Terms: horizon, meridian

The SC002 covers about 25% of the celestial sphere. Most of what it shows is circumpolar and hence always above the horizon, but some of the displayed stars are below the horizon some of the time. To map the part of the SC002 that we can see, we need to display our horizon on the map. Of course, stars are always rising and setting (i.e., the set of stars above the horizon is changing) so any such horizon just displays that moment's visible stars. The red line in the below image is the horizon, with directions NE, N, and NW displayed.

The most important "line" in astronomy is the observer's meridian. Since the meridian runs from pole to pole, it is part of a hour circle and hence a radial line on the SC002. The blue line in the below image is the meridian; the Z shows where zenith is.

Note that all the stars "above" dec=45° are above the horizon (i.e., are circumpolar), whereas some of the stars near the bottom edge are below the horizon now (and so have set). If you were to travel north, the size of the circumpolar zone would increase and and at a latitude of 60° all of the SC002 stars would be circumpolar. At the north pole, the NCP is at zenith. If you were to travel south, the size of the circumpolar zone would decrease, and the horizon would "eat into" the SC002. At the equator the north celestial pole sits on the horizon, and hence the horizon would become a line right through the center of the SC002.