Copernicus dealt only with the six planets then known and the sun and moon. As to the stars, he had no idea that they were suns like our own, at immense and various distances from us. The knowledge of the magnitude of the sidereal universe was reserved for our own century, when it was discovered by the method of parallaxes. We will give Copernicus's own sketch of the planetary system:-- "In the highest place is the sphere of the fixed stars, an immovable sphere, which surrounds the whole of the universe. Among the movable planets the first is Saturn, which requires thirty years to make its revolution. After it Jupiter accomplishes its journey in twelve years; Mars follows, requiring two years. In the fourth line come the earth and the moon which in the course of one year return to their original position. The fifth place is occupied by Venus, which requires nine months for its journey. Mercury occupies the sixth place, whose orbit is accomplished in eighty days. In the midst of all is the sun. What man is there, who in this majestic temple could choose another and better place for that brilliant lamp which illuminates all the planets with their satellites? It is not without reason that the sun is called the lantern of the world, the soul and thought of the universe. In placing it in the centre of the planets, as on a regal throne, we give it the government of the great family of celestial bodies." The hypothesis of the motion of the earth in its orbit appeared simply to Copernicus as a good basis for the exact determination of the ratios of the distances of the several planets about the sun. But he did not give up the excentrics and epicycles for the explanation of the irregular motions of the planets, and certain imaginary variations in the precession of the equinoxes and the obliquity of the ecliptic. According to him the earth was endowed with three different motions, the first about its axis, the second along the ecliptic, and a third, which he called the declination, moving it backwards along the signs of the zodiac from east to west. This last motion was invented to explain the phenomena of the seasons. He thought, like many other ancient philosophers, that a body could not turn about another without being fixed in some way to it--by a crystal sphere, or something--and in this case that the same surface would each day be presented to the sun, and so it requires a third rotation, by which its axis may remain constantly parallel to itself. Galileo, however, afterwards demonstrated the independence of the two motions in question, and proved that the third was unnecessary.