Correct Option
The correct option is 3 only
Explanation
The revolution of the Earth around the Sun, combined with the tilt of the Earth's axis, results in the apparent movement of the Sun between the Tropics. This cyclic movement creates specific astronomical events known as Solstices and Equinoxes, which define the seasons.
Statement-wise Analysis
- Pair 1 is Incorrect. 21st June is known as the Summer Solstice in the Northern Hemisphere. On this date, the Sun is vertically overhead at the Tropic of Cancer ($23.5^\circ$ N), resulting in the longest day of the year for the Northern Hemisphere. The Winter Solstice occurs on this date only in the Southern Hemisphere.
- Pair 2 is Incorrect. 22nd December is known as the Winter Solstice in the Northern Hemisphere. On this date, the Sun is vertically overhead at the Tropic of Capricorn ($23.5^\circ$ S), resulting in the shortest day of the year for the Northern Hemisphere.
- Pair 3 is Correct. 21st March is the Vernal (Spring) Equinox. On this date (and on 23rd September), the Sun is directly overhead at the Equator. This results in equal durations of day and night across the entire globe.
In general geographical conventions (and specifically for UPSC), unless the Southern Hemisphere is explicitly mentioned, seasonal terminology refers to the Northern Hemisphere.
Key Takeaway
21st June is the Summer Solstice, 22nd December is the Winter Solstice, and 21st March and 23rd September are the Equinoxes.