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.
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.
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.