See that term 'constant motion' you used?
In Universal Acceleration motion is not constant. I'd have thought that was stunningly obvious myself.
Take a ship 10m tall and assume a 'horizon' 30km away. This means, according to an accelerating earth model, that in the time light travels 30,000m horizontally the Earth has moved 10m vertically; ie in about 1/10,000th of a second. Meaning that the vertical velocity of the Earth at this point in time is about 100,000m/s.
After a day accelerating at 1g (call it 10m/s/s) the Earth will now be moving at approximately 100,000+(10x3,600x24)=964,000m/s. The horizon distance of the same ship will now be, well about a tenth as far away, obviously.
You said
My example shows the velocity needed for a 30km horizon (which is approximately what we now have IIRC)
In other words, you are claiming that even without any acceleration at work, the
velocity of the Earth would cause a horizon effect. This is complete bullshit. Due to the principle of invariance, which I
just explained, observers on a flat Earth moving at a constant velocity would perceive things
exactly the same way as observers on a stationary flat Earth. In other words, you are completely missing the point by doing your math in a reference frame in which the Earth is moving, and are confusing yourself. The problem with doing the math in a moving reference frame is that you need to consider a) light emitted at an angle so that it is heading towards where the observer will be, not where the observer is now, which the observer will see despite the movement of the earth, and b) the
relativistic beaming effect which explains why this light, although it has traveled further, appears just as bright to the observer. You are insisting on doing the math in a moving reference frame without taking these effects into consideration, which is why your argument is bunk. You either need to take them into account, or else simply think about what would happen in a reference frame in which the Earth is stationary, which is
equivalent anyways. Now do you understand why your calculation of "the
velocity needed for a 30km horizon (which is approximately what we now have IIRC)" is completely wrong and based on incorrect assumptions? Once we have settled this point we can start talking about accelerating reference frames.