Among astronomers, the general consensus for the start of the fall season is when the sun's central disk traverses the celestial equator in a southward direction, normally occurring around the third week of September. However, meteorologists mark the beginning of autumn on September 1.
In 2014, the autumnal equinox marked the onset of fall on September 22, based on astronomical data. Around this time, day and night are roughly of the same duration of 12 hours each. In the Northern Hemisphere, the length of day-night equality stretches out for several days after the equinox.
In meteorology, the seasons are usually divided into 3-month periods. A standard date for the start of each season is vital in comparing and analyzing seasonal weather patterns.