Expert: Courtney Seligman - 8/22/2008

Question
what do we actually see? when we look at things are we seeing the Light or the objects

Answer
Strictly speaking, we the light reflected by or given off by the object. Experience tells us that there has to be an object 'there' for us to see the light that we use to observe it; but it's the light we see, not the object itself.

In astronomy, most of the objects we observe are very far away, and it takes time for the light to cover the distance between us and the object, so we don't even see the object as it is, but as it was at the time the light left it. For objects that are far enough away, by the time the light gets here, the object may no longer exist, or may have moved significantly since the light left them.

In ordinary life, we don't have to worry about such things, because the light-travel time is so short, that what things look like and where they are is essentially the same as what they looked like and where they were when the light we see left them; but engineers at NASA have to worry about such things when they send spacecraft to the other planets, because where we see them is where they were minutes or hours ago, and if that weren't taken into account, the spacecraft would miss their targets by however far the objects moved during that time (which could be thousands of miles, for the more distant planets). And astronomers have taken this effect into account for more than 300 years, when doing accurate calculations of the positions of objects in the Solar System.