What is the vernal equinox? Science behind the astronomical start of spring

After months of cold weather and dark mornings and evenings, the weather is beginning to warm up and the Sun is starting to shine, signalling an end to the long winter months.

Two different dates mark the first day of spring. The meteorological spring starts on 1 March, but the astronomical spring starts on 20 March.

According to the Met Office, the difference is: "Astronomical seasons refer to the position of Earth's orbit in relation to the Sun taking into account equinoxes and solstices.

"Meteorological seasons are instead based on the annual temperature cycle and measure the meteorological state as well as coinciding with the calendar to determine a clear transition between the seasons."

The vernal equinox, known as the spring equinox in the northern hemisphere, marks the arrival of spring on 20 March.

What is an equinox?

As the Earth travels around the Sun along its orbit, the north to south position of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes. The dates of zero tilt of the Earth's equator correspond to the spring equinox and the autumn equinox.

Equinoxes occur when the plane of Earth's Equator passes the centre of the Sun. At that instant, the tilt of the Earth's axis neither inclines away from, nor towards, the Sun.

This happens twice a year, around 20 March and 20 September, which are the only times when the subsolar point – the place on Earth's surface where the centre of the Sun is exactly overhead – is on the equator and consequently, the Sun is at zenith over the Equator. The subsolar point crosses the equator, moving northward at the March equinox and southward at the September equinox.

During an equinox, the Earth's north and south poles are not tilted toward or away from the Sun, and the duration of daylight is theoretically the same at all points on Earth's surface.

This means that day length is exactly the same – 12 hours – at all points on the Earth's surface on these days, with the exception of each pole, where it will be about to change from permanent light to dark, or vice versa. For those in the southern hemisphere, this time is the autumnal equinox that is taking people into their winter.

Why is it called an equinox?

The term equinox comes from the Latin aequus, meaning equal, and nox, meaning night. This is derived from the oldest understanding of an equinox, which suggests it is the day when daytime and night are of equal duration.

On the spring equinox, the Sun will rise in London at 6:03am and set at 6:13pm.