one year. the vernal equinox is on March 21, or the first day of spring. an equinox is when day and night is equal (12 hours daylight, 12 hours nighttime). there is a Vernal equinox in spring, and an autumnal equinox on the first day of autumn (or fall), September 21.
The adjectives vernal and autumnal mean "of spring" and "of autumn (fall)" respectively.
The equinox, when the day and night are even, occurs twice a year. Once in spring and once in autumn (fall)...
A Vernal Equinox is the time when the sun crosses the plain of the earths equator, making night and day of equal length all over Earth and occurring about March 21st.
