Autumn (also known as fall in American English) is one of the four temperate seasons. Autumn marks the transition from summer into winter, usually in March (Southern Hemisphere) or September (Northern Hemisphere) when the arrival of night becomes noticeably earlier.
The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region, so some cultures regard the autumnal equinox as "mid-autumn" whilst others treat it as the start of autumn.
In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September, October and November. However, according to the Irish Calendar which is based on ancient Celtic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In Australia autumn officially begins on 1 March and ends 31 May. The vast diversity of the ecological zones of the Australian continent renders the rigid American seasonal calendar an imposed cultural concept rather than relevant to climactic conditions. The seasonal cycles as named and described by the various indigenous Aboriginal peoples of Australia differ substantially from one another according to their local geographical and ecological environment and are intricately dependent on local environmental events and resources.
The word autumn comes from the Old French word autompne (automne in modern French), and was later normalised to the original Latin word autumnus. There are rare examples of its use as early as the 12th century, but it became common by the 16th century.
Before the 16th century, harvest was the term usually used to refer to the season. However, as more people gradually moved from working the land to living in towns (especially those who could read and write, the only people whose use of language we now know), the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, and autumn, as well as fall, began to replace it as a reference to the season.
The alternative word fall is now mostly a North American English word for the season. It traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are clearly derived either from a common root or from each other. The term came to denote the season in 16th century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year".
During the 17th century, English emigration to the colonies in North America was at its peak, and the new settlers took their language with them. While the term fall gradually became obsolete in Britain, it became the more common term in North America, where autumn is nonetheless preferred in scientific and often in literary contexts.
Association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains that ripen at this time. Most ancient cultures featured autumnal celebrations of the harvest, often the most important on their calendars. Still extant echoes of these celebrations are found in the mid-autumn Thanksgiving holiday of the United States, and the Jewish Sukkot holiday with its roots as a full moon harvest festival of "tabernacles" (huts wherein the harvest was processed and which later gained religious significance). There are also the many North American Indian festivals tied to harvest of autumnally ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, and many others. The predominant mood of these autumnal celebrations is a gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather.