Correct Option
The correct option is21st March and 23rd September
Explanation
The position of the Sun relative to the Earth changes throughout the year due to the Earth's revolution and the tilt of its axis. This results in the migration of the Sun's vertical rays between the Tropic of Cancer and the Tropic of Capricorn. The specific positions where the Sun is vertically overhead at the equator are termed Equinoxes.
Detailed Analysis
- Equinoxes (21st March and 23rd September): On these two days, the direct rays of the Sun fall vertically on the equator. During this position, neither of the poles is tilted towards the Sun, and the circle of illumination passes through both poles. Consequently, the whole earth experiences equal days and equal nights.
- 21st March: Known as the Vernal (Spring) Equinox in the Northern Hemisphere.
- 23rd September: Known as the Autumnal Equinox in the Northern Hemisphere.
- Summer Solstice (21st June): The Sun's rays fall vertically on the Tropic of Cancer (23.5° N). The Northern Hemisphere experiences its longest day.
- Winter Solstice (22nd December): The Sun's rays fall vertically on the Tropic of Capricorn (23.5° S). The Southern Hemisphere experiences its longest day.
Therefore, the dates corresponding to the direct rays falling on the equator are 21st March and 23rd September.
Key Takeaway
Equinoxes occur on 21st March and 23rd September when the Sun is vertically overhead at the equator, resulting in equal day and night duration globally.