It depends on one's point of view and base assumtions. You haven't presented any evidence that ancient Arabs included the Earth when they talked of celestial bodies.
Perhaps it's not mentioned because the Quran is not a science textbook.
The planet Venus can be observed almost every night. It's sometimes erroneously called the "Evening Star", because it's so much brighter than most stars that it's the first celestial body seen in the sky after sunset.
BTW - I'm at a disadvantage here, because I can't read the original Arabic. Would the word that's translated as "rounded" also imply "circular"?
I know I have not included any such evidence. Anyways, imagine if earth was stationary ( just imagine) and planets and sun rotated around it, would it not be called celestial body? I mean it too is in space.
You are assuming that it was believed that earth was considered stationary at that time. Now, first of all, as I have said that if earth was considered stationary why hasn't this been mentioned? Imagine if their is a copier, he will copy everything down including wrong and right. Why did author of Quran only copy the correct things down? Doesn't this imply that he already knew what is wrong and what is right?
I even find this tactic of atheists ( directed at 9 10 penguino) quite disturbing. WHen they ask theists that why they believe in God, and theists reply as their is no evidence against it. Then atheists say, that this is a failed logic. LOOK at what you are saying, If I cannot prove that earth was believed to be moving at that time, then it means at that time it was considered that earth was stationary.
Even if planet venus is observable, I donot think that the verse is referring to only one planet. Go back to the verse and check that it refers to "all celestial bodies". These also include others which are not observable.