The short version
- The autumn equinox occurs on September 23, marking the start of astronomical autumn when the sun crosses the celestial equator.
- True equality in daylight and darkness arrives days later due to atmospheric refraction bending sunlight at the horizon.
- Northern regions experience faster declines in daylight compared to southern areas, yet overall temperatures remain warmer than average.
Astronomical autumn officially begins this week with the arrival of the autumn equinox, a pivotal moment in the Earth's annual orbit. According to data from Royal Museums Greenwich, this celestial event will take place in the early hours of Wednesday, September 23, at precisely 01:06 BST. This specific timestamp marks the instant when the Sun crosses the celestial equator, positioning the planet such that neither the Northern nor Southern Hemisphere is tilted toward or away from the star.
The term equinox derives from Latin roots meaning equal night, suggesting a perfect balance between daylight and darkness. However, this linguistic definition does not perfectly align with observable reality for most observers. The measurement of daylight begins when the center of the Sun reaches the horizon, resulting in a theoretical twelve-hour day and twelve-hour night. In practice, human perception of light differs because the top edge of the solar disk becomes visible before the center crosses the horizon at sunrise and remains visible after it dips below at sunset.
Atmospheric conditions further complicate the notion of equal hours. Light refraction, caused by the bending of sunlight as it passes through the Earth's atmosphere, extends the period of visibility beyond the geometric limits of the Sun's position. This phenomenon means that actual daylight exceeds twelve hours on the day of the equinox itself. Consequently, the true point where day and night are of equal length, known as the equilux or equal light, occurs several days later in the autumn season.
For locations within the United Kingdom, this balance of light is expected to arrive on either Friday, September 25, or Saturday, September 26. The exact date depends on local latitude, with variations occurring across different regions. While the equinox happens simultaneously for all observers on Earth, the impact on daily life varies significantly depending on geographic location. The change in daylight hours is not uniform across the country, creating distinct experiences for residents in different parts of the nation.
The rate at which daylight diminishes accelerates as one moves northward. Areas in northern Scotland are losing daylight more rapidly during this period compared to regions in southern England. This disparity stems from the greater difference between the long days of summer and the short days of winter experienced at higher latitudes. Around the time of the equinox, these changes become particularly noticeable, with sunrise times growing progressively later and sunset times arriving earlier each day.
Despite the astronomical shift signaling the start of autumn, meteorological conditions do not necessarily follow suit immediately. The lowering position of the Sun in the sky and shortening days generally lead to falling temperatures as the season progresses. However, current weather patterns indicate a delay in typical autumnal cooling. Data from the Met Office shows that mean temperatures across the UK have remained above average throughout September so far.
Short-term forecasts suggest some residual warmth will persist, particularly in the south-east of England, before rain moves across the country on Wednesday. Fresher conditions are expected to follow on Thursday, bringing a gradual transition toward cooler weather. As the month draws to a close, changeable conditions are anticipated, with temperatures likely to stay slightly above average overall. Clearer and cooler nights may lead to the formation of fog and even frost in certain areas, hinting at the deeper chill yet to come.
The equinox also holds cultural and spiritual significance for various groups, including druids and pagans who gather at historic sites such as Stonehenge. These gatherings serve to mark the occasion and observe the sunrise, connecting modern observers with ancient traditions tied to the solar cycle. While the astronomical event itself is invisible to the naked eye, its timing provides a framework for these celebrations and reflects humanity's long-standing relationship with the rhythms of the natural world.
Sources behind this briefing
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