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The meridian, like the celestial equator and the poles, is not a real feature of the universe; it is an imaginary line that helps make sense of our perspective.
The meridian is a line, drawn from the north celestial pole to the south pole, bisecting the sky. North has an azimuth of 0 degrees, and South has an azimuth of 180 degrees. Thus, the meridian has an azimuth of 0 degrees in the northern sky, and 180 degrees in the southern sky. Any object must have an azimuth of 180 degrees at this point.
We should note that the term "southern sky" is often conflated with the idea of the sky as viewed from the southern hemisphere. Just as we can see below the celestial equator from the northern hemisphere, so too can southern viewers see part of the northern hemisphere; the meridian of an object in that portion of the sky would be 0.
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