Occasionally I’ve been asked what is the difference between astronomical seasons and meteorological seasons. Deke Arndt of the National Centers for Environmental Information has a nice simple write-up that explains the difference at https://www.ncdc.noaa.gov/news/meteorological-versus-astronomical-seasons.
EarthSky also has a good description of what the autumnal equinox is at https://earthsky.org/astronomy-essentials/everything-you-need-to-know-september-equinox.
