The vernal (spring) equinox occurred this morning at 05.14 GMT. But what actually is the vernal equinox and why does it happen?
During the course of a year the Earth completes one orbit around the Sun. From our perspective, we see this as the Sun moving through the year against the background of stars, along an imaginary line which we call the ecliptic. As the Sun moves (or appears to move) along this path, for half the year it’s seen to be above the celestial equator, which is the projection of the Earth’s equator onto the sky – this is summer in the northern hemisphere. For the other half-year it’s below this equator (northern winter).
So the Sun’s path appears to cross the celestial equator twice a year, in March and September. These times are the vernal and autumnal equinoxes – so called because at these times, day and night are of nearly equal length at all latitudes (equinox means ‘equal night’).
Find out more about equinoxes and solstices in our fact file, including why the equinoxes don’t always occur on the same day and why they don’t occur at the times when day and night are exactly equal.
See also Has spring started yet?