The Universe and the Earth — Practice Questions
UPSC Prelims-pattern practice questions on the Universe and the Earth — solar system, eclipses, tropics, the International Date Line and time calculations.
Questions
These are original UPSC-pattern practice questions. Attempt each one before revealing the answer.
Consider the following statements:
- The Moon always shows the same face to the Earth because its periods of rotation and revolution are equal.
- A lunar eclipse can occur only on a full moon day.
- A solar eclipse can occur only on a full moon day.
Which of the statements given above are correct?
Solution
- Statement 1 — correct. The Moon’s rotation and revolution both take about 27.3 days (synchronous rotation), so one face always points towards the Earth.
- Statement 2 — correct. A lunar eclipse needs the Earth between the Sun and the Moon — the full-moon alignment.
- Statement 3 — wrong. A solar eclipse needs the Moon between the Sun and the Earth — the new-moon alignment.
Eclipses do not happen every month because the Moon’s orbit is inclined to the Earth’s orbital plane; the three bodies line up only when the Moon is near one of the nodes.
Answer: A
Consider the following statements about the solar system:
- Venus rotates from east to west.
- Uranus is the hottest planet of the solar system.
- Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006.
Which of the statements given above is/are correct?
Solution
- Statement 1 — correct. Venus has retrograde rotation; on Venus the Sun would rise in the west.
- Statement 2 — wrong. Venus is the hottest planet, though Mercury is closer to the Sun — its dense carbon dioxide atmosphere traps heat. Uranus is famous instead for its extreme axial tilt (about 98°).
- Statement 3 — correct. The IAU’s August 2006 definition requires a planet to have cleared its orbital neighbourhood; Pluto has not.
Answer: C
How many of the following countries are crossed by the Tropic of Capricorn?
- Brazil
- Kenya
- Indonesia
- Botswana
Solution
The Tropic of Capricorn crosses Brazil and Botswana (along with Chile, Argentina, Paraguay, Namibia, South Africa, Mozambique, Madagascar and Australia).
Kenya and Indonesia are both crossed by the Equator, not the Tropic of Capricorn — a common mix-up when the question lists equatorial countries. So two.
Answer: B
A ship sailing eastward from Japan towards Hawaii crosses the International Date Line on a Tuesday. Immediately after crossing the line, the date on board will be:
Solution
Moving east, a traveller keeps putting clocks forward — one hour for every 15°. The International Date Line cancels the extra day gained: crossing it eastward sets the date back by one day. Tuesday becomes Monday.
Option A (Wednesday) is what happens on a westward crossing, from the Americas towards Asia.
Answer: D
At the same instant, the local time at place P is 6:00 a.m. and at place Q is 8:40 a.m. Which one of the following is correct?
Solution
- Time difference = 2 hours 40 minutes = 160 minutes.
- 1° of longitude = 4 minutes, so the longitudinal difference = 160 ÷ 4 = 40°.
- Q is ahead in time, so it lies to the east (the Sun reaches eastern places first).
Option C confuses minutes with degrees; option D uses the wrong conversion; option B reverses the direction.
Answer: A
Mains
Explain how the shape and the motions of the Earth together determine the distribution of insolation over its surface.
Solution
Introduction: Insolation — incoming solar radiation — is not spread evenly over the Earth. Its distribution is set largely by the Earth’s spherical shape, its rotation, and its revolution with a tilted axis.
Body:
- Shape: on a sphere, the Sun’s rays strike the Equator nearly vertically and the poles very obliquely; oblique rays spread the same energy over a larger area and pass through more atmosphere. Hence insolation decreases from the Equator towards the poles.
- Rotation: produces day and night, so insolation is received only during daylight hours; it also sets up the daily cycle of heating and cooling.
- Revolution with a tilted, parallel axis: moves the overhead Sun between the Tropics of Cancer and Capricorn, changing both the angle of rays and the length of day at each latitude through the year — the cause of seasons.
- Distance: the elliptical orbit (perihelion ≈ 3 January, aphelion ≈ 4 July) makes only a small difference.
- Result: the torrid, temperate and frigid heat zones, and the seasonal shift of pressure and wind belts.
Conclusion: Shape fixes the latitudinal gradient of insolation; tilt and revolution make that gradient shift with the seasons — the starting point for understanding world climate.