The autumn equinox will take place on Wednesday 23 September at 01:06 BST. It is a moment in time when the Earth’s axis is perpendicular to the Sun, and it marks the start of astronomical autumn in the northern hemisphere.
South of the equator, the same September equinox is the first day of astronomical spring. Day and night are nearly equal in length across the world, but in autumn the point when they are actually equal comes after the equinox because light refraction adds extra time to daylight hours.
Earth and the Sun
The word equinox comes from Latin and means equal night. At the equinox, daylight is counted from the point when the centre of the Sun is at the horizon, which is why the day is described as 12 hours from sunrise to sunset and 12 hours from sunset to sunrise.
The Earth is tilted on its axis at 23.5 degrees, and that tilt helps create the seasonal pattern that makes the September equinox a fixed point in the year’s changing light. The autumn equinox almost always falls on 22 or 23 September in the northern hemisphere, which is why this date is the one people in the northern hemisphere use to mark the shift into astronomical autumn.
Meteorological autumn
People in the northern hemisphere who follow meteorological seasons start autumn on 1 September and end it on 30 November. Each of the four meteorological seasons lasts for three calendar months, so that calendar-based season begins weeks before the equinox itself.
Astronomical autumn then runs until the winter solstice in the middle of December. That gives readers two different ways to track the season: one based on the calendar, and one based on the Earth’s position relative to the Sun.
23 September at 01:06 BST
The exact time matters because the equinox is a single instant, not a full day. For anyone watching the seasonal change, 23 September at 01:06 BST is the moment the northern hemisphere crosses into astronomical autumn, while the south crosses into astronomical spring.
The practical answer is simple: if you want the calendar version, autumn already began on 1 September; if you want the astronomical version, it begins at 01:06 BST on 23 September and ends with the winter solstice in the middle of December.
