LECTURE

Galileo Galilei (1564–1642) used a new instrument, the telescope, to show that heavenly bodies contained blemishes (such as sunspots). This disproved Ptolemy’s claim that the heavens were a realm of perfection, and hinted that the same fundamental laws might apply on Earth as in the heavens. Galileo also showed that we do not experience the Earth’s Assuming that our part of the motion through space because, like Universe was typical, Hubble’s passengers on a ship, we share in discovery suggested that the that motion. entire Universe was expanding. During the “Scienti c Revolution” of the 17th century, a new model of the Universe was constructed. In the Principia Mathematica, published in 1687, Isaac Newton (1642–1727) explained the elliptical orbits of the planets as the result of a universal force of attraction (or “gravity”) that existed between all objects, both in the heavens and on Earth. Newton’s cosmology had no obvious limits in time or space, suggesting that the Universe might be in nitely large and in nitely old. But the idea of an in nitely large Universe raised new problems. Kepler had pointed out that if the Universe was in nite it ought to be bathed in an in nite amount of heat and light, which was patently not true. The second law of thermodynamics (see Lecture Three) suggested that the amount of usable energy in the Universe was constantly diminishing. For 19th-century astronomers this posed a fundamental problem, for in an in nitely old Universe there should be no usable energy left. These problems were resolved early in the 20th century with the appearance of an entirely new model of the Universe. In the late 1920s, American astronomer Edwin Hubble (1889–1953) showed that the entire Universe was expanding. His achievement rested on 19th century attempts to map the position and movements of the stars. First, astronomers tried to estimate the distances to the stars and the size of the Universe. Hold your nger up at arm’s length, then move your head from side to side. Your nger will seem to move against the background. That is “parallax.” By measuring those movements you could, in principle, calculate the