The September equinox — commonly called the autumnal equinox in the northern hemisphere and the vernal or spring equinox in the southern hemisphere — takes place at 01:05 BST on 23 September this year. Astronomers mark the moment when the Sun appears to cross the celestial equator as the formal start of the new season in each hemisphere.
What the equinox means
An equinox occurs when the Sun’s apparent path across the sky intersects the extension of Earth’s equator into space, known as the celestial equator. That geometry signals the shift in solar illumination that produces the progression from summer to autumn north of the equator and from winter to spring to the south.
Why day and night are not exactly equal
Although the word equinox comes from the Latin for "equal night," actual day and night lengths are not precisely equal on the calendar date of the equinox. Atmospheric refraction bends sunlight and the Sun’s apparent disk has a measurable size, both of which alter the timing of sunrise and sunset as measured at the ground. As a result, the balance of daylight and darkness typically occurs a day or two after the instant of the equinox — in this case not until Friday.
Observing and understanding the change of seasons
The equinox is an astronomical marker rather than a sudden weather change; seasonal shifts in temperature and daylight accumulate over days and weeks. Skywatchers and cultural calendars around the world have long used the equinox as a reference point for seasonal festivals, planting cycles and navigational calculations, but local climate and weather patterns determine how quickly the new season feels established.