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 Quote by Bjarne Bjarne So when we measure the speed of the Sun to travel 250 km/s, - an observer on Mercury (or on the surface of the Sun) would measure it to be a little more than this? - is this what you saying ? If so Why ?
Someone on the surface of another planet with a different mass than Earth could measure it a little or a lot more or less than ours because their rulers are different lengths than we would if they're based on the same standards that ours are based on and this is because of your stipulation:
 Quote by Bjarne Let us forget everything about SR and only consider the consequence of GR and hence time dilation caused by gravity for 2 observers orbiting the MilkyWay.
 Quote by Bjarne Now imaging the same 2 observers measure the speed of a photon, we on Earth would measure it to be; “c” - would an observer on Mercury also measure that to be faster?
No, everyone measures the speed of light to be "c".
 Quote by Bjarne Can the speed difference, - significant or almost completely solve the "mathematical meltdown" described above, - or is this value almost irrelevant in this context? Does the speed difference, - have anything to do with the cause of the the "mathematical meltdown" described above ?
I thought your issue about a mathematical meltdown would only apply if I answered "yes" to your previous question but since I answered "no", I don't know what you mean by a mathematical meltdown and again, everyone measures the speed of light to be exactly the same value "c" so there's no mathematical meltdown that I can see.
 Quote by Bjarne Bjarne I mean is let’s say the mass of the Sun is exactly 2E30 Kg. Are you saying an observer on Mercury not would agree ? If so, - why?
Yes, he would say the sun had a different mass because all his measuring devices are different due to the difference in gravity between the Earth and Mercury.
 Quote by Bjarne What about the mass of the Milkyway? - would the 2 observers also disagree ?
Yes, they would disagree about everything.
 Quote by Bjarne Can the mass difference significant contribute to solve the "mathematical meltdown" described above, - or is it almost irrelevant in this context? Does the mass difference, - have anything to do with the cause of the the "mathematical meltdown" described above ?
You're going to have to explain to me what this mathematical meltdown is. I didn't know such a thing could be possible.
 Quote by Bjarne What more as (time) speed and mass is different ?
Everything measureable and observable is different:
Temperatures
Colors
Ages
Sizes
Shapes
Frequencies
Pressures
Forces
Directions
I'm having a hard time trying to think of something that wouldn't be put on the list.