Fall does not begin on a single, universally agreed-upon date. Different frameworks define its start, leading to variations in how the transition from summer is measured. Understanding these distinct definitions clarifies when fall truly arrives.
Defining the Astronomical Start
The astronomical start of fall is determined by the autumnal equinox, a moment when the sun crosses the celestial equator. This event occurs in the Northern Hemisphere between September 21 and 23. At this point, the Earth’s axis is tilted neither toward nor away from the sun, resulting in nearly equal hours of daylight and darkness globally.
This definition is rooted in the Earth’s orbit and axial tilt. As the Earth orbits, the sun’s direct rays shift southward after the autumnal equinox, leading to shorter days and longer nights in the Northern Hemisphere. Conversely, for the Southern Hemisphere, this event marks the vernal (spring) equinox, signaling the start of their spring season.
Understanding the Meteorological Start
Meteorologists and climatologists employ a different, fixed definition for the beginning of fall, known as meteorological fall. This system categorizes seasons into three-month periods that align with the Gregorian calendar. In the Northern Hemisphere, meteorological fall consistently spans from September 1 to November 30.
This approach is based on the annual temperature cycle, used for consistent record-keeping, forecasting, and statistical analysis. Grouping months into fixed periods, such as September, October, and November for fall, allows for easier comparison of climate data over time. This provides a standardized framework, unlike relying on celestial events. The Southern Hemisphere observes meteorological fall from March 1 to May 31.
Why the Astronomical Start Date Can Shift
The exact date of the astronomical autumnal equinox can vary slightly each year. This variation occurs because the Earth’s orbit around the sun does not take precisely 365 days; it takes approximately 365.256 days to complete one full revolution. To account for this fractional day, an extra day is added to the calendar every four years during a leap year.
The introduction of a leap day causes the equinox date to shift, moving it backward or forward by a day. For example, the autumnal equinox might occur on September 22 one year and September 23 in another. This annual adjustment reconciles the Earth’s orbital period with our calendar system. While meteorological dates remain constant, the astronomical date reflects the dynamic nature of celestial mechanics and calendar adjustments.