[LeGrande]

Ahh : ) You are afraid to answer my question, that shows progress.

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).

[mrjesse]

Ahh : ) You are afraid to answer my question, that shows progress.

Not at all afraid - but rather your question is irrelevant filibustering :-) You're trying to introduce more aspects then needed in order to confuse the issue. "Go pound a stake in and wait 6 hours," I can already hear you thinking. But we don't need to pound any stakes in or wait 6 hours. Remember, for the use of this thought-experiment, we've got a sensitive gravity meter which we've agreed tells us exactly the current position of pluto, and a nice sundial that tells us the exact optical position of the sun. We can operate both instruments at the same time right next to eachother. We don't need to wait and we don't need to talk about the light's path from sun to Pluto to us -- all we need to do is measure the angular difference between the gravitational pull and optical angle! Just like we did for the sun, where you claim that the gravity is 2.1 degrees off from the optical position at any given instant, for a viewer on earth, due to the fact that the earth rotates 2.1 degrees in the 8.3 minutes it takes the sun's light to reach the earth.

If light was instantaneous then you would be correct. The problem for you is that light is not instantaneous.

I well know that light is not instantaneous and travels at about 300MM/S. I also well know that by the time a given light wavefront reaches me from Pluto, the earth will have rotated 102 degrees and the apparent position of Pluto (along with the stars) will also have appeared to move by 102 degrees -- but the gravity will still be coming from about the same place as the light - because it's the earth that rotated, not Pluto that moved 102 degrees!

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 : ) ).

Yeah yeah I well know that the earth rotates about 102 degrees in 6.8 hours! I also know that the light I would see from Pluto is 6.8 hours old. But that's not the issue here! You claimed that at a given instant, the gravitational(and actual) direction of the sun is 2.1 degrees ahead of its optical/apparent direction for an observer on the earth.

You still didn't answer my question! I asked you how much was the angular seperation between the actual(gravitational) position and optical position! (I also asked about a heavenly body that was 12-light hours away.)

So are you saying that if I were to see Pluto in my telescope (granted my telescope is not good enough I'm sure) that at the instant I saw it, Pluto would actually be 102 degrees off and not even in the night sky, most likely?

You seem afraid to answer this question. Why is that? You answered it readily for the sun. Now for Pluto!

Thanks,

-Jesse