I don't recall seeing a definitive/TFES endorsed answer, but I have seen a few.
I've seen suggested UA is like the Aetheric wind idea, and so it only accelerates objects made of/with a certain material. The bottom of the Earth and the planets/sun/stars/moon all have this material in them.
There's also the idea of 'celestial gears' taking some in this. Works best with the dome hypothesis, as everything is connected to the dome, which rests on the Earth, so everything accelerates together like a snow globe.
Another is about the most fantastical, that being the sun and moon are actually on giant poles coming up out of the North Pole. This accounts for their movement in the sky, and their acceleration. This was suggested along with the dome idea, so the stars and planets are simply dragged along like the prior one.
I'm betting there's some others about, and I'd love to hear if any of our resident FEers have their own ideas, or if perhaps this is one they've filed into 'we don't know'.