Autumnal Equinox Day

Welcome to the Autumnal Equinox Day calculator. Find the exact date and time of the autumnal equinox for any year between 2000 and 2038, with a live countdown, complete seasonal timeline, multi-year comparison, and in-depth astronomical explanations. This tool supports both Northern Hemisphere (September equinox) and Southern Hemisphere (March equinox) calculations.

What Is the Autumnal Equinox?

The autumnal equinox (also called the fall equinox) is the astronomical moment when the Sun crosses the celestial equator heading southward (in the Northern Hemisphere) or northward (in the Southern Hemisphere). At this instant, day and night are approximately equal in length — the word "equinox" comes from Latin aequinoctium, meaning "equal night."

When Is the Autumnal Equinox?

The autumnal equinox typically falls between September 21–24 (Northern Hemisphere) or March 19–21 (Southern Hemisphere). The exact date varies each year because Earth\'s orbital period is approximately 365.2422 days — not a perfect 365. This fractional difference, corrected by leap years, causes the equinox to shift by roughly 6 hours each year, then snap back during leap years.

Why the Date Changes

• Orbital fraction: Each year the equinox drifts later by about 5 hours 49 minutes relative to the calendar.
• Leap year correction: Adding February 29 shifts the equinox earlier by about 18 hours, keeping it anchored to September 22–23.
• Planetary perturbations: Gravitational pull from the Moon, Jupiter, and Venus create minor variations of a few minutes.

Northern vs. Southern Hemisphere

The same astronomical event is experienced differently by each hemisphere:

How to Use This Calculator

1. Select a year: Choose any year from 2000 to 2038 using the dropdown or the quick-access buttons.
2. Choose hemisphere: Northern Hemisphere uses the September equinox; Southern uses the March equinox as the start of autumn.
3. View results: See the exact date, UTC time, live countdown (for future equinoxes), all four seasonal markers, season duration, and a 5-year comparison.

Understanding Your Results

Live Countdown

For upcoming equinoxes, a real-time countdown shows days, hours, minutes, and seconds remaining until the exact moment of the equinox.

All Four Seasons

The seasonal timeline displays the year\'s four astronomical events (two equinoxes and two solstices), labeled appropriately for your chosen hemisphere.

Season Duration

Autumn\'s duration — from the autumnal equinox to the winter solstice — varies between hemispheres. Northern autumn lasts about 89–90 days, while southern autumn lasts approximately 92–93 days, because Earth travels faster in its orbit during January (perihelion) than July (aphelion).

Multi-Year Comparison

The 5-year table shows how the equinox date and time shift across years, making it easy to spot the leap-year pattern and plan ahead.

Frequently Asked Questions

When is the autumnal equinox?

The autumnal equinox occurs in September for the Northern Hemisphere (typically between September 21–23) and in March for the Southern Hemisphere (typically between March 19–21). The exact date and time vary each year because Earth\'s orbital period is not exactly 365 days. In 2025, the Northern Hemisphere autumnal equinox falls on September 22 at 18:19 UTC.

What happens during the autumnal equinox?

During the autumnal equinox, the Sun crosses the celestial equator from north to south (for the September equinox) or south to north (for the March equinox). Day and night are approximately equal in length worldwide, each lasting about 12 hours. After the autumnal equinox, nights become longer than days in the respective hemisphere until the winter solstice.

Why does the autumnal equinox date change each year?

The autumnal equinox date varies because Earth\'s orbital period is approximately 365.2422 days, not exactly 365 days. This fractional difference, combined with the leap year cycle, causes the equinox to shift by about 6 hours each year, then jump back when a leap year adds an extra day. Additionally, gravitational perturbations from the Moon and other planets cause slight variations.

Is the autumnal equinox the same date worldwide?

The equinox occurs at the same instant worldwide in UTC time. However, due to time zone differences, the local date may differ by one day in some regions. For example, if the equinox occurs at 23:00 UTC on September 22, it would already be September 23 in East Asian and Australian time zones.

How long does autumn last?

Astronomical autumn (from the autumnal equinox to the winter solstice) lasts approximately 89–90 days in the Northern Hemisphere. The exact duration varies slightly each year depending on Earth\'s position in its elliptical orbit. In the Southern Hemisphere, autumn (March equinox to June solstice) lasts approximately 92–93 days.

What is the difference between the autumnal equinox and the fall equinox?

There is no difference. "Autumnal equinox" and "fall equinox" refer to the same astronomical event. "Autumnal" is derived from Latin and is the more formal term used in astronomy, while "fall" is the common American English term for the season.

Additional Resources

• Equinox — Wikipedia
• Seasons — Wikipedia