An equinox is commonly regarded as the moment the plane ( extended indefinitely in all directions ) of Earth 's equator passes through the center of the Sun around 21 March and 22 - 23 September .
The Sun occurs twice each year .
An equinox is commonly regarded as the moment the plane ( extended indefinitely in all directions ) of Earth 's equator passes through the center of the Sun , which occurs twice each year , around 21 March and 22 - 23 September .