The day in which the length of day and night are equal everywhere on Earth is

Analyze the question: The question asks for the date when day and night are equal in length everywhere on Earth. This is called an equinox.

Explain the concept:

• The Earth's axis is tilted at about 23½° to its orbital plane.
• Because of this tilt, the length of day and night changes through the year.
• On an equinox, the Sun's vertical rays fall directly on the Equator. Both hemispheres receive equal sunlight, so day and night are each about 12 hours everywhere.
• There are two equinoxes each year: • 21st March (March or Spring equinox in the Northern Hemisphere) • 23rd September (September or Autumn equinox in the Northern Hemisphere)

Check each option:

• A. 21st June: This is the June solstice. The Sun is overhead at the Tropic of Cancer (23½°N). The Northern Hemisphere has its longest day, so day and night are unequal.
• B. 30th August: This is not a special date in the Earth's yearly cycle.
• C. 21st March: This is the equinox. The Sun is overhead at the Equator, so day and night are equal. ✔
• D. 22nd December: This is the December solstice. The Sun is overhead at the Tropic of Capricorn (23½°S). The Southern Hemisphere has its longest day, so day and night are unequal.

Answer: The correct option is C. 21st March.

Exam tip: Remember that "equinox" means "equal night." If you see an equinox question, look for 21st March or 23rd September. If you see "solstice," look for 21st June or 22nd December.