Q. I am looking at a star that is 200 light years away. I know light travels at 186,000 miles per second, which would put this star 1,173,139,200,000,000 miles away. When I look at that star, just how far am I really looking?

A. Asking how far you can see, is like asking how far you can catch. Think of your eyes as a catcher's mitt. The mitt waits for the ball to come to it. Like that catcher's mitt, our eyes just sit there and let the light shine in.

The photons, or units of light energy we see, are like baseballs. They do the traveling. The photons that left a star, or the moon, or the newspaper you are reading, move across short or long distances, over short or long periods of time. When those photons hit your open eye, they trigger photoreceptor cells called rods and cones on the retina.

That turns into an electrical message sent to the back of your brain, to a section of your brain called V1. That's the primary visual cortex, the first part of the brain to receive incoming visual messages. Not until the message from your eyes hits V1 do you see.

If the nerve cell pathways between the eyes and the visual cortex is severed, the eyes may be receiving light, but we never perceive it.

How far away does something have to be before you can't see it? You can see out to infinity, assuming you don't have any health problems with your eyes, says Dr. Dimitri Azar, director of the Cornea Service at the Massachusetts Eye and Ear Infirmary.

If the object is big enough and bright enough to send photons out with enough power to reach your eye, you can see them, no matter how far away the source is.

No matter how long ago they left, or how far they traveled to get to you, if those photons can hit your eye, the catcher's mitt of your vision system, you can see the object from which they came.