Autumn Equinox Signals Shift to Shorter Days in Northern Hemisphere

The autumnal equinox, marking the astronomical start of autumn in the Northern Hemisphere, is set to occur on Wednesday, September 23, 2026, at 01:06 BST. This celestial event signifies the precise moment the Sun crosses the celestial equator, resulting in roughly equal lengths of day and night across the globe.

While the equinox itself is an invisible point in time, it holds cultural and spiritual significance for many. Observances often take place at sites like Stonehenge, where participants gather to witness the sunrise and mark the occasion.

The term "equinox" is derived from Latin, meaning "equal night." At this time, the duration of daylight from sunrise to sunset is approximately 12 hours, with the remaining 12 hours dedicated to night. However, factors such as atmospheric refraction and the visible diameter of the Sun mean that the period of actual daylight extends slightly beyond this equal division. Consequently, the moment when day and night are precisely equal in length, known as the "equilux," occurs a few days after the equinox, around September 25 or 26 in the UK for 2026, varying slightly with latitude.

The impact of diminishing daylight is more pronounced in higher latitudes. In the UK, regions in northern Scotland experience a faster rate of daylight reduction compared to southern England as autumn progresses. This change becomes particularly noticeable around the equinox, with sunrises occurring later and sunsets earlier.

While the equinox heralds astronomical autumn, a sudden drop in temperature is not guaranteed. As the Sun's angle lowers and days shorten, temperatures typically decline into autumn. Despite this general trend, early September 2026 has seen above-average mean temperatures across the UK, according to Met Office data. Some warmth is expected to persist in the week leading up to the equinox, particularly in southeastern England, before rain systems move across the country.

Looking ahead to the latter half of September, a pattern of changeable weather is anticipated, with temperatures likely to remain slightly above average overall. However, clearer and cooler nights may bring localized fog and even frost in some areas. The shift in daylight hours and the subsequent cooling trend are characteristic of the transition into the autumn season.