What makes the spring equinox so special?

By Geoff Maskell
BBC News NI Weather Presenter

Image source, neirfy

Wednesday marks the spring equinox, the midway point between mid-winter and mid-summer.

It takes a year for the earth to orbit the sun, and as it does so our planet spins on its own axis, each revolution taking a day.

If the axis of the earth was at 90° or perpendicular to the plane of its orbit, our planet would be very different.

Sunrise and sunset would occur at the same time every single day.

We would also have no seasons and there would be a huge impact on weather patterns around the globe.

The reason that that is not the case is because the earth is tilted by 23.5 degrees in relation to its plane of orbit. It's that tilt which gives rise to the seasons.

In the northern hemisphere summer we're tilted towards the sun and in winter away from it.

However the equinox is an important staging post on the annual orbit of the earth around the sun, particularly if you live at 54.5°N like we do in Northern Ireland.

In Belfast on the winter solstice, 21 December 2018, there were seven hours and 15 minutes of daylight.

Image caption,
The earth's tilt of 23.5 degrees gives rise to the seasons

On the summer solstice this year, 21 June, the sun will rise at 04:48 (BST) and set at 22:04 (BST), giving Northern Ireland 17 hours and 16 minutes of daylight.

By contrast in Singapore - which is is just 1°N of the equator - the shortest day was 12 hours and three minutes of daylight, and the longest just nine minutes more.

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 name, equinox, which is derived from the Latin meaning equal night.

The northern hemisphere spring equinox - the mid-point between mid-winter and mid-summer - occurs on 19, 20 or 21 March and is also known as the vernal equinox.

The name is derived from the Latin word 'vernalis,' which means "of the spring".

Image source, david010167
Image caption,
The extra daylight each week is the reason why spring is the season when the garden bursts into life

In Northern Ireland at this time of year there is a real stretch in the day.

Each day this week we're adding an extra four minutes 26 seconds of daylight; a difference of more than half an hour from the start of the week to the end.

Because plants need water, light, and warmth to grow, the extra daylight each week is the reason why spring is the season when the garden bursts into life.

The showery weather gives plants the water they need to thrive. The longer days mean they have more daylight and warmth from the sun which raises the temperature of both the air and the soil.

So with the grass going crazy at least the evenings are getting longer and when the clocks go forward at the end of March there will again be plenty of time to get out and cut the lawn after work.