When do the four British seasons dates officially begin?

With no official definitions of the duration of spring, summer, autumn and winter, how do we know when the british seasons dates begin or end?

Some meteorologists adopt a convention, for the purpose of presenting statistics, of grouping the twelve months of the year into four three-month seasons, for example March, April and May being taken as Spring.

The astronomical events closely related to the four uk seasons are the equinoxes and the solstices, and these have been used to define the British seasons dates astronomically.

The equinoxes occur in March and September when the Sun is edgewise to the Earth's axis of rotation so that everywhere on Earth has twelve hours of daylight and twelve hours of darkness.

Bending of light by the atmosphere (refraction) may extend the day by 5 to 10 minutes depending on location. Also, days are about 2 minutes longer because whilst it is 12 hours between the centre of the sun rising and setting, we count sunrise to be when the edge of the sun appears over the horizon, and sunset to be when the final part disappears below it.

The solstices occur in June and December when the Earth's axis is at its extreme tilt towards and away from the Sun, so at mid-day it appears at its highest in one hemisphere and its lowest in the other, giving the longest and shortest days respectively.

These four events repeat every tropical year (365 days, 5 hours, 49 minutes), so they become later by about six hours, or (if there has been an intervening leap day) earlier by about 18 hours, from one year to the next.

They are not equally spaced in the year, because the Earth's orbit around the Sun is elliptical, not circular. Their timings can be obtained from a range of websites.