nd century AD
Ptolemy, working in Alexandria in the 2nd century AD, is one of the great synthesizers of history. In several important fields (cosmology, astronomy, geography) he brings together in encyclopedic form an account of the received wisdom of his time.
His influence derives from the accident that his predecessors' works are lost while his have survived. Their achievements are known only through him, and when he disagrees with them it is usually he who is wrong. Just as in astronomy he wrongly adjusts the degree of precession of Hipparchus, so in geography he rejects Eratosthenes, whose calculation of the circumference of the earth is very close, and prefers instead another estimate which is 30% too small.
Ptolemy's astronomical work is divided into thirteen books. The first proves that the earth is the immovable centre of the universe; the last five describe the movement of the sun, moon and five planets, each attached to its own crystal sphere. By adding adjustments to reflect the erratic behaviour seen in the sky, Ptolemy achieves a system capable of satisfying scientific enquiry in the unscientific centuries of the Middle Ages.
His book becomes known as Ho megiste astronomas (Greek for 'the greatest astronomer'), or Megiste for short. The Arabs call it Al Megiste (the Megiste). Reaching northern Europe through the Arab civilization in Spain, it acquires its eventual title - as Ptolemy's Almagest.
In practical terms the Ptolemaic system proves adequate for everyday purposes. Indeed its very complexity makes it attractive to the exclusive minority of learned men. The details may be hard to master, but once understood they will reveal future positions of the planets. Ptolemy himself prepares charts of the moon's behaviour, more accurate than any previously available, which remain in everyday use until the Renaissance.
But in the long run the complexity is unconvincing (the alternative proposed by Copernicus is simpler); and the orbiting planets of Jupiter, revealed by Galileo's telescope, inconsiderately smash through one of Ptolemy's crystal spheres.
A sudden bright star: AD 1054
Astronomers in China and Japan are excited to observe a new star in the constellation of Taurus. It is so bright that for three weeks it remains visible even in daylight. For a year it can be seen in the night sky. Then it gradually fades from view.
They note this strange phenomenon in their records. Some nine centuries later astronomers identify these notes as the first detailed observation of a supernova. The mysterious event watched with such fascination in the east is the mighty explosion of a star. Its remains, still rapidly flying apart, can be seen now in the night sky as the Crab Nebula.
A moving star: AD 1066
Just twelve years after the sudden bright star of 1054, there is another phenomenon in the sky - a 'long-haired' star, or comet. It is easily visible in Europe in the last week of April 1066. The great significance of that year in Norman history, combined with the omen of the comet, is sufficient for the apparition to feature prominently in the Bayeux tapestry. A group of men point at a star with a blazing tail. The caption explains Isti mirant stella ('these marvel at the star').
The 'star' returns at regular intervals to the night sky. Marvelling at it on one such visit, in 1682, is the English astronomer Edmund Halley.
Copernicus: AD 1497-1543
Nicolaus Copernicus, a Polish canon in the cathedral chapter of Frombork, is interested in the heavenly spheres. He acquires this interest in 1497, as a student in Italy, when he becomes the friend and assistant of an astronomer in Ferrara.
Copernicus' special concern is the orbits of the planets. As he observes and records their positions in the sky, he finds that he has to make ever more detailed adjustments to the already complex contortions imposed upon the 'wanderers' in the established Ptolemaic system.
Copernicus begins to wonder whether Ptolemy's model can indeed be correct. His studies reveal to him that in antiquity, among the Greeks, there were rival theories about the cosmos - including even that of Aristarchus of Samus, who declared that the earth moves round the sun.
Copernicus becomes intrigued by the notion of a planetary system which is heliocentric ('sun-centred'). Testing the idea in relation to his own observations, he finds that it tallies with the evidence much more readily than Ptolemy's solution. (The fit is not yet perfect, because Copernicus still assumes that the planets move in circular orbits - an error which will be corrected by Kepler).
In about 1530 Copernicus begins circulating a manuscript, known as the Commentariolus, giving an outline of his ideas. It creates interest, without the passionate opposition encountered by Galileo in the next century. Plans are made for a printed edition of a fuller work, which is published (under the title De revolutionibus orbium coelestium, 'On the Revolutions of Heavenly Spheres') in 1543. Tradition maintains that the old man, now aged seventy, sees the first copy on his deathbed.
Copernicus places the planets visible to the naked eye in the correct sequence from the sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn). His work launches scientific astronomy.
