The arrival of fall, often called autumn, signifies a period of transition between the warmth of summer and the cold of winter. This seasonal shift is marked by noticeable changes in weather and the duration of daylight. While many recognize this change by dropping temperatures and colorful foliage, the official beginning of the season is defined in two distinct ways. Understanding these definitions clarifies why different sources might cite different dates for the start of fall.
The Astronomical Marker
The astronomical definition identifies the start of fall with a precise moment in Earth’s orbit known as the Autumnal Equinox. This event marks the official end of summer and the beginning of autumn in the Northern Hemisphere. The date is not fixed because Earth’s orbit around the sun takes approximately 365.25 days, causing the equinox to shift slightly each year. The Autumnal Equinox typically falls on September 22nd or September 23rd, though it can occasionally occur on September 21st or September 24th.
The term “equinox” comes from the Latin words aequus (equal) and nox (night), referencing the near-equal length of daylight and darkness worldwide on that date. From this point forward, the Northern Hemisphere experiences progressively shorter days and longer nights until the Winter Solstice.
The Mechanics of the Equinox
Earth’s Axial Tilt
The equinox is a direct result of Earth’s axial tilt and its continuous journey around the sun. Our planet’s axis of rotation is tilted at an angle of approximately 23.5 degrees relative to its orbital plane. This constant tilt is the primary driver of the seasons, as it changes which hemisphere receives the most direct sunlight throughout the year.
Alignment and Sunlight
The Autumnal Equinox occurs when the Earth reaches a specific point in its orbit where the tilt is aligned such that the sun’s rays are shining directly onto the equator. At this exact moment, the planet’s axis is tilted neither toward nor away from the sun. This alignment causes the solar terminator, the line separating day from night, to pass directly through both the North and South Poles, illuminating both hemispheres equally.
Defining Fall by Weather Patterns
Beyond the astronomical measurement, an alternative method for defining the seasons is used by weather professionals. This system, known as the meteorological calendar, divides the year into four seasons based on fixed dates to align with the annual temperature cycle. Meteorological seasons are designed to simplify the tracking and comparison of seasonal weather statistics and climate data.
Under this system, the start of fall is set on September 1st and lasts for three months, concluding on November 30th. This fixed-date approach groups months with similar temperature characteristics, making it easier for climatologists and weather forecasters to calculate seasonal averages. Meteorological fall consistently includes the months of September, October, and November in the Northern Hemisphere.