Cosmology and Astronomy

Tycho Brahe's (1546–1601) observations of comets coursing through the solar system shattered the Aristotelian crystalline spheres, and his observation of a nova, a star flaring up in brightness, pierced Aristotelian belief in an unchangeable heaven. Belief in uniform circular motion died next when Kepler used Brahe's observations to show that planets travel around the sun in elliptical orbits. A new explanation of how the planets retrace the same paths forever around the sun became a central problem of cosmology. Newton showed mathematically how a force of attraction or gravitation toward the sun continually draws planets away from straight-line motion and holds them in Kepler's elliptical orbits.

The Newtonian example of general laws in natural philosophy excited searches for general laws in other realms, including economics. In this branch of philosophy, Adam Smith (1723–1790) played Newton's role. Also inspired by Newton's achievement, Voltaire (1694–1778) and Montesquieu (1689–1755) searched for natural laws of politics. The French Revolution owed much to Newton.

Newton was convinced that his discoveries demonstrated God's wonders. Yet the revolution in thought following from his new cosmology, particularly the concept of a mechanical, clocklike universe, threatened the historic link between cosmology and religion in Western thought. Writers of the Romantic period sought to breathe divine life back into an overly mechanized and increasingly godless universe. Under the sway of the French Enlightenment's atheistic approach to nature, however, Pierre-Simon Laplace (1749–1827) happily replaced the hypothesis of God's rule with a purely physical theory. According to legend, when Napoléon Bonaparte (1769–1821) asked Laplace whether he had left any place for the Creator, Laplace replied that he had no need of such a hypothesis.

The Newtonian solar system provided Immanuel Kant (1724–1804) a model for the larger stellar system. The same cause that gave the planets their centrifugal force, keeping them in orbits around the sun, could also have given the stars the power of revolving. And whatever made all the planets orbit in roughly the same plane could have done the same to the stars. Nebulous-appearing objects in the heavens became, in Kant's mind, island universes, like colossal solar systems. Late in the eighteenth century, William Herschel's (1738–1822) large telescopes expanded the heavens from a starry sphere to a three-dimensional firmament. He observed that most stars seemed to lie between two parallel planes. This stratum, seen from earth, is the Milky Way.