Goodbye summer, Monday, September 22nd is the autumn equinox.

Goodbye summer. At 8:19 PM Italian time on Monday, September 22nd , the autumnal equinox will occur. The Sun will be high above the equator, and the hours of daylight and darkness will be the same across the globe , marking the arrival of autumn in the Northern Hemisphere and spring in the Southern Hemisphere . A spectacular Saturn will greet the event, visible under the best conditions. Today, the planet is at its closest to Earth and in opposition. This means that Earth is aligned between the Sun and Saturn, which therefore appears particularly bright.

At the equinoxes, "the Sun rises exactly in the east and sets perfectly in the west. Also at the equinoxes, sunsets and sunrises are faster, referring to the time it takes for the entire solar disk to disappear, net of the atmospheric effects of refraction and diffraction of sunlight," observes astrophysicist Gianluca Masi, scientific director of the Virtual Telescope.

The equinox occurs when the Sun reaches its highest point ( zenith ) on the equator and its rays fall perpendicular to the Earth's axis of rotation . At that moment, at every point on the planet, day and night have equal length. Since the length of the solar year does not correspond exactly to the time it takes the Earth to describe a complete orbit around the Sun, the day on which the autumnal equinox falls is not always the same every year, but varies between September 21 and 24 .

From the equinox, he adds, "the Northern Hemisphere will see the length of the day steadily shorten in favor of that of the night, to the delight of stargazers, until the culmination of the December solstice, the winter solstice. For the Southern Hemisphere, the night will become increasingly thinner, moving towards summer. At the North Pole, then, the long Arctic night begins, while at the South Pole, instead, the interminable Antarctic day."

Masi also notes that "due to the ellipticity of the Earth's orbit, the four astronomical seasons into which the year is divided do not all have the same length. The longest is our summer: 93.65 days, the shortest is winter, with 88.99 days. In our hemisphere, the spring + summer pair totals 186.4 days, while the autumn + winter pair totals 178.84 days."

Given the Sun's particularly active period, intense aurora borealis is expected to occur at high latitudes due to the so-called " Russell-McPherron " effect. This effect predicts that in the days around the equinox, the geometry of the Earth's magnetic field may change in a way that more easily favors the entry of charged particles from the Sun.