LSAT Explanation PT 22, S4, Q5: Astronaut: Any moon, by definition, orbits

LSAT Question Stem

The astronaut's conclusion follows logically if which one of the following is assumed? 

Logical Reasoning Question Type

This is a Sufficient Assumption question. 

Correct Answer

The correct answer to this question is C. 

LSAT Question Complete Explanation

This LSAT problem is a Sufficient Assumption question. The passage presents an argument made by an astronaut stating that since any moon orbits some planet in a solar system, all the moons in solar system S4 orbit the planet Alpha. We are asked to find an assumption that, if true, would make the astronaut's conclusion logically follow.

Let's break down the argument:

Premise: Any moon orbits some planet in a solar system.

Conclusion: The moons in solar system S4 all orbit the planet Alpha.

To help you understand this argument, think of a simple example: Imagine a school where every student must be in a club. If we know that all students in the school are in the Chess Club, then we can conclude that every student is in the Chess Club.

Now, let's analyze the answer choices:

a) There is only one moon in S4.

This answer choice does not guarantee the conclusion. Even if there is only one moon in S4, it could still orbit a different planet, such as Beta.

b) Every moon in S4 orbits the same planet.

This answer choice also does not guarantee the conclusion. For example, there could be two moons (Moon I and Moon II) and two planets (Alpha and Beta) in S4. Both moons could orbit planet Beta, which is consistent with this answer choice but does not prove the conclusion.

c) Alpha is the only planet in S4.

This is the correct answer choice. If Alpha is the only planet in S4, then it must be the case that every moon in S4 orbits Alpha, as there are no other planets for them to orbit.

d) Every planet in S4 is orbited by more than one moon.

This answer choice does not guarantee the conclusion. For instance, there could be four moons (I, II, III, and IV) and two planets (Alpha and Beta) in S4. Alpha is orbited by Moon I and Moon II, while Beta is orbited by Moon III and Moon IV. This scenario is consistent with the premises and this answer choice, but it does not prove the conclusion.

e) There is at least one moon that orbits Alpha.

This answer choice also does not guarantee the conclusion. For example, there could be two moons (I and II) and two planets (Alpha and Beta) in S4. Alpha is orbited by Moon I, and Beta is orbited by Moon II. This scenario is consistent with the premises and this answer choice, but it does not prove the conclusion.

In summary, the correct answer choice is (c) because if Alpha is the only planet in S4, then it must be the case that every moon in S4 orbits Alpha, making the astronaut's conclusion logically follow.