An observer is placed in the center of a large disc,
and the disk is placed in front of an object 7.5 light years away.

We then send an electromagnetic signal from the center of this disc,
towards the object placed in front of the disc.

After 15 years, we get an answer.

We will then imagine that this disc is a rotating disc of 12 al in
circumference and that it rotates with a tangential speed of 0.8c, over 15
years.

At the exact moment when a point A returns to its starting point, we
receive the response from the distant object.

So far, everything seems normal.

There is nothing complicated, and no bias in the thought experiment.

Let's continue: We know that the relative chronotropy is different
depending on the relative speed of the protagonists, and the central point
(as Sylvia points out) has a greater chronotropy than point A.

For point A, the proper time between the departure of the signal and the
return of the signal is Tr=9ans.

The speed of light being equal to the distance traveled per unit time, we
should have c'=(15/9)c=1.666c

Where then is the invariability of the speed of light? Every small portion
of the disk that can be considered moving
Galilean uniform at all times.

R.H.