Said mrjesse: Does that answer your need for a frame of reference?No. Which instant are you talking about? When the reflected light leaves Pluto or when the reflected light is seen by your observer on earth. Is your observer (observatory) compensating for all the motions of the earth and pluto or not?
In other words when you look at Pluto, you are seeing it about 6.8 hours behind where it actually is, but if you had a sensitive gravity sensor where would it point? At the Pluto you see or 6.8 hours ahead of the Pluto you see?So what's your answer to this question? Would Pluto's apparent optical position be 102 degrees (6.8 hours) behind its actual+gravitational position?
We don't need to compensate for any of the motions because we're measuring an instantaneous difference using two measuring devices. Either the gravity is pulling in the direction of about 102 degrees ahead of Pluto's apparent optical position, or its not.
If light was instantaneous then you would be correct. The problem for you is that light is not instantaneous. It takes time for light to travel from objects to your eyes and things can and do move in that interval of time. Think of slowly rolling a marble across a table. I am able to start the marble and then walk around to the other side to catch it. Just like the marble, light takes time to travel. If the marble was light, I would see myself : )
Like I said before, I think you have a reasonable grasp of classical physics (with some holes, light is not instantaneous).
So what's your answer to this question? Would Pluto's apparent optical position be 102 degrees (6.8 hours) behind its actual+gravitational position?
Using the rotating earth as your frame of reference and recognizing that the light that you see originated 6.8 hours earlier from Plutos actual position, yes, from your perspective, Plutos apparent position is off by 102 degrees (close enough for government work anyway : ) ).
Seriously, the thought experiment that I proposed will help you visualize it. With a telescope pointed at Pluto when it lights up for an instant. How many degrees is the telescope operator going to have to turn the telescope to see the light when it finally gets to him 6.8 hours later? The operator will have to turn his telescope 102 degrees and hopefully he is high enough off the surface or lets start him at a 30 degree angle so that the surface of the earth won't block the light.
If the observer was stationary in relation to Pluto then the apparent position and actual position would be identical. Our earth though, is not a stationary platform it is spinning, orbiting and traveling through space all at the same time. Because of that fact and the fact that light is not instantaneous none of the objects that we see in the sky are where they appear to be, from our frame of reference, the rotating earth.
Let me summarize. Our frame of reference, the earth, is constantly moving and spinning. The speed of light is 300 million meters a second. As a result, everything that we observe is time shifted and nothing that we see is where it actually is (although the Moon is pretty darn close : ) ). Of course all the other objects in the Universe are moving too.
So what is a good astronomer to do? Well for one thing they don't really care : ) All they care about is the apparent position. There is no practical value at all to determining the actual position (well some, like NASA, like to know the actual position, but the math is easy).