Question

Why are stars visible from the northern cities different from the stars that are visible in southern cities?

Answer 1

You would see a completely different set of stars from each pole.

From the North Pole, you can never see any star with a declination below the Equator. At the South Pole, you would theoretically see no star with a declination above it, although the fact that the South Pole is not at sea level, but is instead at 9,301 feet above sea level, does allow stars of a few degrees into the northern sky to be seen on very rare occasions, due to the curvature of the Earth and atmospheric refraction.

Setting aside those effects, a simple rule of thumb is that there is a 90-degree range of stars that can be seen at some time from any latitude on Earth; a person can see all of the stars in their own hemisphere (although not all at once), plus all stars 90 degrees of declination above or below, depending on whether they live in the north or south. A person living at latitude 35N, for example, can occasionally view stars as far south as 55S declination, given an unobstructed horizon. A person living at 35S can see stars as far north as 55N, given the same conditions. This is why most people living in the United States have never seen the Southern Cross--its declination is 60S, and therefore it never rises above the horizon except in Hawaii and parts of Florida and Texas. The Alpha Centauri system, the closest known stars to the Sun, are at 60'50S, and therefore also impossible to view from where I live. And to see Sigma Octantis, the South Star located at 88'57S, one would have to travel to south of 1'03N (and have a good eye, because it's nowhere near as bright as Polaris).

If you lived exactly at the Equator, you could at some point see every star there is, although Polaris and Sigma Octantis would be extremely close to the horizon and difficult to view.