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Q31 (IAS/2005) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Astronomy and astrophysics Answer Verified

Assertion (A): The same face of the Moon is always presented to the Earth.
Reason (R): The Moon rotates about its own axis in 27.3 days which is about the same time that it takes to orbit the Earth.

Result
Your answer: —  ·  Correct: C
Explanation

Assertion (A) is essentially correct: the Moon presents nearly the same face to Earth because it is tidally locked — its rotation period matches its orbital period, a state called synchronous rotation [3]. However, the Reason (R) is false. The Moon does not rotate in 23 days; its rotation (sidereal) period is about 27.3 days, the same as its orbital period around Earth, which is why the same hemisphere is usually visible from Earth [3]. Small apparent “rocking” (libration) occurs because the Moon’s orbit is slightly elliptical and inclined, so we actually see a bit more than half the surface over time, but that does not validate the 23‑day claim [4].

Sources

  1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > Phases of The Moon > p. 257
  2. [3] https://svs.gsfc.nasa.gov/4709/
  3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > Sidereal Period > p. 260
  4. [4] https://www.astronomy.ohio-state.edu/pogge.1/Ast161/Unit2/phases.html
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