2 The clockwork Universe

» 2.1 Mechanics and determinism 1/4

2.1 Mechanics and determinism 2/4

2.1 Mechanics and determinism 3/4

2.1 Mechanics and determinism 4/4

2.2 Energy and conservation 1/2

2.2 Energy and conservation 2/2

3 The irreversible Universe

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

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It is probably fair to say that no single individual has had a greater influence on the scientific view of the world than Isaac Newton. The main reason for Newton's prominence was his own intrinsic genius, but another important factor was the particular state of knowledge when he was, in his own phrase, 'in the prime of my age for invention'.

In 1543, a century before Newton's birth, Nicolaus Copernicus launched a scientific revolution by rejecting the prevailing Earth-centred view of the Universe in favour of a heliocentric view in which the Earth moved round the Sun. By removing the Earth, and with it humankind, from the centre of creation, Copernicus had set the scene for a number of confrontations between the Catholic church and some of its more independently minded followers. The most famous of these must surely have been Galileo, who was summoned to appear before the Inquisition in 1633, on a charge of heresy, for supporting Copernicus' ideas. As a result Galileo was 'shown the instruments of torture', and invited to renounce his declared opinion that the Earth moves around the Sun. This he did, though tradition has it that at the end of his renunciation he muttered 'Eppur si muove' ('And yet it moves').

Figure 1.3 Three views of planetary motion

(a) The Earth-centred view of the ancient Greeks and of the Catholic church in the sixteenth century. Click here for larger image (15.30kb)

(b) The Copernican system, in which the planets move in collections of circles around the Sun. Click here for larger image (8.83kb)

(c) The Keplerian system in which a planet follows an elliptical orbit, with the Sun at one focus of the ellipse. Click here for larger image (14.24kb)

In the Protestant countries of Northern Europe, thought on astronomical matters was more free, and it was there in the early seventeenth century, that the German-born astronomer Johannes Kepler (1571-1630) devised a modified form of Copernicanism that was in good agreement with the best observational data available at the time. According to Kepler, the planets did move around the Sun, but their orbital paths were ellipses rather than collections of circles. This discovery, first published in 1609 in Kepler's book Astronomia Nova (The New Astronomy), was essentially an observational result. Kepler had no real reason to expect that the planets would move in ellipses, though he did speculate that they might be impelled by some kind of magnetic influence emanating from the Sun.
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