PrepTest 140, Section 2, Question 25
At one time, many astronomers assumed that Earth(60 more words not shown)
Which one of the following(9 more words not shown)
- AIf the stars(23 more words not shown)
- BAll stars move(11 more words not shown)
- CEarth does not(8 more words not shown)
- DStars do not(5 more words not shown)
- EA star that(17 more words not shown)
Reading the argument
Why couldn't the stars simply be moving that fast?
Within these astronomers' picture, Earth stays still and the stars make a trip around it each day. A star farther away has a larger circle to travel in the same amount of time, so it would need a greater speed. That part makes sense within their model. But it doesn't establish that the stars are nearby unless those greater speeds are ruled out.
Imagine a closer star making a shorter trip and a farther star making a much longer trip, both finishing in a day. The second one would just have to travel faster. The argument needs to reject that possibility to use speed as a limit on distance.
What we’re looking for
We're asked for an assumption in their reasoning, not for a correction based on modern astronomy. My prediction is that the stars don't travel at the tremendously great speeds that a more distant orbit would require.
Working the answers
- A
This concerns what would be possible if the stars didn't revolve around Earth. The astronomers' argument starts from the assumption that they do. A statement about a different model doesn't supply the missing speed limit within theirs.
- B
Stars at different distances could cover different-sized circles in the same time by moving at different speeds. The astronomers don't need every star to move at exactly the same speed. They need the tremendously great speeds required for much greater distances to be unavailable.
- C
This contradicts the assumed stationary Earth at the center of their account. Whether that assumption is correct in reality is separate from identifying what their argument needs. C isn't an unstated part of their reasoning; it rejects its starting point.
- DCorrect
If stars don't move at tremendously great speeds, they can't be as far away as the distance that would require those speeds for the daily trip. Without that assumption, the astronomers haven't ruled out distant stars completing the longer trip very quickly. D closes the exact possibility raised by our objection.
- E
Being more than one million miles away isn't the distance threshold in the conclusion, which is a few million miles. This possibility doesn't establish the restriction on speed the argument needs. Even if such a star could reappear in roughly the same place each night, we'd still need to know why a much farther star couldn't do so by traveling faster.
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