The beginning of the fall is marked by days that become shorter, leaves that change color, the same length of days and nights and drastic drops in temperature. There is no exact date for when the fall begins. In reality, it really depends on which calendar you are following. However, the fall normally lasts between the months of September and November. To determine the date, you can follow the weather calendar or the astronomical calendar. There is usually a difference of three weeks from the meteorological start day to the astronomical start day. Some people still determine the beginning of autumn by observing the behavioral change of plants and animals. Even with the use of calendars and the observation of signs, the beginning of autumn varies from year to year.
The metrological calendar
The meteorological calendar uses the Gregorian calendar to divide the year into the four seasons, each lasting three months. The dates of the meteorological calendar are usually fixed. When a certain date arrives that marks the beginning of a season, the season is considered to have begun, even if the signs of the season have not yet been revealed. According to the meteorological calendar, autumn should last from September 1 to November 30, before the beginning of winter, which will begin in December 1st and will last until the end of February. These fixed dates make it easier to observe and compare the seasons.
The astronomical calendar
Astronomers determine the dates on which the seasons begin based on the events of the celestial system and the solar system. In case of fall, astronomers determine its onset when the autumn equinox appears to be roughly the same. This is when the length of the day is shortened, thus becoming approximately equal to the length of the night in the northern hemisphere. An equinox occurs when the equator passes through the center of the sun, the north and south poles align towards the sun, a situation that gives the two hemispheres almost the same amount of daylight.
Phenology and natural observations
A third theory of when the beginning of the fall is based on the study of periodic plants and life cycles of animals, behavior and events. The beginning of the fall according to this theory is not dictated by a single date that was previously determined. It is determined by events such as the coloring of trees and the gradual maturation of autumn fruits. An example of fruit that could indicate the beginning of autumn is the maturation of the blackberry. When the leaves begin to change color, it is likely that the fall has begun. The leaves form a red or yellow tint because they stop producing chlorophyll, the substance that gives them the green color. Chlorophyll production is slow and minimal at the end of summer. This phenomenon gives the leaves a reddish and yellowish hue.