Happy Autumnal Equinox!
An equinox is an astronomical event in which the plane of Earth's equator passes the center of the Sun.  Equinoxes occur twice a year, around 21 March and 23 September.
The four key dates in the solar calendar, the September Equinox.
 In the Northern Hemisphere (NH), the day after Equinox the sub-solar point begins its move into the Southern Hemisphere (SH).  So, north of the Equator, our daylight hours decline and night time hours increase.  This is our Autumn.  In the SH, just the opposite.  Their day light hours begin to increase and their hours of night begin to decrease.  For the SH, Spring has begun.
1. There are two equinoxes annually, vernal and autumnal, marking the beginning of spring and fall. They are opposite for the northern and southern hemispheres.

2. The autumnal equinox happens the moment the sun crosses the celestial equator, which is an imaginary line in the sky that corresponds to Earth’s equator. Every year this occurs on September 22, 23, or 24.

3. From hereon, the days get shorter until the winter solstice in December, when the light will begin its slow climb back to long summer days. Winter solstice is technically the shortest day of the year, while the summer solstice in June boasts the most sunlight. Hence, the four season, as illustrated below.
Again, all this courtesy of the earth's oval-ish rotation around the sun.
Below are four seasonal gateway:
Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its normally majestic ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth.

Day arc at 0° latitude (Equator) The arc passes through the zenith, resulting in almost no shadows at high noon.

When the planet Saturn is at equinox, its rings reflect little sunlight, as seen in this image above.

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