Question

the days and nights are equal on 21 March every year

Answer 1

I think it is your answer

Answer 2

Yeah! The days and nights are equal on 21 March because the sun will on Thursday cross the plane of the earth's equator making day and night of approximately equal duration. The event known as equinox is eagerly awaited by space enthusiasts as it takes place twice a year on March 21 and September 23.

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