Measuring north and south of the equator
The celestial equator has declination zero, the north pole +90 degrees and the south pole −90 degrees. The angle is measured along the target’s hour circle in degrees, arcminutes and arcseconds.
The analogy with terrestrial latitude concerns direction, not a geographic location of the source on Earth. Declination also supplies no radial distance.
Combining declination with RA
Declination alone selects a circle parallel to the celestial equator without locating a target along it. RA supplies the longitudinal coordinate needed for an ordinary equatorial direction.
NASA illustrates the pair using approximately RA 6 h 45 m and declination −16°43′ for Sirius. Rounded educational values are not a substitute for frame-, epoch- and motion-labelled pointing data.
Declination is not local altitude
Negative declination means south of the celestial equator, not necessarily below an observer’s horizon. Visibility also depends on latitude, time and local conditions.
A target’s altitude rises and falls during Earth’s rotation while its catalogue declination is approximately stable over a short interval. Local pointing requires RA, site, time and the relevant corrections as well.