Spring equinox 2021: when is the vernal equinox, what it means - and why does it mark the first day of spring?

The spring equinox occurs towards the end of March each year in the UK

The spring equinox officially marks the beginning of spring in the Northern Hemisphere (Getty Images)

The temperature is getting slightly warmer and flowers are beginning to bloom which signals that spring in Scotland is fast approaching.

To determine the first day of the season, astronomers look to the skies.

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The spring equinox - also known as the vernal equinox or the March equinox - officially marks the beginning of spring in the Northern Hemisphere.

So, what is the spring equinox, what does it mean and when is it in 2021?

Here is everything you need to know.

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What is the spring equinox?

Each year, there are two equinoxes in March and September.

The word equinox comes from the Latin words aequi, meaning “equal” and nox, meaning “night”, becoming “equal night”.

The spring equinox is essentially just that, as the length of night and day time become nearly equal - 12 hours - all over the world.

It is caused by the Earth’s tilt on its axis as it orbits the sun.

Because of that tilt - which is about 23.4 degrees - as our planet goes round the sun, different parts are exposed to more and less sunlight, causing the different seasons.

During the autumn and spring equinoxes, the tilt of Earth’s axis is perpendicular to the sun’s rays.

This means the sun shines directly on the equator, resulting in it lighting up the Northern and Southern Hemispheres equally.

While this brings the start of spring in the Northern Hemisphere, it marks the beginning of autumn in the Southern Hemisphere.

What does the spring equinox mean?

The spring equinox brings longer daylight hours and warmer temperatures in the Northern Hemisphere, along with softer winds and sprouting plants.

As a result of the sun crossing the equator and becoming more northerly in the sky, more hours of daylight occur.

Meanwhile, the opposite season occurs in countries south of the equator, like Australia - later sunrises, earlier sunsets, colder winds and falling leaves.

As the Northern Hemisphere tilts more towards the sun, the Southern Hemisphere tilts away from it, which causes the start of autumn and the colder temperatures.

The spring equinox has been celebrated for centuries by different cultures, as it has become a symbol for rebirth, fertility and new beginnings.

People often gather to watch the equinox in different parts of the world.

When is the spring equinox in 2021?

It always falls toward the end of March, any time between the 19th and 21st of the month.

In 2021, astronomers have predicted that the phenomenon will occur on the morning of Saturday 20 March.

On this day, the official time for the start of astronomical spring is 9:37am GMT (5:37am EDT).

Spring will then last until the summer solstice - the longest day of the year - which falls on Monday 21 June 2021.

Some people count spring as beginning on 1 March, according to the meteorological calendar.

The Met Office states that the meteorological seasons consist of splitting the seasons into four periods which are made up of three months.

"These seasons are split to coincide with our Gregorian calendar, making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics,” the forecaster says.