LSAT Explanation PT 28, S1, Q14: The axis of Earth's daily rotation

LSAT Question Stem

If the statements above are true, which one of the following must also be true on the basis of them? 

Logical Reasoning Question Type

This is a Must Be True question. 

Correct Answer

The correct answer to this question is B. 

LSAT Question Complete Explanation

Let's first analyze the passage and break it down into simpler terms. The passage states that Earth's daily rotation is tilted at an angle of about 23 degrees, and this angle is kept stable due to the gravitational influence of Earth's large Moon. A stable and moderate axis tilt is necessary for a planet's climate to be suitable for life. Mars, on the other hand, has small moons and fluctuating tilt angles, making it unable to support life.

To better understand this, let's use a simple example. Imagine a spinning top that represents Earth. The top is tilted at a certain angle, and this angle is maintained by a magnet (the Moon) that keeps it stable. If the magnet were removed, the top would wobble and become unstable, making it unsuitable for life.

Now, let's represent the passage's logic using formal logic diagrams:

AS = angle stable

GI = gravitational influence of Earth's large, nearby Moon

PCE = planet's climate too extreme and unstable to support life

Sentence 2: AS -> GI

Sentence 3: PCE -> /AS

Chain: PCE -> /AS -> /GI

Now, let's analyze each answer choice:

a) If Mars had a sufficiently large nearby moon, Mars would be able to support life.

This answer choice is incorrect because it is the Mistaken Reversal of the chain inference. The diagram for this answer choice would be:

GI (Mars) -> PCE (Mars)

b) If Earth's Moon were to leave Earth's orbit, Earth's climate would be unable to support life.

This is the correct answer choice because it represents the contrapositive of the chain inference.

c) Any planet with a stable, moderate axis tilt can support life.

This answer choice is incorrect because it is the Mistaken Reversal of the third sentence. The diagram for this answer choice would be:

AS -> PCE

d) Gravitational influences other than moons have little or no effect on the magnitude of the tilt angle of either Earth's or Mars's axis.

This answer choice is incorrect because it discusses issues not raised in the passage.

e) No planet that has more than one moon can support life.

This answer choice is incorrect because it contradicts the information in the passage. The passage states that a large moon is necessary for a stable angle, which is necessary for a climate that can support life. It is possible that a planet with more than one moon could have a stable angle as long as at least one of the moons is of sufficient size. Therefore, a planet can have more than one moon and support life, making this answer choice incorrect.

In conclusion, the correct answer is choice (b): If Earth's Moon were to leave Earth's orbit, Earth's climate would be unable to support life.