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This is about what the observers SEE in their telescopes.If you are anxious about making a fool of yourself in the face of the future, do as you please.
They are co-located when they receive the light from the supernova,
so they obviously both see the star as it goes supernova.
Since earth observers have _measured_ (via parallax ?) the distance
to the star to be 15000 ly, and the speed of light is c in the
Earth frame, the earth observer can deduce that the light must
have left the star 15000 years ago.
The rocket observer will see in his telescope the star
as it goes supernova in the direction RA = 12.7⁰ and DEC = 0⁰.
He has no possibility of knowing the distance to the star or
when it went supernova, but the Earth observer know that
since they were co-located at the reception, they both have
received the light that was emitted 15000 years ago.
That means that since the speed of light is c in the rocket frame,
the distance in the rocket frame must be 15000 ly.
But the rocket observer can't know this without being told.
But we can calculate that the coordinates of the star as
it went supernova must have been:
E '= (x' = 14633 ly, y' = 3297 ly, z' 0 ly, t' = -15000 y)
Case closed.
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