Statement (A) : Only one side of the moon is visible to us on Earth. Statement…
2019
Statement (A) : Only one side of the moon is visible to us on Earth.
Statement (B) : The moon moves around the Earth in about 27 days and takes approximately the same time to complete 1 spin.
Select the correct option from the given alternatives.
- A.
Both (A) and (B) are true and (B) is the correct explanation of (A).
- B.
Both (A) and (B) are true, but (B) is not the correct explanation of (A).
- C.
(A) is true, but (B) is false.
- D.
(A) is false, but (B) is true.
Show answer & explanation
Correct answer: A
Synchronous rotation (tidal locking) occurs when a body's axial rotation period exactly equals its orbital period around the body it revolves around; the direct consequence is that essentially the same hemisphere continues to face the object being orbited, while the far hemisphere stays effectively hidden from there.
The Moon's sidereal orbital period around Earth and its axial rotation period are both close to 27.3 days -- a case of tidal locking produced by Earth's gravity acting on the Moon over billions of years. Because these two periods are nearly equal, the Moon presents essentially the same face to Earth at every point in its orbit; this closely matches Statement (A) (only one lunar hemisphere is ever visible from Earth) and confirms that the near-equal-period fact stated in Statement (B) is the mechanism that produces it -- Statement (B) is therefore the correct explanation of Statement (A).
This is independently confirmed by observation -- humanity had never seen the Moon's far side until a spacecraft flew around it and photographed it in 1959, which would be impossible if the visible face changed substantially over time (a minor wobble called libration lets Earth-based observers glimpse a limited sliver beyond the mean near-side edge over a month, but the far-side hemisphere as a whole remains unseen from Earth). If the Moon's rotation and orbital periods were NOT nearly equal, the visible face would shift noticeably over time, as happens for many other rotating bodies observed from a distance -- the near-equality described in (B) is precisely what rules that out.
Both statements true but (B) not the explanation: wrongly treats the roughly 27.3-day period match as an unrelated coincidence rather than the actual tidal-locking mechanism behind the fixed-face phenomenon.
(A) true, (B) false: incorrectly assumes the Moon's spin period and orbital period differ; precise tracking shows both average about 27.3 days.
(A) false, (B) true: incorrectly assumes different lunar hemispheres become visible from Earth over time; continuous observation confirms the same broad hemisphere remains visible, aside from minor peripheral shifts.
Hence, both statements are true and (B) correctly explains (A).