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 – eg 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 cloud gets 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 2.25 million light years away.



Answered by Luis Villazon, How It Works contributor.