When fall officially begins is often confusing because there are two separate, yet valid, systems for defining the start of the seasons. These two definitions offer different starting dates based on focusing on either a precise celestial event or a practical method for tracking climate data. Understanding the logic behind each system helps to clarify why the seasonal calendar appears to have a dual nature.
The Scientific Start: Fall Equinox
The most traditional and globally recognized start of fall is determined by the Earth’s position as it travels around the sun, an event known as the autumnal equinox. This specific moment occurs when the sun’s rays are directly overhead at the equator, meaning the Earth is not tilted toward or away from the sun. The term “equinox” comes from the Latin for “equal night,” reflecting that daylight and nighttime hours are nearly balanced across the globe.
The equinox marks the exact astronomical transition from summer to fall in the Northern Hemisphere. Its precise date is not fixed, typically falling on September 22nd or 23rd. This slight annual shift happens because an Earth year is not exactly 365 days long, requiring the adjustment of leap years.
The Earth’s orbit around the sun is elliptical, which causes the length of the astronomical seasons to vary slightly. This variability presents a challenge for consistent, long-term data collection, but the definition remains rooted in the physical mechanics of the Earth-Sun system.
The Practical Start: Meteorological Fall
A second, more practical definition for the start of fall is used by climate scientists and weather forecasters, called meteorological fall. This system was created to impose consistency on seasonal data for easier comparison and statistical analysis. It divides the year into four standardized seasons, each lasting exactly three full months.
Under this system, fall for the Northern Hemisphere is always defined as the three-month period of September, October, and November. The start of meteorological fall is fixed on September 1st every year, concluding on November 30th. This structure aligns the seasons with the annual temperature cycle.
The primary benefit of this standardized approach is that it allows climatologists to calculate and compare seasonal statistics, such as average temperature and total precipitation, with greater ease and reliability. By removing the yearly variability of the astronomical calendar, this method provides a consistent framework for tracking climate trends and creating seasonal forecasts.
Resolving the Dual Definition
The existence of two starting dates for fall means that both are technically correct, but they serve different purposes. The autumnal equinox represents the true physical moment when the Earth reaches a specific point in its orbit, making it the globally recognized astronomical start of the season. This definition is based on the objective, measurable mechanics of the solar system.
In contrast, meteorological fall, beginning on September 1st, is a construct designed for statistical convenience and operational consistency. It is the definition most often cited in media and government climate reporting due to its utility in data comparison and forecasting. The dual system is necessary because the precision of celestial mechanics does not lend itself well to the practical needs of statistical analysis.