In non-scientific terms the Autumnal Equinox marks the start of Autumn.
Equinoxes occur twice a year, in Spring and Autumn around 21 March and 23 September.
These are the only times during the whole duration of the year when the Earth is not titled at a 23.5 degree angle away from the plane of the ecliptic resulting in equal day and night on both sides of the Earth's hemisphere, hence the word 'equinox' meaning 'equal night'.
The illustration show Earth's orbit around the sun and the beginnings of the seasons
As well as being an important astronomical event the beginning of Autumn also means we get to take out our wooly jumpers, start thinking about our Christmas festivities and more importantly enjoy the seasonal bloom of autumnal colours around us.