> "for example, the Southern Cross is only visible in the southern hemisphere" It's in the same place everywhere; it's the position of the Earth, which you're

> "for example, the Southern Cross is only visible in the southern hemisphere" It's in the same place everywhere; it's the position of the Earth, which you're implicitly measuring by pointing out that it's obstructing your view, that's different. The things above the horizon are visible; the things below the horizon are not. That's not facetious; the core idea of this type of navigation is to measure the position of the horizon with high precision, which is hard. Very hard if you're surrounded by mountains. ("Which hemisphere am I on?" is... low precision, if easy to measure). Those classic methods aren't pure astrometry: they're differential astrometry that measure the horizon against stars, the relative orientation of the two.