It happens on 21st March and 23rd September.
Explanation
An Equinox is an astronomical event where the Sun is positioned vertically overhead at the Equator. During this phenomenon, the circle of illumination passes directly through both the North and South Poles, resulting in an equal duration of day and night across the entire globe.Option Analysis:
- It happens only once a year. is incorrect: The Equinox occurs twice a year, marking the change of seasons.
- It happens when the Sun is at the Tropic of Cancer. is incorrect: When the Sun is vertically overhead at the Tropic of Cancer (23.5° N), it is known as the Summer Solstice (typically June 21st), not the Equinox.
- It happens on 21st March and 23rd September. is correct: The Equinoxes occur on approximately 21st March (Vernal or Spring Equinox) and 23rd September (Autumnal Equinox). On these dates, the Sun’s rays fall vertically on the Equator.
- It results in the longest day in the Northern Hemisphere. is incorrect: The longest day in the Northern Hemisphere occurs during the Summer Solstice. During an Equinox, day and night are of approximately equal duration (12 hours each) everywhere on Earth.
Key Takeaway: The term "Equinox" translates to "equal night." It occurs when the Sun is directly over the Equator, happening twice annually on March 21st and September 23rd.