Logical Reasoning

PT106 · S1 · Q14 The axis of Earth’s daily rotation

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The axis of Earth’s daily rotation is tilted with respect to the plane of its orbit at an angle of roughly 23 degrees.

Statements

Earth's axis is tilted about 23 degrees from the plane of its orbit.

Conditional (only by) Stable axis tilt → gravitational influence of a large, nearby moon

Conditional (without) Planet can support life → stable, moderate axis tilt

Mars has only small moons, tilts at wildly fluctuating angles, and cannot support life.

Evaluate

Two rules share the term "stable tilt," so they chain: supporting life requires a stable tilt, and a stable tilt requires a large nearby moon. Life → stable tilt → large moon. Mars is the worked example: small moons, unstable tilt, no life, exactly as the chain predicts.

The contrapositive is where the guaranteed inferences live. No large nearby moon → no stable tilt → cannot support life. Take away a planet's large moon and the chain says its climate can no longer support life. What the rules do not say is that a large moon or a stable tilt is enough for life; they are requirements, and a planet could meet both and still be lifeless for other reasons. The statements also say nothing about other gravitational influences or about planets with several moons.

Goal

Find the claim that runs the chain in the guaranteed direction: losing the moon means losing the stable tilt, which means losing the ability to support life.

14.

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

  1. If Mars had a sufficiently

    Reversal / Negation

    A large moon is required for a stable tilt, and a stable tilt is required for life, but neither is guaranteed to produce life. Mars with a big moon might gain a stable tilt and still be unable to support life for other reasons. This reads the requirements as guarantees.

    4% picked this

  2. Correct

    If Earth’s Moon were to

    Why this is right

    Earth's tilt can be kept stable only by the Moon's gravity. If the Moon left orbit, the tilt would no longer be stable. Without a stable, moderate tilt, a planet's climate cannot support life. So Earth's climate would be unable to support life. This is the contrapositive of the chain, read in the guaranteed direction.

    Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.

    86% picked this

  3. Any planet with a stable,

    Reversal / Negation

    A stable, moderate tilt is something a life-supporting planet must have, not something that by itself makes a planet able to support life. "Any planet with a stable tilt can support life" turns the requirement around.

    6% picked this

  4. Gravitational influences other than moons

    Unsupported

    The statements say a large moon is needed to keep the tilt stable. They do not say other gravitational influences have no effect on the tilt. The Sun or other planets could affect it while the Moon still does the stabilizing.

    3% picked this

  5. No planet that has more

    Unsupported

    Mars has small moons and lacks a stable tilt, but the statements say nothing about planets with multiple moons in general. A planet with several moons, one of them large, could satisfy every requirement given.

    1% picked this

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