UPSC Prelims 2024 Q15: June 21 Day Length by Latitude — Explained
On June 21 every year, which of the following latitude(s) experience(s) a sunlight of more than 12 hours?
1. Equator
2. Tropic of Cancer
3. Tropic of Capricorn
4. Arctic Circle
Select the correct answer using the code given below:
- 1 only
- 2 only
- 3 and 4
- 2 and 4
Answer: (d) 2 and 4
Statement-wise explanation
- 1: incorrect — The Equator always cuts the circle of illumination into two equal halves, so it gets about 12 hours of day and 12 hours of night all year, including June 21. That is not 'more than 12 hours' in the textbook sense. Atmospheric refraction adds a few minutes to the day everywhere, but UPSC and NCERT treat the Equator's day as 12 hours.
- 2: correct — On June 21 the Sun is overhead at the Tropic of Cancer (23°30'N), and the whole Northern Hemisphere has days longer than nights. The Tropic of Cancer gets about 13 hours 35 minutes of daylight on this date.
- 3: incorrect — The Tropic of Capricorn (23°30'S) is in the Southern Hemisphere, which has its winter solstice on June 21. Its day is shorter than its night, at about 10 hours 41 minutes of daylight.
- 4: correct — The Arctic Circle (66°30'N) is the southernmost parallel where the Sun stays above the horizon for a full day at the June solstice. It gets 24 hours of daylight on June 21, which is far more than 12 hours. Every latitude north of it has continuous daylight for longer, up to six months at the North Pole.
How to eliminate
You need only one fact: on June 21 the North Pole leans towards the Sun, so the Northern Hemisphere has days longer than 12 hours and the Southern Hemisphere has days shorter than 12 hours. That settles two statements straight away. The Tropic of Cancer and the Arctic Circle are both in the Northern Hemisphere, so both are in. The Tropic of Capricorn is in the Southern Hemisphere, so it is out. That removes the option pairing statements 3 and 4. The Equator gets 12 hours of daylight on every day of the year, so 'more than 12 hours' does not fit it. Once you see that two northern latitudes qualify, every option that names only one statement is also out. The question is built on hemispheres, so ask which side of the Equator each line lies on and which hemisphere is having summer.
Know more
Why day length changes
The Earth's axis is tilted about 23½° from the perpendicular to its orbit, and it points the same way all year. As the Earth goes round the Sun, the circle of illumination (the line between day and night) cuts each parallel into unequal day and night parts. The Equator is the only parallel it always cuts in half.
June 21: summer solstice (Northern Hemisphere)
- The Sun's rays fall vertically on the Tropic of Cancer.
- The North Pole tilts towards the Sun, so the day gets longer the further north you go.
- The area inside the Arctic Circle gets continuous daylight, and the area inside the Antarctic Circle gets continuous night.
- Seasons are reversed in the Southern Hemisphere. December 22 is its summer solstice, when the Sun is overhead at the Tropic of Capricorn.
| Latitude | Daylight on June 21 (approx.) |
|---|---|
| North Pole (90°N) | 24 h (six months of day) |
| Arctic Circle (66½°N) | 24 h |
| Tropic of Cancer (23½°N) | ~13 h 35 min |
| Equator (0°) | ~12 h |
| Tropic of Capricorn (23½°S) | ~10 h 41 min |
| Antarctic Circle (66½°S) | 0 h (Sun does not rise) |
Equinoxes (March 21, September 23)
Neither pole tilts towards the Sun. The Sun is overhead at the Equator, and every latitude gets about 12 hours of day and 12 hours of night.
Exam angles to remember
- Day length depends on the hemisphere and the distance from the Equator.
- Insolation also depends on day length. This partly explains why summer insolation at higher latitudes can be high even though the Sun's rays are slanting.
- The Earth is near aphelion (farthest from the Sun) in early July. So northern summer is caused by the tilt, not by the Earth's distance from the Sun.
NCERT Concepts to Revise
- Summer solstice: The question's date, June 21, is the Northern Hemisphere's summer solstice. It decides which hemisphere has longer days.
- Tropic of Cancer: Statement 2 names this parallel. Its position at 23°30'N tells you which hemisphere's seasons it follows.
- Inclination of the Earth's axis: The tilt of the Earth's axis is why day length on June 21 differs between the Equator, the two Tropics and the Arctic Circle.