The official start of autumn is not a singular event, with multiple calendars defining the season's beginning on different dates. This divergence can lead to confusion about when the autumnal period truly commences.
The astronomical calendar marks the start of autumn with the autumnal equinox, which typically falls around September 22 or 23 in the Northern Hemisphere. This date is determined by the Earth's tilt and its position in its orbit around the sun.
In contrast, the meteorological calendar defines the seasons based on annual temperature cycles. This system divides the year into four three-month periods, with autumn beginning on September 1 and concluding on November 30. BBC NI weather presenter Barra Best explained that this approach allows meteorologists to "compare one season with another and look back through decades of weather records." He uses this framework to contextualize current weather conditions, such as comparing summer temperatures to long-term averages or assessing how a season measures up against previous years.
However, Best cautions that the weather does not abruptly change with the calendar. "We can still have very summery weather well into meteorological autumn, or wintry conditions during spring. The calendar simply gives meteorologists a consistent framework for recording and comparing our climate," he stated.
A third perspective comes from the Celtic calendar, which traditionally observed Samhain on October 31 as the end of summer and the beginning of the darker half of the year. While not a direct start date for autumn in the same way as the other calendars, it marks a significant transition in the seasonal cycle.
These differing definitions highlight the varied ways societies and scientific disciplines interpret and categorize the passage of time and the natural world. The astronomical start is tied to celestial mechanics, the meteorological start is based on climate patterns, and traditional calendars often reflect cultural observances and agricultural cycles. This multiplicity of dates underscores the different needs and perspectives that shape our understanding of the seasons.