Copernican cosmology, though revolutionary in important respects, clung to Aristotelian circular motion, whose cause generally was attributed to God. Kepler, using Tycho Brahe’s (1546–1601) observations, showed that planetary orbits are elliptical.
Kepler also found several mathematical relationships, such as the proportionality of the cubes of the mean planetary distances to the squares of the periodic times. Initially he attributed planetary motion to moving souls, but within a few years was searching for a physical principle. Kepler’s cosmology was strongly Christian. He was convinced that the creator had used mathematical archetypes to design the universe, and this religious belief drove his cosmological research and shaped his results, which were “a sacred sermon, a veritable hymn to God the Creator,” showing “how great are His wisdom, power, and goodness.” An explanation of how the planets continue to retrace the same paths forever around the sun became a central problem of seventeenth- and eighteenth- century cosmology. Newton showed mathematically that Kepler’s elliptical orbits as well as several mathematical relationships, including the proportionality of distances and times, were consequences of a universal inverse-square law of gravity. For Newton, the medium conveying action must be immaterial. The omnipresence of God, an immaterial ether, pervaded the Newtonian cosmos, offering no resistance to bodies, but moving them.
Theological implications of Newton’s cosmology were criticized in 1715 in a letter to Caroline, Princess of Wales, from the philosopher and mathematician Gottfried Leibniz (1646–1716). Leibniz was a rival of Newton in the invention of the calculus, each accusing the other of plagiarism. Newton’s friend Samuel Clarke answered in a letter to Caroline, which she forwarded to Leibniz. In the course of the debate Leibniz wrote five letters and Clarke five replies, which were published in 1717.
Newton thought that his discoveries provided new evidence of the existence and providence of God. Irregularities in planetary motions caused by the disturbing influence of other planets would increase until the system wanted reformation. Leibniz charged that Newtonian views were contributing to a decline of natural religion in England. The implication that God occasionally intervened in the universe, much as a watchmaker has to wind up and mend his work, derogated from God’s perfection.
Clarke admitted that God had to intervene in the universe, but only because intervention was part of God’s plan. Eighteenth-century belief in the orderliness of the universe made determination of that order an important theological, philosophical, and scientific endeavor. William Whiston (1667–1752), Newton’s successor at Cambridge University in 1703, argued that the system of the stars, the work of the creator, had a beautiful proportion, even if frail humans were ignorant of the order. In 1750 the English astronomer Thomas Wright (1711–1786) proposed a model for the Milky Way (a luminous band of light circling the heavens). Inspired by an incorrect summary of Wright’s book, Immanuel Kant (1724–1804) explained the Milky Way as a diskshaped system viewed from the Earth, which was located in the plane of the disk. Thoroughly imbued with a belief in the order and beauty of God’s work, Kant went on to suggest that nebulous patches of light in the Heavens are composed of stars and are other Milky Ways, or island universes. In the absence of large telescopes and revealing observations of distant stars, philosophical and theological speculations dominated cosmology.
This situation began to change after the English astronomer William Herschel (1738–1822) proposed a cosmological model rooted in observations. From the 1780s onward, the heavens, penetrated by Herschel’s large telescopes, increasingly were understood as an expanded firmament of three dimensions. The universe was also thought of as a clock, which Newtonians had argued required God’s occasional reformation. But in 1786 the last major problem in celestial mechanics was solved when Pierre-Simon Laplace (1749–1827) demonstrated that the gravitational interactions of Jupiter and Saturn were self-correcting, not in need of divine intervention.
Laplace also proposed a plausible mechanism for the formation of the solar system, which Newton had cited as reason for belief in divine providence, given the small likelihood that random chance could have been responsible. Reflecting the atheistic approach to nature of the French Enlightenment, Laplace attempted to replace the hypothesis of God’s rule with a purely physical theory that could also explain the observed order of the universe. He was successful, at least in his own mind. According to legend, when Napoleon asked Laplace whether he had left any place for the creator, Laplace replied that he had no need of such a hypothesis.