What is the spring equinox?

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The March (spring) equinox happens on Tuesday 20 March, and marks the start of astronomical spring for the Northern Hemisphere.

It takes a year for the Earth to orbit the Sun and as it does so it spins on its own axis, one revolution being a day.

Image source, Calios/Weather Watchers
Image caption,
Calios captured this misty sunrise in Aberdeenshire on last year's spring (vernal) equinox

However the Earth is titled by 23.5 degrees in relation to its plane of orbit and it's that tilt that gives rise to the seasons.

During summer in the Northern Hemisphere we are tilted towards the Sun and in winter we are tilted away from it.

Image source, Michiko Sunshine/Weather Watchers
Image caption,
The March equinox marks the start of astronomical spring. Photo taken in Wakefield by Michiko

However during an equinox, the Earth's north and south poles are not tilted towards or away from the Sun and the duration of daylight is theoretically the same at all points on the Earth's surface. Hence the term "equinox" derived from the Latin meaning "equal night".

Image source, kevfromsaltburn/Weather Watchers
Image caption,
A beautiful sunset to end the spring equinox last year. Photo taken in Saltburn by Kev

The spring equinox happens on the 19, 20 or 21 March every year, so the equinox is indeed a moveable feast.

This year it will happen on 20 March and on this date day and night will be of equal length all over the world.

For more information on when spring starts, have a look at our video.

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