Astronomical autumn is set to begin this week with the autumn equinox. Royal Museums Greenwich states that the equinox will occur at 01:06 BST on Wednesday, 23 September.
The equinox is the precise moment when the Sun crosses the celestial equator, meaning neither hemisphere is tilted towards or away from the Sun. This results in day and night being roughly equal in length across the world.
The term 'equinox' originates from Latin, meaning 'equal night'. However, due to light refraction, the actual time when day and night are equal, known as the 'equilux', occurs after the equinox. In the UK this year, the equilux is expected on Friday, 25 September, or Saturday, 26 September, varying by latitude.
While the equinox marks the start of astronomical autumn, it does not guarantee an immediate shift in weather. The Met Office reports that UK mean temperatures have been above average overall for September so far. Some warmth is forecast for this week, particularly in south-east England, before rain moves across the UK on Wednesday, followed by fresher conditions on Thursday. Towards the end of September, changeable weather is anticipated, with temperatures likely to remain slightly above average overall, though clearer and cooler nights could bring fog and frost in some areas.