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So -- what happened to all the other 100 billion stars in our galaxy (with proper motions independent of ours) -- not to mention all the other galaxies in His created universe?
It's God's universe. He -- and only He -- can change the "rules or laws" by which it operates. (When He does so, we call it a "miracle"...)
HE changed mankind's temporal reference frame for a day.
He once did the same for me for about a picosecond of His time. Seemed like hours for me and changed my entire life's direction. In fact, that is why I'm a physical chemist. (Long, true story...)
BTW, did Joshua and/or Jashar even know what a "galaxy" was? If so, how many could he/they see? Also -- did mankind's language even have a word for "galaxy"?
All interesting but irrelevant to the fact that in Joshua the Bible says the Sun move around the Earth. If we are to accept the Bible as the literal truth, as young Earth creationists would have us do, then the literal fact is that astronomy is wrong and it is the Sun that is moving and not the Earth. So it’s not hard to believe that a good percentage of the people the story speaks of would be of the YEC variety.