# How can our 'ability' to see something be a valid indicator of distance(lightyears)?

1. Jan 25, 2013

### neugie92

Light years-I just dont understand how something as subjective as visibilty, is used as a major indicator of distance. Isnt that subjective. What if some species had a million times the eye power than us humans, wouldnt they view the distance of lets say stars very differently because it wouldnt be as many 'light years' away from them.

Maybe I'm confused about how light years are actually measured, it just seems like using somethings visiblity to us, is a poor way to try to figure out actual distance.

Thoughts?

2. Jan 25, 2013

### mathman

Re: How can our 'ability' to see something be a valid indicator of distance(lightyear

A light year is the distance that light travels in a year. It has nothing to do with visibility.

3. Jan 25, 2013

### neugie92

Re: How can our 'ability' to see something be a valid indicator of distance(lightyear

but to meaure the distance traveled we base it on an increase or decrease in visibility? am I wrong?

4. Jan 25, 2013

### Jimmy

5. Jan 25, 2013

### Staff: Mentor

Re: How can our 'ability' to see something be a valid indicator of distance(lightyear

There is no relation to visibility.

A light year is the distance light travels in one year. As the speed of light in vacuum is constant, this length is constant, too. It does not matter how good some eyes are.

Edit: Looks like two topics got merged, as I posted the other replys were not in the thread.

Last edited: Jan 25, 2013
6. Jan 25, 2013

### Drakkith

Staff Emeritus
Re: How can our 'ability' to see something be a valid indicator of distance(lightyear

Yes, you are wrong. I can turn on a laser beam and bounce it off of a mirror at a known distance from me to measure the speed of light. Then, knowing this speed, I just multiply it out and the resulting distance is 1 light-year. It has nothing to do with visibility. The laser could be a bazillion watts or 1 microwatt, it does not matter as that does not affect the speed of the beam.

Maybe you are confused over how we measure the distance to objects in space. There are a variety of methods, and Jimmy's links go over some of them.