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Earth
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Q.1
WBCS Prelims 2021
The ratio of the diameters of moon and earth is nearly
A. 1 : 2
B. 1 : 5
C. 1 : 4
D. 1 : 8
Explanation
Core Formula/Logic: The moon's diameter is about 27.3% of Earth's diameter, which converts to a ratio of approximately 1:4.
Step-by-Step Solution: 1. Earth's diameter ≈ 12,742 km. Moon's diameter ≈ 3,474 km. 2. Ratio = Moon diameter ÷ Earth diameter = 3,474 ÷ 12,742 ≈ 0.2727. 3. Convert to ratio: 0.2727 ≈ 1/3.67, closest to 1:4 among given options.
Common Pitfall: Confusing diameter ratio with mass or volume ratio gives 1:2 or 1:8. Misreading 27.3% as 1/3.7 and rounding to 1:5.
Shortcut/Takeaway: Moon's diameter is roughly 1/4 of Earth's. Remember: 27% ≈ 1/4 for quick ratio estimation.
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Q.2
WBCS Prelims 2015
The difference in time between Kolkata and London is
A. Kolkata is more than 5 hours ahead
B. Kolkata is more than 3 hours ahead
C. Kolkata is 15 minutes ahead
D. Kolkata is 10 hours behind
Explanation
Why Correct: Kolkata follows Indian Standard Time (IST) at UTC+5:30, while London follows Greenwich Mean Time (GMT) at UTC+0. The standard time difference is 5 hours 30 minutes ahead for Kolkata, making it "more than 5 hours ahead."
Distractor Analysis: More than 3 hours ahead underestimates the actual 5.5-hour difference. 15 minutes ahead applies only to locations within narrow longitudinal bands, not intercontinental differences. 10 hours behind would place Kolkata west of London, which is geographically impossible since Kolkata is east of London.
Takeaway: India's single time zone (IST) uses the 82°30'E meridian as reference, creating a +5:30 offset from UTC that remains constant throughout the year, unlike some countries with daylight saving adjustments.
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Q.3
WBCS Prelims 2013
On a map of the world some lines are drawn joining places of equal dip angles. These lines are known as –
A. Isogonic lines
B. Isoclinic lines
C. Agonic lines
D. Isothermal lines
Explanation
Why Correct: Isoclinic lines connect points with identical magnetic dip angles, which measure the vertical tilt of Earth's magnetic field.
Distractor Analysis: Isogonic lines connect equal magnetic declination values, agonic lines mark zero declination points, and isothermal lines connect locations with the same temperature.
Takeaway: Magnetic declination is the horizontal angle between true north and magnetic north, crucial for navigation.
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Q.4
WBCS Prelims 2011
The latitude of a place is expressed by its angular distance in relation to
A. South Pole
B. North Pole
C. Equator
D. Axis of the Earth
Explanation
Why Correct: Latitude measures angular distance north or south of the equator, ranging from 0° at the equator to 90° at the poles.
Distractor Analysis: The South Pole and North Pole are endpoints of latitude lines, not the reference plane. The axis of the Earth defines longitude, which measures angular distance east or west of the Prime Meridian.
Takeaway: Important latitude lines include the Tropic of Cancer (23.5°N), Tropic of Capricorn (23.5°S), Arctic Circle (66.5°N), and Antarctic Circle (66.5°S).
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Q.5
WBCS Prelims 2007
Map of India is best suited to_________ projection.
A. Polyconic
B. Lambert Conical Orthomorphic
C. Albert’s Conical Equal Area
D. Sinusoidal
Explanation
Why Correct: The Lambert Conical Orthomorphic projection preserves shapes and directions accurately, making it ideal for mid-latitude countries like India with a north-south extent.
Distractor Analysis: Polyconic projections suit large-scale topographic maps, not small-scale national maps. Albert’s Conical Equal Area preserves area but distorts shapes, used for thematic maps. Sinusoidal projections are equal-area projections best for global or continental mapping.
Takeaway: For countries with significant east-west extent (like Russia), cylindrical projections are preferred, while conical projections are standard for mid-latitude countries.
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Q.6
WBCS Prelims 2001
There is no local time at
A. London
B. North and South Poles
C. International Date Line
D. Magnetic Poles of Earth
Explanation
Why Correct: All longitude lines converge at the geographic poles, making every time zone meet there simultaneously, so local time becomes undefined.
Distractor Analysis: London observes Greenwich Mean Time (GMT) or British Summer Time (BST). The International Date Line marks where calendar days change but has defined time zones. Magnetic poles indicate Earth's magnetic field orientation but have no relation to time measurement.
Takeaway: Research stations at poles typically use the time zone of their supplying country or Coordinated Universal Time (UTC) for coordination.
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Q.7
WBCS Prelims 2001
The Earth's magnetic field
A. never changes
B. has reversed itself many times
C. is centred exactly at the Earth's centre
D. originates in a permanently magnetized iron core
Explanation
Why Correct: Paleomagnetic records show the Earth's magnetic field has reversed polarity at least 183 times in the last 83 million years, with the last reversal occurring 780,000 years ago.
Distractor Analysis: The magnetic field constantly changes in strength and orientation, drifting westward at 0.2° per year. The magnetic dipole is offset about 500 km from Earth's geometric centre. The field originates from convection currents in the outer liquid iron core, not from permanent magnetization.
Takeaway: The magnetic north pole currently moves about 55 km annually toward Siberia, requiring frequent updates to navigation systems.
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Q.8
WBCS Prelims 2000
Period of revolution is exactly 12 years for
A. Mars
B. Jupiter
C. Saturn
D. Uranus
Explanation
Why Correct: Jupiter's orbital period around the Sun is 11.86 Earth years, which rounds to exactly 12 years for most exam purposes.
Distractor Analysis: Mars completes one revolution in 687 Earth days (1.88 years). Saturn takes 29.5 Earth years for one orbit. Uranus requires 84 Earth years for a full revolution.
Takeaway: Planetary revolution periods follow Kepler's third law: T2 ∝ R3, where T is orbital period and R is average distance from the Sun.
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