When Does Fall Start – Astronomical vs Meteorological Dates

Fall begins on two different dates depending on which seasonal system applies. The astronomical definition ties the start of autumn to the autumnal equinox — a precise celestial event — while the meteorological definition sets a fixed calendar date of September 1 each year in the Northern Hemisphere. Both are widely accepted, each serving distinct purposes in science, forecasting, and daily life.

In 2024, the astronomical first day of fall in the Northern Hemisphere arrived on September 22 at 8:43–8:44 a.m. EDT. At that exact moment, the center of the sun aligned directly with Earth’s equator, producing roughly equal hours of daylight and darkness across the planet. For meteorologists, however, autumn had already been underway since September 1.

The distinction affects more than just terminology. It shapes weather forecasting, aurora borealis viewing windows, agricultural calendars, and how seasonal records are compiled year over year. Knowing which definition applies in a given context clarifies what is actually being measured when someone asks when fall truly begins.

When Does Fall Start?

• In 2024, astronomical fall began on September 22 at 8:43–8:44 a.m. EDT in the Northern Hemisphere.
• Meteorological fall always starts on September 1, every year, without variation.
• The word “equinox” comes from Latin words meaning “equal night,” reflecting the near-equal daylight and darkness that occur at that moment.
• After the autumnal equinox, nights gradually grow longer until the winter solstice on December 21.
• The exact date of the autumnal equinox can shift by one to two days from year to year due to Earth’s orbital position relative to the sun.
• The autumnal equinox is associated with elevated geomagnetic activity, making it a favorable period to observe the aurora borealis.
• In the Southern Hemisphere, the equivalent seasonal shift — autumn — begins in March.

Meteorological vs. Astronomical Fall: Key Differences

What Defines the Astronomical Start of Autumn?

Astronomical fall is determined by Earth’s position relative to the sun. The autumnal equinox occurs when the sun crosses the celestial equator, moving from north to south. At that moment, both hemispheres receive approximately equal amounts of sunlight, producing roughly equal periods of day and night worldwide.

According to the Royal Observatory Greenwich, this solar alignment defines the equinox itself. Because Earth’s orbit is not a perfect circle, the precise date of the equinox shifts slightly from one year to the next, typically landing on September 22 or September 23 in the Northern Hemisphere.

How Does Meteorological Fall Differ?

Meteorological fall is not connected to any astronomical event. It runs for exactly three months: September 1 through November 30 in the Northern Hemisphere, every year. This fixed structure allows scientists and forecasters to compile and compare weather data consistently across different years.

As reported by Mental Floss, meteorologists rely on this consistent three-month calendar system to simplify weather pattern tracking and long-term record-keeping. Because the dates never change, year-over-year comparisons of autumn temperatures, precipitation, and storm activity remain statistically reliable.

When Does Fall End and What Are the Full Season Dates?

The end of fall mirrors the same definitional split as its beginning. Astronomically, autumn concludes at the winter solstice — the shortest day of the year. In 2024, that date falls on December 21. After the solstice, daylight hours gradually begin to lengthen again in the Northern Hemisphere.

For meteorologists, fall ends on November 30, with meteorological winter beginning December 1. This three-month block — September through November — corresponds to the period when temperatures in the Northern Hemisphere consistently follow autumn-like patterns.

Resources such as The Old Farmer’s Almanac and Time and Date publish precise annual equinox and solstice times, which shift slightly each year. For those in Canada tracking what follows autumn, the Canada Winter Weather Forecast – 2024-2025 La Niña Outlook provides detailed seasonal context for the months ahead.

Fall Start Dates by Hemisphere

The Northern and Southern Hemispheres experience opposite seasons simultaneously. When autumn begins in the North, spring is starting in the South — and vice versa. This occurs because Earth’s axial tilt means the two hemispheres receive sunlight at different angles at different points in the planet’s orbit around the sun.

In the Northern Hemisphere, autumn begins around September 22–23 astronomically, or September 1 meteorologically. In the Southern Hemisphere, autumn begins in March — the period the Northern Hemisphere refers to as spring. The precise equinox dates in the Southern Hemisphere follow the same astronomical logic, shifted by approximately six months.

How Do the Four Seasons Progress Through the Calendar Year?

1. June 20 — Summer Solstice (Northern Hemisphere): The longest day of the year in the North and the shortest in the South. Marks the astronomical start of Northern Hemisphere summer.
2. September 22 — Autumnal Equinox (2024, Northern Hemisphere): The astronomical first day of fall, occurring at 8:43–8:44 a.m. EDT. Day and night are approximately equal in length worldwide at this moment.
3. December 21 — Winter Solstice (Northern Hemisphere): The shortest day of the year in the North. Marks the end of astronomical fall and the astronomical start of winter.
4. March 20 — Spring Equinox (Northern Hemisphere): Day and night again reach approximate equality. This marks the astronomical start of spring in the North and the astronomical start of autumn in the Southern Hemisphere.

What Is Fixed and What Shifts Year to Year in Autumn’s Timing?

The U.S. Naval Observatory maintains detailed equinox records for each year, calculated using precise astronomical models. The USNO equinox data shows these shifts clearly: while the autumnal equinox reliably falls near September 22, the exact date and time follow Earth’s orbital path around the sun rather than a fixed calendar rule.

What Causes the Seasons to Change?

Seasons are caused by the tilt of Earth’s axis relative to its orbital plane around the sun. As Earth moves through its yearly orbit, the Northern and Southern Hemispheres are tilted toward or away from the sun at different times. This tilt — not Earth’s distance from the sun — determines how directly sunlight strikes the surface.

When the Northern Hemisphere tilts toward the sun, sunlight arrives more directly, producing the warmth of summer. When it tilts away, sunlight strikes at a shallower angle, reducing its heating effect and producing the cooler temperatures and shorter days of autumn and winter.

At the equinox, neither hemisphere is tilted significantly toward or away from the sun. Solar energy is distributed more evenly across the planet, which is why both hemispheres experience approximately equal day lengths at that moment. NASA details the mechanics of seasonal change through Earth’s axial tilt and its relationship to the sun across the orbital year.

Where These Seasonal Definitions Originate

The autumnal equinox occurs when the sun crosses Earth’s equator moving southward, resulting in nearly equal hours of daylight and darkness at all latitudes across the globe.

— NASA, on the mechanics of the equinox

Meteorological seasons are defined to coincide with the calendar months that best reflect seasonal temperature cycles, providing a consistent and comparable framework for weather pattern tracking and climate records.

— NOAA, NOAA JetStream

The Royal Observatory Greenwich further notes that geomagnetic activity is statistically more elevated during spring and autumn equinoxes than at other points in the year. This documented pattern makes the autumnal equinox period one of the best opportunities to observe the aurora borealis — a natural connection between Earth’s orbital position and atmospheric physics.

The Core Facts on When Fall Begins

Fall begins on September 1 by the meteorological calendar and on or around September 22 by the astronomical calendar — with the 2024 equinox confirmed at September 22, 8:43–8:44 a.m. EDT. Neither definition is incorrect; each serves a different purpose. As autumn transitions toward winter across the Northern Hemisphere, the Canada Child Benefit November 2025 – Payment Date and Rates is among the fall-season milestones that Canadian households track as the colder months approach.

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