William RevilleObjections to a ‘sun-centred’ system not only religious
Most people are familiar with the bones of the story of Nicolaus Copernicus (1473 – 1543) who proposed in his book On the Revolution of the Heavenly Spheres in1543 that the Earth rotates daily and revolves annually around the sun (heliocentric model), thereby overturning over 1,000 years of astronomical and theological wisdom, and of Galileo Galilei (1564 – 1642) who championed Copernicus's proposal thereby raising the ire of the Church. Galileo was tried before the Inquisition and famously forced to recant his position in 1633. The Church was wrong to prosecute Galileo, who was making a scientific argument – religion has no competence to adjudicate on scientific matters. Galileo was subsequently proved to be right, but even if he had been wrong, this would not justify the interference of religion in a scientific arena.
However, that said, the situation at the time was far more complex than the simple story I have outlined above and I would bet that most people are unaware of this.
Proposal
Most scientists refused to accept the revolutionary proposal of Copernicus for many decades, even after Galileo made his famous telescope observations in 1609 that lent support to the Copernican position. By 1600, 47 years after Copernicus made his proposal, only 12 serious astronomers had given up belief in an unmoving Earth. This opposition was not just on religious grounds but was supported by observational evidence that favoured another new competing cosmological hypothesis proposed by Tycho Brahe (1546 – 1601) in 1588. The story is told by Dennis Danielson and Christopher Graney in Scientific American, January 2014.
The classical pre-Copernican geocentric model of the universe placed an unmoving Earth at its centre, circled by the moon, sun, planets and an outermost sphere of stars. The heliocentric model of the universe proposed by Copernicus, placed an unmoving sun at the centre, circled by the planets and the outermost sphere of stars. In 1588 Tycho Brahe, spurred on by aspects of the Copernican model that he could not explain, proposed a competing geoheliocentric model of the universe that was a mixture of the other two models, with the sun, moon and outermost stars orbiting an unmoving earth and the planets orbiting the sun.
Tycho Brahe is a giant in the historical pantheon of astronomers. For his day this Danish astronomer was extremely well-funded and had the best of equipment. Brahe’s observations of Mars later allowed his assistant Johannes Kepler (1571 – 1630) to formulate the elliptical nature of planetary motion.
Brahe was impressed by the elegance of the Copernican proposal but several aspects of the Copernican model bothered him, including the mechanism whereby the Earth moves and the apparent size of stars. Brahe was well acquainted with the size of the Earth and was puzzled as to how such a massive object could fly perpetually at great speed around the sun, when it was clearly so difficult to pull a loaded wagon along the street. The physics discovered later by Isaac Newton (1642 – 1727) explained the motion of the Earth but this knowledge was unavailable to Brahe.
On the other hand, Brahe had no trouble explaining the motion of the planets and stars. Since the time of Aristotle, astronomers had proposed that these celestial bodies were made of a special aetherial substance not present on Earth that has a natural tendency towards rapid circular motion.
Copernicus had noticed that no annual parallax – changes in the relative position of stars due to observations from the Earth in different positions in its orbit – could be observed from Earth when observing the stars. (You can demonstrate parallax by holding your index finger about 10 cm in front of your face. Close one eye and observe the finger with the other eye. Now open the closed eye, close the other and look at the finger again. The image of the finger will change position. The extent of the change in position of the image as you switch from eye to eye gets less as the distance of the finger from the face is increased).
Distance
Since no annual star parallax could be measured on Earth, this must mean, if the Earth is rotating around the sun as in the heliocentric model, that the diameter of the orbit of the Earth around the sun (orbis magnus) is but a point compared to the distance from Earth to the stars, placing the stars at an ‘immeasurably large’ distance from the Earth. In both the classical and Brahe’s geocentric models the stars lie just beyond the planets and Brahe found the enormous dimensions of the heliocentric model almost impossible to believe. Neither does the stationary Earth in the geocentric models pose any parallax problems.
Stars observed in the night sky appear to have fixed widths. Knowing this width and the distance to the stars, the diameter of the star can be calculated from simple geometry. Brahe made these measurements and calculations, concluding that, since the stars are so enormously far away in the Copernican model, they must also be absurdly large – hundreds of times bigger than the sun. On the other hand, calculations made using geocentric cosmological models showed star sizes comparable to the size of the sun, a result that was much easier to accept. But what Brahe didn't know was that the star discs visible in the sky are artifacts resulting from the wave nature of light as it passes through circular viewing apertures. The wave nature of light was not discovered until the 19th Century. We now know that the stars are actually point sources of light located extremely far away, and are not necessarily of enormous size.
Another problem noted by Brahe with the Copernican system, was that a rotating Earth should cause a deflection in falling bodies away from a straight path, but nobody could detect such deflection.
Deflections
These deflections were not observed until the 19th Century when the Frenchman Gaspardi Gustave de Coriolis (1792 – 1843) mathematically described such effects. And the annual stellar parallax, which is far too small to be detected from Earth with the unaided eye, was not convincingly recorded until Friedrich Bessel (1784 – 1846) did so in 1838 when telescopes of sufficient resolving power became available.
So, in Galileo's day, and for a considerable period after that, those astronomers who opposed Copernicus had some respectable scientific observation on their side. As Danielson and Graney point out, the fact they were eventually proved wrong did not mean that they were bad scientists. Disputing the strong arguments of others is part of normal science.
William Reville is an Emeritus Professor of Biochemistry at UCC http://understandingscience.ucc.ie