The duration of fall depends entirely on the method used to define the season. Two primary definitions exist: the astronomical, tied to Earth’s orbit, and the meteorological, based on fixed calendar months. The difference between these systems creates variability in the start, end, and total length of the season, which also changes depending on the hemisphere.
Astronomical Fall: The Changing Dates
The astronomical definition of fall is governed by the Earth’s position relative to the Sun, marking the period between the Autumnal Equinox and the Winter Solstice. In the Northern Hemisphere, this season begins around September 22nd or 23rd and concludes around December 21st or 22nd. This timing is rooted in the planet’s axial tilt and elliptical path around the Sun.
Because the Earth’s orbit is not a perfect circle, its speed changes throughout the year, causing the time between the equinox and solstice to vary slightly. The Earth moves faster when it is closer to the Sun, making the astronomical seasons unequal in length. Consequently, the duration of astronomical fall is not fixed, but shifts each year, typically lasting about 89 to 90 days. This variability is also influenced by leap year adjustments.
Meteorological Fall: The Fixed Calendar
In contrast to the astronomical system, the meteorological definition of fall uses fixed calendar months. This approach simplifies the collection and comparison of seasonal weather statistics. The system divides the year into four seasons of three full months each, aligning with the annual temperature cycle. This provides a consistent framework for climate record-keeping and forecasting.
In the Northern Hemisphere, meteorological fall is defined as the three-month period from September 1st through November 30th. This fixed schedule means the season always lasts for exactly 91 days. This definition is preferred by weather scientists and climatologists. It groups months with similar average temperature patterns, offering a more practical measure than the shifting astronomical dates.
Seasonal Reversal in the Southern Hemisphere
The concept of fall and its duration is directly reversed in the Southern Hemisphere due to the Earth’s axial tilt. When the Northern Hemisphere experiences autumn, the Southern Hemisphere experiences spring, and vice versa. Therefore, Southern Hemisphere fall occurs when the North is experiencing spring.
Astronomically, fall south of the equator begins with the March Equinox (around March 20th or 21st) and concludes with the June Solstice (around June 20th or 21st). Because of orbital mechanics, the astronomical duration of fall in the Southern Hemisphere is longer than in the North, averaging about 92.8 days. The meteorological season for the Southern Hemisphere is also shifted, covering the fixed period of March 1st through May 31st. This three-month block results in a fixed duration of 92 days for meteorological fall, regardless of the year.