When the observer is at the equator, as in Fig. 10, the poles of the heavens are on the horizon, and the diurnal circles are _perpendicular_ to the horizon. When the observer is at one of the poles, as in Fig. 11, the poles of the heavens are in the zenith and the nadir, and the diurnal circles are _parallel_ with the horizon. [Illustration: Fig. 10.] [Illustration: Fig. 11.] 6. _Elevation of the Pole and of the Equinoctial._--At the equator the poles of the heavens lie on the horizon, and the celestial equator passes through the zenith. As a person moves north from the equator, his zenith moves north from the celestial equator, and his horizon moves down from the north pole, and up from the south pole. The distance of the zenith from the equinoctial, and of the horizon from the celestial poles, will always be equal to the distance of the observer from the equator. In other words, the elevation of the pole is equal to the latitude of the place. In Fig. 12, _O_ is the point of observation, _Z_ the zenith, and _SN_ the horizon. _NP_, the elevation of the pole, is equal to _ZE_, the distance of the zenith from the equinoctial, and to the distance of _O_ from the equator, or the latitude of the place. Two angles, or two arcs, which together equal 90°, are said to be _complements_ of each other. _ZE_ and _ES_ in Fig. 12 are together equal to 90°: hence they are complements of each other. _ZE_ is equal to the latitude of the place, and _ES_ is the _elevation_ of the equinoctial above the horizon: hence the elevation of the equinoctial is equal to the complement of the latitude of the place. [Illustration: Fig. 12.] Were the observer south of the equator, the zenith would be south of the equinoctial, and the south pole of the heavens would be the elevated pole. [Illustration: Fig. 13.] _7. Four Sets of Stars._--At most points of observation there are four sets of stars. These four sets are shown in Fig. 13. (1) The stars in the neighborhood of the elevated pole _never set_. It will be seen from Fig. 13, that if the distance of a star from the elevated pole does not exceed the elevation of the pole, or the latitude of the place, its diurnal circle will be wholly above the horizon. As the observer approaches the equator, the elevation of the pole becomes less and less, and the belt of circumpolar stars becomes narrower and narrower: at the equator it disappears entirely. As the observer approaches the pole, the elevation of the pole increases, and the belt of circumpolar stars becomes broader and broader, until at the pole it includes half of the heavens. At the poles, no stars rise or set, and only half of the stars are ever seen at all.