Spring is in the air and this Sunday, 20 March, officially marks the arrival of the new season in the Northern Hemisphere with the vernal equinox.
The vernal equinox is one of two moments in the year when the sun is directly above the equator and day and night are almost equal in length.
After the vernal equinox, the Northern Hemisphere will start seeing more hours of daylight than darkness each day until the summer solstice on June 21.
The astronomical event, which has been celebrated for centuries, is seen as heralding new beginnings and the renewal of the natural world.
Sunday marks the start of ‘astronomical spring’, which is different from ‘meteorological spring’, which always commences on March 1 and ends May 31.
There are two equinoxes every year – one in March and another in September – when the sun is directly above the equator and day and night are of equal length
The seasons occur because the Earth is tilted over slightly on its axis, so for half of the year the Northern Hemisphere is tilted slightly towards the sun, meaning we have longer daylight hours and warmer temperatures.
For the other half of the year, we are tilted slightly further away from the Sun so we have longer nights and colder weather.
However, twice a year, in March and September, the sun is directly over the equator, so neither the Northern or Southern Hemisphere is tilted towards the sun.
This year, the sun will pass over the equator at precisely 15:33 GMT on March 20.
There are two equinoxes each year – one in March, known as the vernal equinox, and one in September called the autumnal equinox.
The word equinox comes from the Latin words aequus, meaning equal, and nox, meaning night.
In the Southern Hemisphere, the seasons are flipped, with the March equinox marking the start of autumn and the September equinox marking the beginning of spring.
For the Northern Hemisphere, the vernal equinox kicks of the countdown to the summer solstice – the longest day of the year and the start of summer – which this year will be on June 21.
Solstices occur when the planet’s geographical pole in the Northern or Southern Hemisphere is most greatly inclined towards the Sun.
On these dates, depending on which hemisphere you live in, you will either experience midsummer, with the longest day of the year, or midwinter, with the longest night.
The vernal equinox, which has been celebrated for centuries, is seen as heralding new beginnings and the renewal of the natural world
Sunday marks the start of ‘astronomical spring’, which is one of two definitions of when the seasons start and end.
Astronomical spring is different from meteorological spring, which always commences on March 1 and ends May 31.
Meteorological seasons are based on the annual temperature cycle and are more rigidly defined by the months of the year.
The Met Office explains: ‘These seasons are split to coincide with our Gregorian calendar, making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics.’