“Kepler was not merely an observer and calculator; he inquired with great diligence into the physical causes of every phenomenon, and made a near approach to the discovery of that great principle which maintains and regulates the planetary motions. He possessed some very sound and accurate notions of the nature of gravity, but unfortunately conceived it to diminish simply in proportion to the distance, although he had demonstrated that the intensity of light is reciprocally proportional to the surface over which it is spread, or inversely as the square of the distance from the luminous body.” Great names follow in rapid succession. One of Kepler’s contemporaries was Galileo Galilei, the discoverer of the “three laws of motion” and the relation of time and space in falling bodies, the first to apply the newly invented telescope to the observation of the heavens and the discoverer of four satellites of Jupiter (named by him the “Medeiran Stars” in honor of his patron). He also detected spots on the sun’s disk, the phases of Venus, and irregularities on the moon’s surface, and declared the Milky Way to be composed of a countless tract of separate stars. When we remember the limited power of the telescope of the age, we can but marvel, not at how little, but how much was known regarding the starry skies. During this period, numerous observers rendered great service to Astronomy, and other scientists were engaged in making useful drawings, charts, maps, tables, and catalogues of stars. To this period also belongs John Bayer of Augsburg, who published a description of the constellations with maps upon which the stars were marked with the letters of the Greek Alphabet—a convenient method that was universally adopted and is still in use. Other names include Gassendi, Riccioli, Grimaldi, and Hevelius—the latter a rich citizen of Dantzig, who had a fine observatory of his own, where he worked for forty years. His drawings and descriptions of the moon, his researches on comets, which he still believed moved in parabolas, and his celestial charts engaged most of his attention. The Dutch astronomer Huygens (born in 1629) is famous for his improvements in the telescope use of the pendulum clock and developments in the machinery of astronomical instruments. He discovered the ring of Saturn and four of his satellites. Edmund Halley, an English astronomer (born in 1656), also took a great interest in the telescope, and went to Dantzig to settle a controversy between Robert Hooke and Hevelius regarding the best glasses for use in astronomical observations; for Hevelius still worked with the ancient instruments, while Hooke believed in the lens. Halley revived the ancient idea that comets belonged to the Solar System, and predicted that the comet of 1681 would return to its perihelion in 1759. This was the first prediction of its kind verified.