UPSC Prelims 2005 · General Studies

Question 83: Geography

Assertion (A): The same face of the Moon is always presented to the Earth. Reason (R): The Moon rotates about its own axis in 23½ days which is about the same time that it takes to orbit the Earth.

aBoth A and R are individually true and R is the correct explanation of A

bBoth A and R are individually true but R is not the correct explanation of A

cA is true but R is false

dA is false but R is true

AnswerThe answer is (c).

The Moon always shows the same face to Earth because its rotation period equals its orbital period, about 27.3 days (tidal locking), so A is true. R is false: the Moon rotates in about 27⅓ days, not 23½ days; 23½ degrees is the tilt of Earth's axis, which is probably the source of the confusion.

A. The Moon is tidally locked, so one face always points at Earth.
R. Its rotation takes about 27.3 days, not 23½ days.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong

(a) R is false, so it cannot explain A.
(b) R is false (about 27.3 days, not 23½).
(c) Correct: A is true, R is false.
(d) A is true, so this is wrong.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2005, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.