Dust, water vapour and pollution in the air will rarely let you see more than 20 kilometres (12 miles), even on a clear day. Often, the curvature of the Earth gets in the way first. For example, at sea level, the horizon is only 4.8 kilometres (2.9 miles) away. On the top of Mt Everest, you could theoretically see for 339 kilometres (211 miles), but in practice clouds get in the way. For a truly unobstructed view, though, look up. On a clear night, you can see the Andromeda galaxy with the naked eye, which is 2.5 million light years away – that’s 24 quintillion kilometres (15 quintillion miles), in case you were wondering.
Answer by Luis Villazon