The vernal - or spring - equinox marks the start of spring in
the northern hemisphere. This year - 2012 - the spring equinox was
on March 20.
Why does the equinox happen?
The equinoxes - and our seasons - are caused by the Earth
orbiting the Sun and the Earth's tilt.
The Earth is tilted 23.5 degrees on its vertical axis. During
the northern hemisphere's spring and summer, the northern
hemisphere is tilted towards the Sun.
During the northern hemisphere's autumn and winter, the northern
hemisphere is tilted away from the Sun.
During the spring and autumn equinoxes, the Earth is neither
tilted towards or away from the Sun.
This means that both the northern and southern hemispheres
receive equal sunlight. Hours of daylight and night are roughly
equal.