In a group discussion on shape and size of the Earth, three students stated the following points: 1. Student 1 : The shape of the Earth is basically an oblate spheroid. 2. Student 2 : The polar diameter of the Earth is more than the equatorial diameter. 3. Student 3 : Bulge along the equatorial region is due to revolution of the Earth. Who among the above students is/are correct?
- (a)Student 1 only
- (b)Student 1 and Student 2 only
- (c)Student 2 and Student 3 only
- (d)Student 1, Student 2 and Student 3
Answer
Why
Correct — A, Student 1 only. The Earth is basically an oblate spheroid — flattened at the poles and bulging at the equator — so Student 1 is right. Student 2 is wrong because the polar diameter (about 12,714 km) is smaller, not larger, than the equatorial diameter (about 12,756 km). Student 3 is wrong because the equatorial bulge is caused by the Earth's rotation on its axis, not its revolution around the Sun. Hence only Student 1 is correct.
Why the others are wrong
- (b)Student 1 and Student 2 only — Student 2 is incorrect — the polar diameter is shorter than the equatorial diameter, so this pairing is wrong.
- (c)Student 2 and Student 3 only — Both of these statements are false — the polar diameter is smaller, and the bulge is due to rotation, not revolution.
- (d)Student 1, Student 2 and Student 3 — Statements 2 and 3 are false, so not all three students can be correct.
Concept
The Earth is not a perfect sphere but an oblate spheroid (a geoid): it is slightly flattened at the poles and bulges outward at the equator. This shape arises from the centrifugal effect of the Earth's daily rotation on its axis, which throws material outward most at the equator.
Two common errors are tested here: thinking the poles are 'longer' (they are actually flattened, so the polar diameter is smaller) and attributing the bulge to revolution (the yearly orbit) rather than rotation (the daily spin). Getting both facts straight leaves only Student 1 correct.
Key facts
- The Earth's shape is an oblate spheroid — flattened at the poles, bulging at the equator.
- Equatorial diameter (~12,756 km) is greater than polar diameter (~12,714 km).
- The equatorial bulge is caused by the Earth's rotation (daily spin), not revolution.
- 'Revolution' is the yearly orbit around the Sun; 'rotation' is the daily spin on the axis.
Only Student 1's statement is correct — the Earth is an oblate spheroid.
Study next
Common traps
- Assuming the polar diameter is larger than the equatorial diameter.
- Blaming the equatorial bulge on revolution instead of rotation.
A statement-evaluation question mixing one true fact (oblate spheroid) with two classic errors.
Related PYQs
Which one of the following is the longest parallel of latitude?
- (a) Tropic of Cancer
- (b) Tropic of Capricorn
- (c) Arctic Circle
- (d) Equator
Answer(d) Equator
Same underlying fact — the Earth's equatorial bulge. That NDA item asks which parallel is longest (the Equator, because the Earth bulges there); this one asks why that bulge exists and what shape it gives the Earth.
Practice
- practice — not a real PYQ
The equatorial bulge of the Earth is primarily a result of the Earth's
- (a)revolution around the Sun
- (b)rotation on its axis
- (c)gravitational pull of the Moon
- (d)tilt of the axis
Answer(b) rotation on its axis — the daily spin produces the equatorial bulge. - practice — not a real PYQ
Which of the following correctly describes the Earth's dimensions?
- (a)Polar diameter is greater than equatorial diameter
- (b)Equatorial diameter is greater than polar diameter
- (c)Both diameters are exactly equal
- (d)The Earth is a perfect sphere
Answer(b) Equatorial diameter is greater than polar diameter — the Earth is an oblate spheroid.