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From the 16th century
     Tycho Brahe and Kepler
     Distance of the sun
     Speed of light
     Newton and gravity

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Copernicus: 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: 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: 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: 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: 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).

Newton and gravity: 1684-1687

In 1684 Edmund Halley visits Newton in Cambridge. Hearing his ideas on the motion of celestial bodies, he urges him to develop them as a book. The result is the Principia Mathematica (in full Philosophiae Naturalis Principia Mathematica, Mathematical Principles of Natural Philosophy), published in 1687. When lack of funds in the Royal Society seems likely to delay the project, Halley pays the entire cost of printing himself.

The book, one of the most influential in the history of science, derives from the young Newton's speculations about the moon during his time at Woolsthorpe Manor two decades earlier.

The question which stimulated his thoughts was this: what prevents the moon from flying out of its orbit round the earth, just as a ball being whirled on a string will fly away if the string breaks? The ball, in such an event, flies off at a tangent. Newton reasons that the moon can be seen as perpetually falling from such a tangent into its continuing orbit round the earth.

He calculates mathematically by how much, on such an analogy, the moon is falling every second. He then uses these figures to calculate, on the same principle, the probable speed of a body falling in the usual way in our own surroundings. He finds that theory and reality match, in his own words, 'pretty nearly'.

The word gravity is already in use at this time, to mean the quality of heaviness which causes an object to fall. Newton demonstrates its existence now as a universal law: 'Any two particles of matter attract one another with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.'

With this observation he introduces the great unifying principle of classical physics, capable of explaining in one mathematical law the motion of the planets, the movement of the tides and the fall of an apple.

This History is as yet incomplete.

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