The autumnal equinox is an astronomical event that happens every year and officially marks the beginning of fall. On the day of the autumnal equinox, the day and night are both approximately of equal length.
There are two equinoxes every year, and they are often referred to as the March (spring or vernal equinox) and the September (autumn) equinox. They usually occur on the same date or a day later or before every year. The autumnal equinox usually starts on either September 22 or 23, depending upon the year. When this happens, it is officially the beginning of autumn.
Equinoxes are the only astronomical times when the sun shines directly above the equator, and then moves south after the autumnal equinox and north in the case of the spring equinox. During both annual equinoxes, both day and night are about the same length all over the world, and as such, equinox comes from the Latin word aequinoctium (basically meaning equal night). It is when the center of the sun spends an approximately equal amount of time both above and below the horizon.
The sun crosses the celestial equator (an imaginary line that parallels the equator on earth) from north to south (in the northern hemisphere). Since seasons are the opposite on each side of the equator, the autumnal equinox in the northern hemisphere actually corresponds to the spring or vernal equinox in the southern hemisphere.
During every day of the year, the earth either tilts its axis away from or towards the sun, on an equinox however, the earth does not tilt away or towards the sun, and so this is why there is an approximate balance between day and night in different places around the world during such a time.
It is a phenomenon that has happened as long as the earth and sun have been in existence, even Julius Caesar, who established his calendar in 45BC (and which was used in the western world at least, before Pope Gregory XIII established the Gregorian calendar and replaced it) marked the dates of both annual equinoxes.
Dates have varied slightly over time, but they have always occurred during the months of March and September.
Pagans often conduct celebrations on the first day of fall, and so they celebrate the autumnal equinox, which in this case, corresponds to the festival called Mabon. Mabon is a mid-harvest festival and is often regarded as a time of reflection, as light and dark become equal in balance. It’s often a time to honor and welcome the change in seasons and the second harvest.
The autumnal equinox happens each year during September 22 or 23, depending upon that particular year. It is a phenomenon when the day and night are of approximately equal length all over the world, as the earth does not tilt in any direction away from the sun, and the sun shines directly above the equatorial line.