how far can a human see
The human eye can see an almost unlimited distance. Looking up into the sky on a very clear night, the Triangulum Galaxy can sometimes be seen. This is a distance of 3.14 million light years. The Andromeda Galaxy is also sometimes visible. This is a distance of 2.5 million light years. However, it must be pointed out that when we see these galaxies what we actually see is something at a somewhat lesser distance. What we see is light from the far away object rather than the object itself.