Fall in the Northern Hemisphere officially begins with the Autumnal Equinox, the day (actually a moment in time) when the position of the sun in our sky crosses to the southern side of the celestial equator, having spent the previous six months shining more directly on the northern half of our globe. For those who dwell in the Northern Hemisphere, this means a period of transition: around the time of the Autumnal Equinox, day and night are of more or less equal length, following the longer days and shorter nights of summer and preceding the longer nights and shorter days of the winter season. (The word equinox comes from a Latin compound meaning “equal night,” although, due to the complex nature of astromechanics, the actual date on which the length of daytime and nighttime are closest to being equal tends to run a few days ahead of the spring equinox and lags a few days behind the fall equinnox, and also varies considerably according to the latitude of a given location.
In our modern Gregorian Calendar (which is a 16th century slight modification of the Julian Calendar, established way back in the time of Julius Caesar), the Autumnal Equinox falls, in any given year, on a date as early as September 21 and as late as September 24. The shifting of the sun’s apparent position throughout the year is caused not by any actual motion of the sun itself, but rather is the result of an effect created by the fact that earth’s axis is not perpendicular to the plane of its orbit around the sun, but is tilted at an angle of a little more than 23 degrees. This means that, for one half (six months) of the earth’s annual journey around the sun, one hemisphere is pointed more directly at the sun than the other, and then they trade places at the Equinoxes. (The corresponding Equinox in the month of March is called the Vernal Equinox by those who inhabit the Northern Hemisphere.) The hemisphere receiving more direct sunlight (and for a longer portion of each 24-hour day) experiences summer, and the hemisphere receiving less direct sunlight (and for a shorter portion of each 24-hour day) experiences winter.
The Equinoxes also mark the only 2 days in each year when, as seen from every position on the globe (except the poles), the sun rises exactly due east and sets exactly due west. Also, in the weeks immediately before and after the equinoxes, there is a more noticeable and rapid change in the length of daylight hours than at other times of the year. At its peak, the difference is as much as 3 minutes from one day to the next. That means that, around the time of the Autumnal Equinox, the length of daylight time on a given day is about 20 minutes shorter than it was just a week before – a noticeable difference that prompts the oft-heard “My, the days are certainly getting shorter!” and other similar comments during this season.
