A season is a particular period of the year. In the West, astronomers and climatologists knew long ago there are four seasons in a year, including winter, spring, summer and fall or autumn. Winter is known to occur between December and February, spring between March and May, summer between June and August, while autumn will occur between September and November. No day of the month is precise to signify the beginning of any of the seasons; but the particular one that sparks the beginning of autumn is what is known as autumnal equinox.
So, on what day exactly is autumnal equinox? There are only indicators but no clear fix. A popular indicator gives the autumnal equinox a date near September 22 in the northern hemisphere.
Another indicator predicts a date when night and day are nearly of the same length. Of course, this prediction is justified going by the meaning of equinox, the Latin form of which was formed from the two words “æquus” meaning “equal” and “nox” meaning “night”. Just after this period, the people in the northern hemisphere start to experience a longer night and shorter day.
In the northern hemisphere, a third indicator predicts autumnal equinox as a date when the Sun crosses the celestial equator moving southwards. When this occurs, the earth experiences a declination equal to 0 degrees. Declination was measured as the angular difference between true north and the horizontal trace of the magnetic field of the equator.
In the southern hemisphere, autumnal equinox is the date when the center of the Sun crosses the celestial equator moving northwards and occurring on the day of the vernal equinox. When this occurs, the Sun is directly above the equator.
The autumnal equinox may be predicted using the Gregorian calendar, released in 1582 and widely used in the Western world. In this calendar, the occurrence of leap year once in four years has a tremendous effect on the correct prediction of the date of autumnal equinox. Clearly, there should be no argument about this. If one adds one day every four years, then there is no way the prediction would
remain the same. So, the autumnal equinox gets a new date and time every year
The astronomy of the equinoxes may be used to predict the meaning of autumnal equinox. This equinox signifies a key turning point in the orbiting pattern of the earth; the ecliptic apparent path of the Sun intersect the celestial
equator to usher in the autumn season in the northern hemisphere.
Finally, it is clear the autumnal equinox is the sole determinant of the seasons of the world. It is a very important parameter in determining many of the predictions
creditable to weather forecasters as well as astronomers.