Views of the world’s structure. Scale of the Solar System

Ideas about the structure of the world changed gradually as observations accumulated. In the 4th century BCE Aristotle argued with evidence that the Earth is a sphere and placed it at the centre of the world; in the 2nd century CE Ptolemy gave this geocentric system a mathematical form: a planet moves on a small circle, the epicycle, whose centre moves on a large circle round the Earth, the deferent. Because this model roughly explained the looping paths of planets against the stars, it was used for about 14 centuries. Nicolaus Copernicus (1473–1543) proposed the heliocentric system: the Sun stands at the centre, and the Earth, as an ordinary planet, revolves round it and also rotates on its axis; the looping motion turned out to be an apparent effect of the Earth and the planet moving at different speeds. Giordano Bruno (1548–1600) put forward the idea that the stars are distant suns that may have planets of their own – today more than six thousand such exoplanets have been confirmed. Biruni (973–1048) seriously discussed the possibility that the Earth rotates on its axis and noted that Mercury and Venus never move far from the Sun, but on the whole he kept to the geocentric view; the heliocentric system was later confirmed by Galileo’s observations and Kepler’s laws.