Tycho Brahe and Kepler: AD 1600-1609
During 1600 two of Europe's leading astronomers are guests of the emperor Rudolf II in the castle of Benatky near Prague. Each is a refugee. The older man, Tycho Brahe, has spent twenty years making astronomical observations in Uranienborg, a custom-built observatory created for him on an island near Copenhagen by the Danish king Frederick II. But in 1596 his lavish funding is cut by Frederick's successor. Tycho moves, with his instruments, to the hospitality offered by Rudolf II in Bohemia.
The younger astronomer, Johannes Kepler, has had to leave his post in Graz, in Austria. He is expelled from the university in 1600 on religious grounds as a Protestant.
Tycho Brahe, after inviting Kepler to Prague in 1600, dies in the following year. Kepler inherits his instruments and the detailed results of a lifetime of observation. In 1602-3 Kepler edits and publishes Tycho's work (Astronomiae instauratae progymnasmata, 'Beginnings of a New Astronomy'), giving the precise position of 777 stars.
With Tycho's information on planetary movements over many years, together with his own continuing observations, Kepler is in a position to publish - in Prague in 1609 - his own most significant finding. His Astronomia nova puts forward the radical and correct proposition that the planets move in elliptical rather than circular orbits.
With this insight, the last anomaly is removed from the heliocentric model of Copernicus. It is now unmistakably a simpler explanation of observable phenomena than the Ptolemaic version. But the Copernican theory remains theoretical; it has not yet dented the orthodox acceptance of Ptolemy. The leading astronomers are by now convinced Copernicans, but they discuss and develop the theme in privacy. The church establishment, guardian of the truth, is not yet involved in the debate.
This situation changes abruptly in 1610, when Galileo discovers firm proof of the Copernican thesis.
Galileo and Ptolemy: AD 1609-1632
In the summer of 1609 the professor of mathematics at Padua, Galileo Galilei, hears news of a recent invention in the Netherlands - the telescope. He immediately makes a telescope for himself to test the principle, soon following it with a much improved version which he presents to the doge in Venice. This is an astute career move. Padua is ruled from Venice. The Venetian senate, much impressed, doubles Galileo's salary and confirms him in his post for life.
With this much satisfactorily achieved, Galileo settles down in Padua to make serious use of the new instrument. He trains his lens on the night sky.
Within a year Galileo has so much improved the instrument that he has a telescope magnifying thirty-three times. With this, during 1610, he makes some startling astronomical discoveries.
Like many other scientists, Galileo has long been privately convinced that the heliocentric system of Copernicus is correct and the traditional Ptolemaic account of the universe a much repaired misconception (he expresses this view in a letter to Kepler in 1597). What he now observes disproves, beyond any scientific doubt, the theories enshrined by Ptolemy.
Focussing his telescope on Jupiter, Galileo sees four moons circling the planet; if Jupiter were fixed to a crystal sphere, as Ptolemy maintains, these moons would shatter it. When Galileo observes the sun, he sees spots which over a period move across its surface. The evident implication is that the sun itself is revolving, not fixed to its own sphere as Ptolemy would have it.
In 1610 Galileo publishes a general account of his observations, with the title Sidereus Nuncius (Star Messenger). It brings him immediate fame. He is invited to Florence to work at the Medici court. He is even well received in 1611 in papal Rome.
Feeling encouraged to be more explicit, Galileo publishes in Rome in 1613 a work which tackles Ptolemy head on. Istoria e dimostrazioni intorno alle machie solari ('Account and evidence of the sun spots') directly states that the movement of the spots across the sun proves Copernicus right and Ptolemy wrong.
This time there is outrage in traditional circles, culminating in 1616 in a papal decree placing Copernicus and his theory on the index of censored material. Galileo is forced to busy himself for the next seven years with other studies. But in 1623 he seems to be given another chance.
In 1623 a new pope, Urban VIII, gives Galileo permission to compare the Copernican and Ptolemaic systems. The pope makes one condition. No conclusion is to be reached as to the truth of either theory, since only God knows how he created the universe. Nine years later, with the approval of the censors in Rome, Galileo publishes his great work - Dialogo sopra i due massimi sistemi del mondo (Dialogue on the two chief world systems).
Although the final chapter prevaricates, as required, the weight of the argument makes the scientific conclusion unmistakable. With the book widely hailed as a masterpiece, and Rome's authority undermined, Urban VIII overreacts. He orders the Inquisition to investigate Galileo as a heretic.
Galileo is convicted in 1633 of having held the Copernican heresy. Shown the instruments of torture, he recants and is sentenced to life imprisonment. This takes the form of house arrest at his home near Florence, where he spends the remaining years of his life.
