The Star Garden
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Geocentric universe Heliocentric universe
Danish astronomer Tycho Brahe made some of the most accurate ever observations with the naked eye in the late 1500s. He disproved Aristotle's concept of an unchanging universe (Wesley, pp.42), which had been accepted by the Church, when he saw a new star in the constellation of Cassiopeia in 1572, this was later identified as a supernova. Tycho also showed that Aristotle's transparent spheres do not exist as he proved that comets would have to travel through them.
Tycho's student, German astronomer Johannes Kepler had first suggested that the planets move within spheres. He extended Aristotle's theory by arguing that the spheres were separated by five polyhedrons. Polyhedrons are three dimensional objects whose sides are all made up of the same shape, like a cube, a tetrahedron or a pyramid made of equilateral triangles. All of these shapes could be placed inside of a sphere so that the edges just touch the surface. Kepler thought that the motion of the planets could be described with a pitch and frequency and so produced musical notes as they spun. This was based on Pythagoras' idea that the universe could be represented in musical terms.
Kepler's first universe
Tycho later assigned Kepler the task of analysing his observations of Mars. Of all the planets, the predicted position of Mars had the largest errors. This is because it crosses the sky in an erratic way, sometimes appearing to move backwards and then forwards again in large loops.
Tycho's data was accurate enough for Kepler to show, in 1609, that Mars' orbit is not circular but fits the shape of an ellipse which orbits a centre of mass determined by the Sun and another focal point. Kepler also showed that planets move faster when they are closer to the Sun. By 1619, Kepler was able to determine the relationship between a planets distance from the Sun and the time it takes to orbit, its period (Kepler, 1992 and 1997).
Kepler's contemporary, Italian astronomer Galileo Galilei added further proof to the heliocentric theory of the universe in 1610, when he made a number of observations that contradicted Aristotle. Galileo was the first person to use the telescope as an astronomical instrument and when he turned it to the sky he became the first to observe a number of phenomena, including the moons of Jupiter, the phases of Venus, sunspots and the depth of the craters on the Moon, which he illustrated by showing the change in shadows across a day (Galileo, 1610). Galileo showed that the heavens are not perfect and unchanging and not everything orbits the Earth or Sun.
Sketch of craters on the Moon by Galileo Sketch of Sunspots and Jupiter's moons by Galileo
Galileo's claims were accepted within nine months of the publication of his findings in Sidereus nuncius (Starry Messenger) in 1610 and he moved from being an instrument maker to becoming a discoverer and eventually a court philosopher. At first, Galileo was incredibly secretive about his new instrument, concerned that others would soon learn to build better telescopes and make all of the possible new discoveries before him. He only sent sample telescopes to dukes and cardinals and did not give away any technical details of his own instrument which he would not leave with anyone. Galileo's fears were not unjust, he knew from experience that after receiving a description, a twenty power instrument could be created in four months. He also knew that these instruments were being constructed in Venice and that the Jesuits had already turned their telescope into a thirty four power instrument. The issue was not whether others would catch up with him but how long it would take (Biagioli, pp.70).
Galileo's observations were verified three times in 1610 but none did this with his help. Kepler had already publicly endorsed Galileo's claims, despite not being able to replicate them. He had requested a telescope to further his own project but never received one. Kepler became the second person to verify Galileo's findings when he received a telescope from Italian instrument maker Antonio Santini, the first person to have verified them. The third confirmation was by a Jesuit mathematician who received his telescope from Kepler (Biagioli, pp.88).
Until 1632, the heliocentric theory was easily dismissed because there was no explanation for why the Earth does not appear to move. Galileo explained this using the principle of relativity which states that all uniform motion is relative and there is no absolute state of rest. When we are on a ship, for example, we are at rest with respect to the floor even though we are moving with respect to the sea.
Galileo stated: "Shut yourself up with some friend in the main cabin below decks on some large ship, and have with there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed these things carefully (though there is no doubt that when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still" (Galileo, 1976).
Telescopic observations led to more contradictions of Aristotle's theory. In 1655, Dutch mathematician Christiaan Huygens discovered Saturn's moon Titan, he was also the first to show that the budges Galileo had observed on Saturn were rings. Jupiter's Great Red Spot was first observed by English physicist Robert Hooke in 1664.
English astronomer James Bradley provided further evidence for the heliocentric theory of the universe in 1725, when he proved that the Earth moves. If the Earth moves around the Sun, then the stars should appear to move as we view them from slightly different positions throughout the year. Those that are closer should appear to move more than those that are very far away, this phenomenon is known as stellar parallax. Bradley knew that if he could measure the angle that the stars appeared to move, then he could determine how far away they are using trigonometry. Tycho had failed to observe this movement and Bradley's observations were not able to provide evidence of parallax either, but he did observe stellar aberration. Stellar aberration does not depend on the Earth's change in position but its change in velocity. It cannot be used to determine the distance to the stars, but it does show that the Earth is accelerating, which is consistent with orbital motion.
References
Biagioli, M., 2006, 'Galileo's instruments of credit: telescopes, images, secrecy', University of Chicago Press, Chicago
Copernicus, N., 1939, 'On the Revolutions of the Celestial Spheres', The St. John's bookstore, New York
Galilei, G., 1610, 'The Starry Messenger'
Galilei, G., 1976, 'Dialogue Concerning the Two Chief World Systems', Drake, S. (ed.), University of California Press, California
Kepler, J., 1992, 'New Astronomy', Donahue, W.H. (trans.), Cambridge University Press, Cambridge
Kepler, J.,1997, 'The Harmony of the World', Aiton, E.J., Duncan, A.M. and Judith Veronica Field, J.V. (trans.), Diane Publishing, Darby
Wesley, W. G., 1978, 'The Accuracy of Tycho Brahe's Instruments', Journal for the History of Astronomy, Vol.9
White, M., 2003, 'The Pope and the Heretic: The True Story of Giordano Bruno, the Man Who Dared to Defy the Roman Inquisition', HarperCollins, New York
Copernicus, Kepler and Galileo (1500-1600s)
The concept of a heliocentric universe, first considered by Heraclides Ponticus and Aristarchus in around 300 BC, was reintroduced to Europe by German astronomer Nicolaus Copernicus in On the Revolutions of the Celestial Spheres first published in 1543 (Copernicus, 1939). This would not have been accepted as a realist theory by the Church and so was proposed as being merely instrumental. It provided an easier mathematical system for calculating where planets would be but it was not to be taken literally. Italian philosopher, Giordano Bruno was burned at the stake for heresy almost sixty years later after suggesting, amongst other things, that the Sun is just another star (White, 2003).