The equinoxes, when day and night are nearly equal all over the world, fall on about:

On the equinoxes, the Sun is directly overhead at the Equator at noon, and day and night are nearly equal (about 12 hours each) everywhere on the Earth.

Meaning of the word:

"Equinox" comes from the Latin "aequus" (equal) and "nox" (night).

The two equinoxes:

March equinox (around 20 or 21 March): called the vernal (spring) equinox in the Northern Hemisphere. After this date, the Sun's direct rays move into the Northern Hemisphere.

September equinox (around 22 or 23 September): called the autumnal equinox in the Northern Hemisphere. After this date, the Sun's direct rays move into the Southern Hemisphere.

In the Southern Hemisphere, the names are reversed: the March equinox marks autumn and the September equinox marks spring.

Why equinoxes happen:

• The Earth's axis is tilted relative to its orbit.
• As the Earth goes around the Sun, the hemisphere tilted towards the Sun changes.
• On the equinoxes, neither hemisphere is tilted towards the Sun, so the circle of illumination passes through both poles and divides every latitude into equal halves of day and night.

"Equal" day and night is approximate:

Because of the bending of sunlight by the atmosphere and because sunrise is counted from the top edge of the Sun, day is actually a few minutes longer than night on the equinox.

Equinoxes and festivals:

• Many cultures celebrate the March equinox; for example, Nowruz, the Persian New Year, falls on it.
• The Sun rises due east and sets due west on the equinoxes.

Why not the other options?

• ✗3 January and 4 July are around perihelion and aphelion.
• ✗15 June, 15 December, 1 May and 1 November are not equinox dates.