At one time, many astronomers assumed that Earth remains moti

Question

At one time, many astronomers assumed that Earth remains motionless while the stars revolve around it. They concluded from this that the stars were not more than a few million miles from Earth. They reasoned that if the stars were farther away, they would have to move at tremendously great speeds in order to circle Earth during the day and reappear in roughly the same positions each night.

Which one of the following is an assumption required by the reasoning described above?

(A) If the stars do not revolve around Earth, it is possible for at least some stars to be more than a few million miles from Earth.

(B) All stars move at exactly the same speed when they are revolving around Earth.

(C) Earth does not remain motionless while the stars revolve around it.

(D) Stars do not move at tremendously great speeds.

(E) A star that is more than a million miles from Earth could reappear in roughly the same position each night.

LSAT 70

AshutoshB · Answer

QUESTION TYPE:  Necessary Assumption

CONCLUSION:  Astronomers thought that the stars were within a few million miles of the Earth.

REASONING:  The astronomers thought that the stars moved around the Earth. If the stars were very far away, they would have to move very fast.
 
ANALYSIS:  The flaw is not thinking that the stars revolve around the Earth. That’s just context to set up the situation. You can reason correctly from false hypotheticals, i.e. “If unicorns exist, then….”.

The conclusion is the second sentence (“They conc,luded”). The astronomers think the stars can’t be far away, otherwise stars would move very fast. So what….why is it a problem for stars to move fast? The astronomers only implied that great speeds were a problem. If an argument implies something but doesn’t state it, then that is an assumption.

You can use some outside knowledge on this question. Stars appear in the same position each night. Everyone agrees on this – it’s why sailors could navigate by the stars. It’s why astronomers thought stars would have to travel fast each night.

___________

A. The astronomers were assuming that the stars do revolve around the Earth. This talks about what happens if they don’t. It’s as though you’re planning for how to take the CAT, and I make an argument about what happens if you weren’t required to take the CAT. Irrelevant.

B. Why would it matter if one star moved 0.00001 mph faster than another star?

C. If you negate this, you get “Earth remains motionless while the stars revolve around it”. That doesn’t contradict the astronomers.

D.  CORRECT.  The astronomers’ only reason for saying the stars couldn’t be far away was that the stars would move quickly if they were far away.
Negation: Stars move at very great speeds.

E. If you negate this, you get “stars more than a million miles away could not reappear in the same position”. This strengthens the argument: the stars have to be even closer. The astronomers were predicting a maximum, not a minimum.

Src:LSATHACK

sssanskaar · Answer

Read the lines:

Straight go to point D.

KarishmaB · Answer

In assumption questions, we should try to identify the gap between premises and conclusion. Premises: At one time, many astronomers assumed that Earth remains motionless while the stars revolve around it. If the stars were more than a few million miles away, they would have to move at tremendously great speeds in order to circle Earth during the day and reappear in roughly the same positions each night. Conclusion: They concluded from this that the stars were not more than a few million miles from Earth. What is missing in the logic? The reasoning given is that if the stars were more than a few million miles away, they would have to move at tremendously great speeds to circle Earth in a day. So they are not more than a few million miles away. Well, the argument has not mentioned that the stars cannot move at tremendously great speeds. So it is a missing premise. It has been taken to be obvious. (D) Stars do not move at tremendously great speeds. Negated (D): Stars move at tremendously great speeds. Then our logic of the argument and the conclusion fail. Then the stars could be much farther away. (A) If the stars do not revolve around Earth, it is possible for at least some stars to be more than a few million miles from Earth. There is no discussion of this possibility - stars do not revolve around Earth. Out of scope. (B) All stars move at exactly the same speed when they are revolving around Earth. This is not assumed. The word 'exactly' should be enough to make you eliminate it. We are given that they are a few million miles away and come back to 'roughly' the same positions each night. There is no implication of all stars being at exactly the same distance away and all coming back to exactly the same relative point. Hence this is not assumed. (C) Earth does not remain motionless while the stars revolve around it. It is given that they assumed that Earth stays motionless. Their reasoning does not assume that Earth does not remain motionless. (E) A star that is more than a million miles from Earth could reappear in roughly the same position each night. The assumption is that a star that is much more than a few million miles away from Earth (say 1000 million miles away) will not be able to come back to its original position each night because it cannot move at tremendous speeds. 'more than a million lines' includes a few million miles as well as 1000 million miles. Hence this statement makes no distinction between the two. Hence it will not be the answer. Answer (D) Discussion on Assumption Questions: https://youtu.be/O0ROJfljRLU A Hard Assumption Question: https://youtu.be/0j4tovGifIg

Advait01 · Answer

A. The astronomers were assuming that the stars do revolve around the Earth. This talks about what happens if they don’t. It’s as though you’re planning for how to take the CAT, and I make an argument about what happens if you weren’t required to take the CAT. Irrelevant.

B. Why would it matter if one star moved 0.00001 mph faster than another star?

C. If you negate this, you get “Earth remains motionless while the stars revolve around it”. That doesn’t contradict the astronomers.

D. CORRECT. The astronomers’ only reason for saying the stars couldn’t be far away was that the stars would move quickly if they were far away.
Negation: Stars move at very great speeds.

E. If you negate this, you get “stars more than a million miles away could not reappear in the same position”. This strengthens the argument: the stars have to be even closer. The astronomers were predicting a maximum, not a minimum.

At one time, many astronomers assumed that Earth remains moti

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At one time, many astronomers assumed that Earth remains moti

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