First Day of Autumn 2025: Date, Time & Equinox Facts

For about a week each year, the question of when autumn actually starts sparks genuine confusion — partly because two official systems define the season differently, and partly because the precise moment shifts every year. In 2025, astronomical autumn arrives on Monday, September 22, and the UK will experience that moment at exactly 19:20 BST. This piece unpacks what that means, why the date can vary, and what separates this equinox from the one your weather app uses to start the season.

Autumn 2025 start: September 22, 2025 · Equinox time (BST): 19:20 on September 22 · Astronomical end: December 21, 2025 · Season type: Autumnal equinox · Daylight balance: 12 hours day and night

The table above captures the essential dates for tracking astronomical autumn through the 2025 season.

What day does autumn officially start in 2025?

Astronomical autumn in 2025 begins on Monday, September 22. The UK will witness the equinox at 19:20 BST, when the Sun crosses the celestial equator moving southward — a moment that divides the year into roughly equal halves of light and dark (Sky at Night Magazine). This is the official start according to how astronomers track Earth’s seasons.

In UTC terms, the equinox occurs at 18:20 — one hour earlier, reflecting the UK’s current British Summer Time. The moment is universal, but your local clock determines when you personally experience it (Space.com). For those on the US East Coast, that’s 2:20 p.m. EDT on the same calendar day.

The Met Office confirms this date explicitly: astronomical autumn begins on Monday, 22 September 2025, and ends on Sunday, 21 December 2025 (Met Office). The season runs roughly 91 days, aligning the arrival of crisp mornings with the winter solstice.

Northern Hemisphere date

The Northern Hemisphere as a whole marks autumn’s arrival on September 22. This covers North America, Europe, most of Asia, and parts of northern Africa — billions of people share the same celestial calendar on this date.

Exact equinox time

The precise timing of 19:20 BST means the equinox happens late afternoon in the UK — well after sunset approaches. The Sun will rise due east and set due west on that day, a characteristic feature of equinoxes that makes them visually distinct (Space.com).

USA and Ireland specifics

Americans experience the equinox at 2:20 p.m. EDT, 1:20 p.m. CDT, 12:20 p.m. MDT, and 11:20 a.m. PDT on September 22 (Country Living). Ireland shares the same BST/GMT framework as the UK, so Dublin sees the equinox at the identical 19:20 Irish Standard Time.

Is September 22 always the first day of fall?

September 22 is the most common date for the autumnal equinox, but it’s not fixed. The equinox typically falls on September 22 or 23, with rare occurrences of September 21 or 24 (Country Living). This variability stems from how Earth’s 365.25-day orbit doesn’t divide evenly into the calendar year.

Equinox date shifts

The Gregorian calendar creates these shifts. Because we insert leap days every four years (except century years not divisible by 400), equinox dates drift slightly across the calendar. This explains why 2025 lands on September 22 while some years push into September 23.

Role of axial precession

Earth’s axial precession — a 26,000-year wobble in how the planet tilts — gradually shifts which zodiac constellations align with equinoxes over millennia. The September equinox used to mark the start of Libra; today it falls in Virgo. This cycle affects precise equinox timing, though its annual impact is minimal.

Historical variations

In the Julian calendar era (before 1582), equinoxes occurred earlier in September on average. The Gregorian reform adjusted this, and modern astronomical calculations keep dates aligned with solar reality. The Met Office notes that September 24 equinoxes are so rare that most people alive today will never see one (Met Office).

What does equinox mean?

“Equinox” comes from Latin — aequus (equal) and nox (night). It literally means “equal night,” referring to the roughly 12-hour balance of daylight and darkness when the Sun crosses the celestial equator (Royal Museums Greenwich).

Etymology and science

Astronomically, the equinox is the moment when Earth’s axis points neither toward nor away from the Sun. Neither hemisphere leans into solar heating; the Sun’s rays strike equally across both halves of the planet (Sky at Night Magazine).

Day-night balance

At the precise equinox moment, everywhere on Earth experiences approximately 12 hours of daylight and 12 hours of darkness. However, geography and atmospheric refraction mean some locations see slightly more or less than exactly 12 hours on that calendar date. At 50°N latitude, there are roughly 10 extra minutes of daylight on equinox day (Sky at Night Magazine).

Autumnal vs vernal

The September equinox is the autumnal equinox for the Northern Hemisphere. The March equinox is the vernal (spring) equinox. They are mirror events: the September version marks summer’s end and autumn’s beginning; the March version marks winter’s end and spring’s arrival in the north.

What is the official first day of autumn?

Two competing definitions exist, and confusion between them drives many of the questions people ask about seasonal start dates.

Astronomical definition

Astronomical autumn begins at the autumnal equinox — around September 22 in 2025. This definition ties seasons to Earth’s position in its orbit around the Sun and the tilt of the planet’s axis (Met Office). Because Earth’s orbit isn’t perfectly circular and the axial tilt shifts slightly over time, the exact date varies year to year.

Met Office stance

The Met Office uses both definitions but distinguishes them clearly. Astronomical autumn begins on the equinox; meteorological autumn runs from September 1 to November 30 — fixed dates that let meteorologists compare seasonal data consistently across years (Met Office).

Global differences

Most countries follow either the astronomical or meteorological system — or both. The Northern Hemisphere shares the September equinox as autumn’s start; the Southern Hemisphere experiences spring beginning on the same date. In the tropics, seasons may follow rainfall patterns rather than equinoxes at all.

What happens on September 22, or 23rd?

Beyond the precise astronomical moment, September 22 in 2025 triggers several observable changes that mark autumn’s arrival.

Earth-sun alignment

On the equinox, Earth’s axis tilts perpendicular to the Sun’s rays. The terminator line — where daylight meets darkness — passes directly over the North and South Poles. This is the only time when the Sun’s light reaches both poles simultaneously (Royal Museums Greenwich).

Seasonal changes

After the equinox, nights grow longer than days in the Northern Hemisphere. Tree leaves begin their colour shift. Daylight hours shorten noticeably — by roughly 2–3 minutes per day in mid-latitudes. The longer nights create ideal conditions for stargazing as autumn progresses (Space.com).

Cultural notes

Harvest festivals and autumnal celebrations cluster around the equinox in many cultures. The Moon’s cycle around this date can amplify or dampen the seasonal feel. US daylight saving time ends on November 2 in 2025, adding an hour of morning light but shortening afternoon daylight further (Country Living).

Timeline of the 2025 autumn season

The timeline above reveals how astronomical autumn outlasts its meteorological counterpart by roughly three weeks, giving sky watchers extra time to track seasonal constellations.

Confirmed facts vs unclear aspects

What experts say

In 2025 the autumn equinox occurs on 22 September at 19:20 BST.

— Sky at Night Magazine (Astronomy publication)

Astronomical autumn will begin on Monday, 22 September and end on Sunday, 21 December.

— Met Office (UK National Weather Service)

The autumn equinox marks the end of summer and the beginning of autumn in the Northern Hemisphere.

— Royal Museums Greenwich (UK astronomy authority)

For readers in the UK, the takeaway is straightforward: meteorological autumn already started on September 1, but astronomical autumn — the one tied to Earth’s position in space — officially begins on September 22 at 19:20 BST. If you’re planning outdoor activities, both dates matter; if you’re tracking celestial events, the equinox moment is your anchor.

Related reading: local weather updates

The autumnal equinox on September 22 at 19:20 BST aligns precisely with worldwide equinox timing confirming 18:19 UTC across the northern hemisphere.

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