First Day of Fall in 2018: Date and Exact UTC Time
The first day of fall in 2018 was September 22, when the autumnal equinox occurred at 21:54 UTC, marking the moment the Sun crossed the celestial equator moving southward. This date is determined by Earth's axial tilt and orbital position, not by a calendar rule, and it shifts by one day each year around September 22 or 23.
For observers in the Northern Hemisphere, this moment signaled the start of astronomical autumn, while the Southern Hemisphere began astronomical spring at the same instant. The precise second of the equinox is calculated by astronomical observatories and published in almanacs and on official timekeeping pages.
How the Autumnal Equinox Is Defined and Measured
An equinox occurs when the center of the Sun lies directly above the Earth's equator, resulting in nearly equal lengths of day and night worldwide. In 2018, this happened on September 22 at 21:54 UTC, which corresponds to 5:54 PM Eastern Daylight Time in the United States.
The exact time is based on precise astronomical models and is confirmed by institutions such as the U.S. Naval Observatory, which provides official sunrise, sunset, and equinox data for civil and scientific use.
Why the Date Varies Slightly Each Year
The first day of fall shifts between September 22 and 23 because Earth's orbital period around the Sun is not a whole number of days. The Gregorian calendar uses leap years to keep the seasons aligned, but the equinox moment still moves within a narrow window each year.
In 2018 the autumnal equinox fell on September 22, while in 2019 it moved to September 23, illustrating the typical one-day drift caused by the mismatch between the calendar year and the tropical year.