Darkness at Night
Edward Harrison, a professor of physics and astronomy at the University of Massachusetts at Amherst, became intrigued by the riddle of the dark night sky more than twenty years ago. To the uninitiated, the riddle’s answer seems simple: The sun has set and now shines on the Earth’s other side. If one thinks of infinite space as uniformly populated with stars, however, then the entire night sky ought to be ablaze with light, because the more distant stars, though fainter, are correspondingly more numerous. By adding the light of the fewer near stars to the light of the more numerous far stars, one finds that the night sky should be intensely illuminated. Nevertheless, the night sky is dark. Why?
Astronomers have proposed two basic solutions: Either the dark gaps are filled with stars that remain unseen and the missing starlight must be explained (the covered-sky interpretation) or the dark gaps are mostly devoid of stars and the missing stars must be explained (the uncovered-sky interpretation). For Harrison, the riddle has an answer falling within one of these interpretations, and surprisingly, the first person to give a qualitatively correct solution was not a scientist but a poet.
Harrison divides Darkness at Night: A Riddle of the Universe into three parts. In the first part he traces the emergence of ideas in the ancient and medieval periods that led to the origin of the riddle of darkness. Three systems of natural philosophy dominated the pre-Christian period: the atomist system of infinite space populated with numberless worlds composed of atoms; Aristotle’s orderly geometric system of celestial spheres; and the Stoic system of a finite cosmos of moving worlds surrounded by an infinite void. Harrison suggests that the Stoic worldview, which pictured a place where the starry universe ends, offers a natural explanation of the night sky: The dark spaces between the stars are the darkness of the infinite void.
During the medieval period, ecclesiastical authorities were concerned about the spread of pagan beliefs involving the nature of the universe, and the condemnations of Aristotle’s ideas in the thirteenth century led to new cosmologies, such as Nicholas of Cusa’s unbounded universe. Nicholas Copernicus wrote much about a sun-centered universe but little about what lay beyond the sphere of fixed stars. Shortly after Copernicus, Thomas Digges, an English mathematician, thought that the stars were not affixed to a sphere but scattered throughout endless space. He confronted the riddle of darkness and answered it by proposing that the most distant stars, despite their great numbers, were too faint to be seen. The true nature of the riddle could not emerge, however, until new systems of astronomy were developed, particularly through the work of Galileo Galilei and Johannes Kepler. By using the telescope, Galileo made visible unseen stars that greatly outnumbered the stars observable with the naked eye. Kepler, an early Copernican who discovered the elliptical orbits of the planets, believed passionately in a finite universe. He argued that the large number of new stars discovered by Galileo proved that the fixed stars must be less bright than the Sun, otherwise the night sky would be more luminous than the Sun. Kepler therefore believed that the sky was dark at night because the universe simply contained too few stars to cover the whole sky.
In the second part, Harrison shows how the development of the great systems of modern science, particularly the Cartesian and Newtonian, revealed new solutions to the age-old riddle. In René Descartes’ system, space extended in all directions to indefinite distances, and continuous matter under the aegis of mechanical laws pervaded the whole of space. The Cartesian system rejected the existence of space by itself along with the atomicity of matter, but it accepted the unlimited extent of space. Sir Isaac Newton rejected an unlimited expanse of stars, but he accepted space by itself (the vacuum) and the atomicity of matter. Although he began by believing in a finite cosmos, he accepted, later in his career, an infinite, star-filled universe. The reason for this belief derived from his theory of gravity: If stars formed a finite system, then they would all be attracted into the middle of the universe. Since that is obviously not the case, the stars must be uniformly distributed throughout infinite space.
Edmond Halley, Newton’s friend, explained the darkness of the night sky by the great decrease in apparent luminosity of the stars with distance, but his argument lacked clarity and accuracy. For example,...
(The entire section is 1888 words.)