The Inquisition prevents Galileo from publishing, but he continues to write. His assistants save from the censors his last work, the Discorsi, the culmination of lifelong research into the laws of mechanics. Published in Leiden in 1638, it becomes a cornerstone of the newly developing science of physics. Meanwhile, in cosmology and astronomy, Galileo has provided the basis for scientific research along newly validated lines.
Distance of the sun: AD 1672
Giovanni Domenico Cassini, director of the newly established Royal Observatory in Paris, sends a colleague on a 6000-mile journey to French Guiana. At an agreed time the position of Mars in the sky is to be recorded both in Guiana and in Paris.
When Cassini receives the information back in Paris, and can compare the two readings, he is able to calculate the distance of Mars from the earth. He does this by geometry based on the effect of parallax (the result of viewing an object from two positions, familiar to all of us when we look through one eye and then the other).
Once Cassini has this first astronomical distance, he is able to apply it to each of the other planets by means of Kepler's work on their elliptical orbits. But his real quarry is the distance between the earth and the sun - a crucial measurement known to scientists as the astronomical unit.
Cassini's calculation of the astronomical unit, made in 1672, is creditably close. He arrives at a figure of 87 million miles. This is only about 7% out, the real figure being a little more than 93 million miles.
Speed of light: AD 1676
The Danish astronomer Ole Roemer, working with Cassini in Paris to compile tables of Galileo's moons of Jupiter, notices that eclipses of the moons (when they pass into the shadow of Jupiter or go behind the planet) occur at irregular intervals. The eclipses are later than expected when Jupiter is moving away from the earth, earlier when Jupiter is approaching - and the difference in time relates exactly to the variation in distance.
Roemer concludes that the rays reflected from each moon must take a finite time to reach us, implying that light travels at a fixed speed.
Work recently done by Cassini in Paris has revealed with considerable accuracy the distance of each planet from the earth. Figures on the distance of Jupiter's moons, compared with the observed variations in the times of the eclipses, enable Roemer to calculate the speed of light.
In 1676 he presents to France's newly founded scientific academy a Démonstration touchant le mouvement de la lumière (Demonstration concerning the movement of light). The figure he arrives at is 140,000 miles per second. This is about 25% too little (the established figure is 186,000 mps), but is an impressive first attempt given the nature of Roemer's instruments and the small variations on which he is working (see Scientific academies).
Halley and the comets: AD 1680-1758
An impressive comet, appearing in the sky in 1680, first kindles the interest of the young astronomer Edmund Halley in these strange intermittent celestial phenomena. He determines to study them and is rewarded, just two years later, with another spectactular example. It is in the hope of gleaning information about predicting their orbits that he visits Isaac Newton in Cambridge in 1684.
It is poetic justice that Halley's generosity in subsidizing Principia Mathematica is scientifically rewarded. Newton's discoveries enable Halley to calculate the orbits, often from fairly scant observations, of twenty-four known comets.
The result of his researches is published in 1705 as Synopsis Astronomiae Cometicae. The book would be little remembered - as perhaps would Halley himself by the general public - but for one startling discovery and prediction. Calculating the orbits of comets observed in 1456, 1531, 1607 and 1682, Halley notices that they are very similar. He concludes that this must be the same comet returning at fixed intervals and predicts that it will reappear in 1758. He would be 102 in that year, so he contents himself with an appeal 'to candid posterity to acknowledge that this was first discovered by an Englishman'.
Halley's comet duly returns, on Christmas Day 1758, and his fame is secure.
Herschel and Uranus: AD 1781
William Herschel is a musician from Hanover, earning a successful living as an organist in Bath. But his private passion is the construction of ever larger telescopes with which to search the heavens. By 1774 he has made himself a reflecting telescope, on Newton's principle, with a focal length of six feet.
While searching the heavens during the night of 13 March 1781, Herschel observes what he takes at first to be a comet. Subsequent investigations reveal it to be a planet, the first to be added to the six (including earth) known since antiquity.
Sensing an opportunity to give up music and to make his private passion his future career, Herschel takes the prudent decision to name his discovery after the monarch. He calls it Georgium Sidus (Georgian Star) in honour of George III. The international scientific community soon changes its name to one more in keeping with its fellow planets. Mars, Mercury, Venus, Jupiter and Saturn are joined by Uranus.
But flattery has done the trick. In 1782 George III appoints Herschel his private astronomer. Five years later the king accompanies the archbishop of Canterbury through the tube of a new 40-foot telescope, under construction near Windsor for the use of his talented star-gazer.
to be completed.....
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