Regions of the sky

At any given northern latitude, the sky can be divided into three regions:

1. The north circumpolar region, where stars never set.
2. The equatorial region, where stars rise and set.
3. The south circumpolar region, where stars will never rise.

The declinations involved can be determined with the aid of the diagram below.

The horizontal line represents your horizon. The semicircle represents the dome of your sky. The vertical line is the direction to your zenith. If you are at 42 degrees north latitude, the angle from a point due north, on your horizon, to the north celestial pole (NCP) is same as your latitude, 42 degrees. Since the angle from a point on your horizon to your zenith is 90 degrees, the angle from the NCP to your zenith must be 48 degrees. Since the angle from the NCP to the celestial equator is 90 degrees, the angle from your zenith to the celestial equator must be 42 degrees. Since the angle from your zenith to a point on your south horizon is 90 degrees, the angle from the celestial equator to your south horizon must be 48 degrees.

Since the declination of the NCP is 90 degrees, the declination of a point due north of you, on the horizon must be 48 degrees (See diagram: 90 - 42 = 48). The declination of your zenith must be 42 degrees (See diagram: 90 - 48 = 42). Since the declination of the celestial equator is 0 degrees, the declination of a point due south of you, on the horizon, must be -48 degrees (See diagram: 0 - 48 = -48).