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

 P: 5 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?
 Sci Advisor P: 6,112 A light year is the distance that light travels in a year. It has nothing to do with visibility.
P: 5
 Quote by mathman A light year is the distance that light travels in a year. It has nothing to do with visibility.
but to meaure the distance traveled we base it on an increase or decrease in visibility? am I wrong?

 P: 692 How can our 'ability' to see something be a valid indicator of distance(lightyears)? Brightness is only one way the distance of an object may be determined. http://hyperphysics.phy-astr.gsu.edu...o/stdcand.html There are several other techniques used: http://hyperphysics.phy-astr.gsu.edu.../distance.html http://www.talkorigins.org/faqs/astronomy/distance.html
 Mentor P: 12,111 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.
Mentor
P: 12,013
 Quote by neugie92 but to meaure the distance traveled we base it on an increase or decrease in visibility? am I wrong?
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.

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