The southern constellations are those that fall within the Southern Celestial Zone, which is located at the bottom, or southern, portion of the sky map below. This zone is bounded by the celestial equator (0° declination) to the north, and the southern horizon to the south.
Stars in this region tend to hang low in the southern sky, never rising very far above the horizon. From our N39° latitude on the terrestrial sphere, these constellations generally rise in the southeast, make a brief appearance above the horizon, and then set in the southwest. As a the southern declination of a star increases, the amount of time that it will appear above our horizon decreases.
To gain a sense of how a star’s declination affects its time above the horizon, examine the -20° and -40° declination lines in the chart below. The -20° declination lines spans from the 0th hour (0h) of right ascension line to the 10th hour (10h) of right ascension line, which means that a star traversing the sky at -20° will be above the horizon for ten hours. Now consider an object just 20 degrees farther south on the -40° declination line. This line spans from the 2nd to the 8th hour of right ascension, which means that the star will appear above the horizon for only six hours. The effect accelerates as the declination increases even more. At five degrees farther south, or -25° declination, we approach the southern limit of our view from N39° latitude. An object at this declination would only appear above a perfect southern horizon for just a few minutes.
In contrast, objects that have a more northerly declination remain above the horizon increasingly longer. At +39.14° declination and higher on the celestial sphere, which corresponds to my N39.14° latitude on the terrestrial sphere, objects remain above the horizon all night!