This year's autumnal or September equinox occurs at 03:09 Universal Time (UTC) on September 23. In my local time, Eastern Daylight Time (EDT), it's 23:09, 11:09 PM, on September 22.
The Earth's seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honor, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes.
- Wikipedia: Equinox
This image shows the orientation of the Earth from the perspective of the Sun at the March/Vernal Equinox: North is to the upper right, and Earth orbits to the left. At the September/Autumnal Equinox, the only difference is that North would appear to the upper left from the same perspective. Illustration: Dennis Nilsson
Bas-relief in Takht-e Jamshid (Throne of Jamshid), Parseh, also known as Persepolis. On the day of an equinox, the power of an eternally fighting bull (personifying the Earth) and that of a lion (personifying the Sun) are equal. The September equinox marks the first day of Mehr or Libra in the Persian calendar. Photo: Anatoly Terentiev
So, what's with the chick with the fruit at the top of this post? Persephone/Proserpina was the daughter of Demeter/Ceres and Zeus/Jupiter. Demeter hid her daughter from the other gods, but Hades/Pluto abducted her:
She was innocently picking flowers with some nymphs — Athena, and Artemis, the Homeric hymn says — or Leucippe, or Oceanids — in a field in Enna when Hades came to abduct her, bursting through a cleft in the earth. Later, the nymphs were changed by Demeter into the Sirens for not having interfered. Life came to a standstill as the devastated Demeter, goddess of the Earth, searched everywhere for her lost daughter. Helios, the sun, who sees everything, eventually told Demeter what had happened.But it's not Persephone's return to the underworld that brings on Winter. Demeter was the goddess of the harvest, agriculture, forests, and the earth. It's Demeter's grief for her daughter that that cools the nights, shortens the days, triggers the harvest, and brings on the "sleep" of the earth.
Finally, Zeus, pressed by the cries of the hungry people and by the other deities who also heard their anguish, forced Hades to return Persephone. However, it was a rule of the Fates that whoever consumed food or drink in the Underworld was doomed to spend eternity there. Before Persephone was released to Hermes, who had been sent to retrieve her, Hades tricked her into eating pomegranate seeds, which forced her to return to the underworld for the winter each year.
- Wikipedia: Persephone
LinksU.S. Naval Observatory: The Seasons and Earth's Orbit
Wikipedia: Equinox, Persephone