The term "fall" was popular at the time. However, the names for the season did not stop with autumn. Poets were still awestruck by the changes that autumn brought, and the phrase "the fall of the leaves" became synonymous with the season throughout time.
"Fall" is the elder of the two terms, having entered English in the 1300s from the Latin word "autumnus." This was abbreviated to autumn in the 1600s. The other term is "winter," which comes from the Latin word "venter," meaning "nose." When it is cold outside, we say that it is wintertime.
The changing colors of autumn are caused by the fact that green plants use light wavelengths between 400 and 700 nanometers for photosynthesis. Their pigment molecules are most sensitive to light within this range, so plants color themselves a warning signal: avoid any more of this light exposure or you'll suffer damage to your equipment.
As far as why we call it autumn instead of summer or winter, there is no real reason. These names just seem right to us.
The Latin term "autumn" first entered in English in the late 14th century and progressively surpassed "harvest." "Fall" entered usage in the 17th century, very definitely as a poetic counterpart to "spring," and it competed with the other words. At that time, "autumn" meant the season corresponding to autumn, while "fall" referred to any sharp decline or fall.
Nowadays, most people think of autumn as the time of falling leaves and decreasing daylight hours, but this tradition dates only from 1872, when Charles Francis Jenkins published an album of photographs he had taken during his travels around the world. This is when trees started changing color ahead of the actual season, and so photographers began calling it "the time of falling leaves." Before then, people just used whatever word they wanted for the season, which was usually based on what was growing wild where they lived.
There are many different theories about how "fall" came to mean the seasonal change we see now. Some say it has something to do with agriculture; others, with mourning. Some believe it's derived from the Norse word for fall, which is faaL, some say it comes from the Latin word for fall, which is fallere, and so on. But no one really knows for sure.
The origin of the term "fall" as a season name is unknown, however it is considered to have originated from the notion of leaves falling off trees (particularly the contraction of the English saying "fall of the leaf"). However, before the season was known as autumn or fall in English, it was known as "harvest."
Another theory is that it is because people used to call out "the fall of night" when darkness fell. Yet another theory is that it is because people would say that winter has fallen if it was actually cold outside instead of just feeling that way due to the presence of ice and snow. The final theory is that it is because of the idea that as the days get colder, the nights get warmer, and then one day in November when the temperatures drop suddenly, everyone says that fall has fallen.
Harvest time is when farmers harvest their crops. So, "fall" originally meant harvest time. Then, later on, it became associated with other things like colors of the foliage, sounds of nature, etc.
Autumn is the season after spring and before winter. It is named after the Latin word for free, which refers to the fact that plants release their seeds then. Also, there are many beautiful flowers in autumn that don't appear in spring such as roses and rhododendrons.
Fall is the period from autumn to winter defined by the absence of summer heat and the presence of frost.