If we watch the sky for some time, or make several observations on the same night, we notice, by observing the changing positions of the constellations, that the stars move very slowly across the blue dome above us. The stars that rise due east of us do not, in crossing the dome of the sky, pass directly over our heads but, from the moment that we first see them, curve some distance to the south, and, after passing their highest point in the heavens, turn toward the north and set due west. A star rising due east appears to move more rapidly than one rising some distance to the north or south of the east point, because it crosses a higher point in the heavens and has, therefore, a greater distance to traverse in the same length of time. When we observe the stars in the northern sky, we discover that many of them never set but seem to be moving around an apparently fixed point at somewhat more than an angle of 40°[194] above the northern horizon and very near the north star. These are called _circum-polar stars_. The whole celestial sphere, in other words, appears to be revolving about an imaginary axis passing through this fixed point, which is called the _north pole_ of the heavens, through the center of the earth and through an invisible pole (the south pole of the heavens) exactly opposite the visible one. This apparent revolution of the whole star sphere, as we know, is caused by the earth's rotation on its axis once every twenty-four hours from west to east. Chaucer and his contemporaries believed it to be the actual revolution of the nine spheres from east to west about the earth as a center. [Illustration: Fig. 1.] For determining accurately the position of stars on the celestial sphere astronomers make use of various circles which can be made clear by a few simple diagrams. In Figure 1, the observer is imagined to be at O. Then the circle NESW is the celestial horizon, which we have described above. Z, the point immediately above the observer is called the _zenith_, and Z', the point immediately underneath, as indicated by a plumb line at rest, is the _nadir_. The line POP' is the imaginary axis about which the star-sphere appears to revolve, and P and P' are the poles of the heavens. The north pole P is elevated, for our latitude, at an angle of approximately 40° from the north point on the horizon. PP' is called the _polar axis_ and it is evident that the earth's axis extended infinitely would coincide with this axis of the heavens.