# How far can we see if unobstructed?

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.

• Real AF/Sub-Zero

2.6 million light-years

• Cathy

Do we actually see Andromeda, or do we see the light that is being emitted from it?

• Weston Liveit

Cathy, I have wondered about that as well. It is more like the light from the Andromeda Galaxy have traveled through time and because it shines brightly we can see. We do not see it because we have such great vision. If we did, then we would be able to see it during the day as well. But getting back to the question of how far we can really see unobstructed, I think a related question is how well we can make out objects from a distance.

• deviheart13

Why do people keep saying we can see far based n stars. We don’t SEE stars, we see light that has already traveled most if not all of the way too us. Saying we can see millions of light years away because we can see stars would be like taking credit for an alien coming to our planet to say hi. Just like we don’t have the technology for light year traveling, we do not have the eyesight for it either, thank the starlight for being nice enough to come to us. Can someone use a better estimate, like an airplane in the distance or a average person looking at another average person.

• Peter Edwardson

By that Logic, “I’m not actually SEEING the computer monitor on my desk, only the light that has traveled most if not all of the way too us. Saying I can see a few feet away because I can see my Computer monitor would be like someone walking up to me to say hi. Even though we have the technology to produce Computer Monitors, we don’t have the eyesight for it. Thank the photons being emitted by the monitor for being nice enough to come to us.”

Just like a train leaving New York, is still the same train when it arrives in Los Angeles, Visible Light is Visible light, no matter how far it travels, it just takes 2.54 Million Years to get here from the Andromeda Galaxy, rather than 2*10^-9 seconds from a computer monitor.

• TMG

Actually, your cynicism and sarcasm is precisely correct.

• oldjohn

But you must accept that a star we can “see” may not still exist….

• TMG

We don’t see stars. We see the light that was emitted from it. Of course it may not exist at this very moment. That’s not a hard concept to understand. The point was that everything that we see is light. Peter was trying to be sarcastic, but the irony was that he chose his computer monitor as his counterpoint, which is amusing because displays emit light, not just reflect it. They are designed to do exactly what he refutes.

# Why are howler monkeys so loud?

What makes these creatures make such a racket?
Environment