For the other measurements used by astronomers in observations of the stars still other circles on the celestial sphere must be imagined. We know that the earth's surface is divided into halves, called the northern and southern hemispheres, by an imaginary circle called the _equator_, whose plane passes through the center of the earth and is perpendicular to the earth's axis. If the plane of the earth's equator were infinitely extended it would describe upon the celestial sphere a great circle which would divide that sphere into two hemispheres, just as the plane of the terrestrial equator divides the earth into two hemispheres. This great circle on the celestial sphere is called the _celestial equator_, or, by an older name, the _equatorial_, the significance of which we shall see presently. A star rising due east would traverse this great circle of the celestial sphere and set due west. The path of such a star is represented in Figure 2 by the great circle EMWM', which also represents the celestial equator. All stars rise and set following circles whose planes are parallel to that of the celestial equator and these circles of the celestial sphere are smaller and smaller the nearer they are to the pole, so that stars very near the pole appear to be encircling it in very small concentric circles. Stars in an area around the north celestial pole, whose limits vary with the position of the observer never set for an observer in the northern hemisphere. There is a similar group of stars around the south pole for an observer in the southern hemisphere. [Illustration: Fig. 2.] The angle of elevation of the celestial equator to the horizon varies according to the position of the observer. If, for example, the observer were at the north pole of the earth, the north celestial pole would be directly above him and would therefore coincide with the zenith; this would obviously make the celestial equator and the horizon also coincide. If the observer should pass slowly from the pole to the terrestrial equator it is clear that the two circles would no longer coincide and that the angle between them would gradually widen until it reached 90°. Then the zenith would be on the celestial equator and the north and south poles of the heavens would be on the horizon.