It is true that the present idea of the earth’s place in the system is different. Euclid imagined the earth to be at the centre of the universe. It is now known that the earth is one of various planets which revolve round the sun, and further, that the journey of the earth round the sun is so even and beautifully regulated that its motion is confined to a single plane. Year after year the earth goes on revolving round the sun, never departing, except to a very small extent, from this plane, which is one of the fundamental planes of the astronomer and called the Plane of the Ecliptic. [Illustration: FIG. 4.—The Plane of the Ecliptic. ] Suppose this plane to be a tangible thing, like the surface of an infinite ocean, the sun will occupy a certain position in this infinite ocean, and the earth will travel round it every year. If the axis of the earth were upright, one would represent the position of things by holding a globe with its axis upright, so that the equator of the earth is in every part of its revolution on a level with this ecliptic sea. But it is known that the earth, instead of floating, as it were, upright, as in Fig. 4, has its axis inclined to the plane of the ecliptic, as in Fig. 5. It is also known that by turning a globe round, distant objects would appear to move round an observer on the globe in an opposite direction to his own motion, and these distant objects would describe circles round a line joining the places pointed to by the poles of the earth, _i.e._, round the earth’s axis. [Illustration: FIG. 5.—The Plane of the Ecliptic, showing the Inclination of the Earth’s Axis. ] It is now easy to explain the observations referred to by Euclid by supposing the surface of the water in the tub to represent the plane of the ecliptic, that is, the plane of the path which the sun apparently takes in going round the earth; and examining the relative positions of the sun and earth represented by two floating balls, the latter having a wire through it inclined to the upright position; it will be seen at once by turning the ball on the wire as an axis to represent the diurnal motion of our earth, how Euclid finds the Bear which never sets, being the place in the heavens pointed to by the earth’s pole; and how all the stars in different portions of the heavens appear to move in complete circles round the pole-star when they do not set, and in parts of circles when they pass below the horizon. By moving the ball representing the earth round the sun and so examining their relative positions, during the course of a year it will be seen how the sun appears to travel through all the signs of the zodiac in succession in his yearly course, remaining a longer or shorter time above the horizon at different times of the year.