Sign up for NPL updates
Sign up for NPL updates

NPL and UKRI investment for innovation in time resilience, dissemination and application.
Funding of up to £4.7 million available through joint initiative, call for applications now open.

Questions and answers

When do the four seasons officially begin?

There are no official definitions of the duration of spring, summer, autumn and winter

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 seasons are the equinoxes and the solstices, and these have been used to define the seasons astronomically.

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

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.

For 2021 they are:



March 20 at 09:37 UTC

Vernal (Spring) equinox

June 21 at 04:32 BST

Summer solstice

September 22 at 20:21 BST

Autumnal equinox

December 21 at 15:59 UTC

Winter solstice

There are many sources, such as diaries, which use these dates as if they were the boundaries of the seasons. For example ‘June 21, Summer begins’, but this leads to confusion when, in many European countries, June 24 is celebrated as midsummer day.

For information on past and future solistices and equinoxes visit this external webpage.

You may also like

Environmental monitoring


Earth observation, climate and optical


Temperature and humidity

Time and frequency

Work with us

Our research and measurement solutions support innovation and product development. We work with companies to deliver business advantage and commercial success.
Contact our Customer Services team on +44 20 8943 7070