The spring equinox is here, signalling the official commencement of astronomical spring. In the northern hemisphere, this celestial transition

consistently occurs between 19 and 21 March. This year, the event falls on 20 March at 14:45.

The specific timing is dictated by Earth's axial tilt during its solar orbit, occurring twice annually — once in March and again in September. It happens simultaneously for everyone worldwide; however, for those south of the equator where seasons are flipped, this marks the start of autumn.

During the equinox, the Earth's axis (indicated by a black line) is positioned neither toward nor away from the Sun. The planet maintains a 23.5-degree tilt, and its orientation relative to the Sun shifts as it completes its orbital path.

At the equinox, Earth reaches a specific junction where the tilt is neutral toward the Sun, resulting in equal solar radiation reaching both the northern and southern hemispheres. The term "equinox" originates from Latin, where equi signifies "equal" and nox denotes "night".

On this date, one might naturally assume that daylight and darkness are perfectly balanced at 12 hours each. However, that assumption is not entirely accurate. In the UK, the true balance of 12 hours of light and dark occurs several days before the equilux.

What is the equilux? During an equinox, measurements are calculated from the moment the Sun's centre aligns with the horizon, providing 12 hours between sunrise and sunset. However, the Sun's upper edge is visible first during dawn and last during dusk. Because of atmospheric refraction, light remains present briefly even after the sun is no longer visible. This contributes slightly more time to the daylight total.

In springtime, this ensures that the date when day and night are truly identical happens before the actual equinox. In the UK, this will take place on Wednesday, 18 March — the equilux.

What is astronomical spring? For meteorologists, spring commences on 1 March. This allows weather data and observations to be compared more consistently year to year. These seasonal months include March, April, and May.

Conversely, the spring equinox defines the start of astronomical spring, with the date shifting slightly every year, based on Earth's orbit around the Sun. Astronomically, spring concludes on 21 June at the summer solstice, when summer begins.