The September equinox signifies the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere. It occurs when the Sun is directly over the equator, leading to nearly equal day and night lengths worldwide.
Complemented by cooler temperatures and falling leaves, the September equinox heralds the official beginning of the fall season for the Northern Hemisphere. For 2022, this year’s autumnal equinox (for the Northern Hemisphere) or spring equinox (for the Southern Hemisphere) occurs on September 22 at 8:04 p.m. CDT.
During an equinox, the Sun shines directly over the equator, according to Alphonse Sterling, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama. He says that this results in nearly equal amounts of day and night throughout the world – except for the North and South Pole where the Sun approximately straddles the horizon for the entire day. Following the autumnal equinox, the Northern Hemisphere gradually experiences later sunrises and earlier sunsets – making the days shorter and the nightfall longer. In the Southern Hemisphere, the opposite is true – the days begin to last longer.
Seasons are the results of Earth’s tilted axis, which always points in the same direction. As Earth orbits around the Sun, the angle of sunlight that the Northern and Southern Hemispheres receive is different.
“On the June solstice (summer) in the Northern Hemisphere, sunlight is more direct, so it warms the ground more efficiently,” said Mitzi Adams. She is an Assistant Manager in the Heliophysics and Planetary Science Branch at NASA’s Marshall Space Flight Center. “In the Southern Hemisphere, sunlight is less direct (winter), which means that the ground is not heated as easily.”
While astronomical seasons are defined by the Earth’s journey around the Sun, meteorological seasons are guided by annual temperature cycles. Meteorologists group the seasons into time periods that line up with the weather and monthly calendar: December through February is winter, March through May is spring, June through August is summer, and September through November is fall.
On the other hand, astronomical seasons are marked by the equinoxes and solstices that each happen twice a year. Solstices are when the Sun appears to reach the lowest or highest point in the sky all year; they mark the beginning of summer or winter. Solstices are commonly referred to as the longest (summer solstice) or shortest (winter solstice) day of the year.
The September equinox is a time that welcomes Earthlings to a new season. To those in the Northern Hemisphere, enjoy the beginning of milder weather after a hot summer and say hello to early sunsets and late sunrises.
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