According to the principles of gravitation as unfolded by Newton, the power of attraction decreases directly as the square of the distance between the sun and a planet. Neptune, being on the outer rim of the system and hence farthest away from the sun, moves in its orbit around the sun more sluggishly than any other planet. Life such as we know it on the earth could not exist on Neptune; it would be too cold. The light and heat from the sun on Neptune are only one nine hundredth part of what we get on the earth. But even so, the sunlight falling upon Neptune is equal in power to seven hundred of our full moons. It was thought that Uranus was the last planet of the solar system until Neptune was found. Whether Neptune is the last, or whether other worlds are roaming around beyond it, is not known. Ptolemy, who was one of the most celebrated astronomers of earlier times, was born in Egypt about a century and a half after Christ. According to the Ptolemaic system of astronomy, which Ptolemy expounded but did not originate, the earth was considered the center of the universe, and around it the other planets and the sun were believed to revolve. A passage in the Bible in which Joshua commanded the sun to stand still indicates that the old Hebrews believed the sun circled around the earth. The Ptolemaic theory did not account for all the facts observed by astronomers, but for nearly fifteen centuries it held practically universal sway over the belief of men, until another thinker set the matter right. Nicholas Copernicus was born in Prussia, February 19, 1473. He studied mathematics, medicine, theology, and painting, but his greatest achievements were in astronomy. He made holes in the walls of his room, through which he might observe the stars. Copernicus did not believe in the theory of Ptolemy that the earth was the center of the universe, but held that the solar system had for its center the sun, and that around it the planets, including the earth, revolved. In working out this belief, which science has subsequently shown to be correct, he laid the foundations of the modern system of astronomy. The book in which Copernicus expounded his theory was begun in 1507 and was completed in 1530. He could not be induced to publish it, however, until shortly before his death. On May 24, 1543, he lay dying in Frauenburg. A few hours before his death, when reason, memory, and life were slipping away from him, the first printed copy of his book was borne to Frauenburg and placed in the great astronomer's hands. He touched the book, looked at it for a time, and seemed conscious of what it was. Quickly afterward he lapsed into insensibility and was gone.