91. In the horizon system of coordinates what is equivalent to the declination of the equator system?

Altitude in the horizon system is equivalent to declination in the equatorial system because both measure angular height above a reference plane. Declination measures the angular distance north or south of the celestial equator, while altitude measures the angular distance above the observer's horizon. Azimuth measures direction, nadir indicates a point opposite the zenith, and zenith distance is related to altitude but not directly equivalent to declination.