September Equinox Marks the Start of Fall 2022

Complemented by cooler temperatures and falling leaves, the September equinox marks the beginning of the fall season for the Northern Hemisphere. This year’s autumnal equinox (for the Northern Hemisphere) or spring equinox (for the Southern Hemisphere) occurs on Sept. 22 at 8:04 p.m. CDT.

An illustration of the March (spring) and September (fall or autumn) equinoxes.
An illustration of the March (spring) and September (fall or autumn) equinoxes. During the equinoxes, both hemispheres receive nearly equal amounts of daylight. (Image not to scale) Credits: NASA/GSFC/Genna Duberstein

During an equinox the Sun shines directly over the equator resulting 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, according to Alphonse Sterling, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama. Following the autumnal equinox, the Sun gradually continues to rise later and set earlier in the Northern Hemisphere – making the days shorter and the nightfall longer. The opposite is true in the Southern Hemisphere where the days begin to last longer.

Seasons are caused by 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 an Assistant Manager in the Heliophysics and Planetary Science Branch at Marshall. “In the Southern Hemisphere, sunlight is less direct (winter), which means that the ground is not heated as easily.”

A visual aid to better understand how the Earth's tilted axis causes the different seasons throughout the year in the Northern and Southern Hemispheres.
A visual aid to better understand how the Earth’s tilted axis causes the different seasons throughout the year in the Northern and Southern Hemispheres. Credit: NASA/Space Place

Astronomical seasons are defined by the Earth’s journey around the Sun, while 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. 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 and say hello to early sunsets and late sunrises.

by Lane Figueroa

September Equinox 2021 is Coming!

In meteorology, the fall season begins on Sept. 1, however, the September (or fall) equinox gives us the green light to welcome the astronomical fall season in the Northern Hemisphere (and astronomical spring season in the Southern Hemisphere). This happens Sept. 22, 2021, at 19:21 UTC, which is 2:21 p.m. CDT for us in North America.

illustration of the March (spring) and September (fall or autumn) equinoxes
An illustration of the March (spring) and September (fall or autumn) equinoxes. During the equinoxes, both hemispheres receive equal amounts of daylight. Credit: NASA/JPL-Caltech

Along with marking the beginning of astronomical fall, the Sun will be exactly above Earth’s equator, moving from north to south, making day and night nearly equal in length – about 12 hours – throughout the world.

At the North Pole, over the upcoming days, the Sun will sink below the horizon for a kind of twilight from now until sometime in October when it will be completely dark, according to NASA solar scientist Mitzi Adams. Spring twilight at the North Pole begins a few weeks before the vernal, or spring, equinox in March, when the Sun rises above the horizon again.

This only happens twice in Earth’s year-long trip around the Sun. The rest of the year, the Sun shines unevenly over the Northern and Southern Hemispheres. That’s because Earth’s axis is tilted with respect to the Sun-Earth plane. But on these special days – the spring and fall equinoxes – the Sun shines almost equally on the Northern and Southern hemispheres.

Equinox Solstice Info Graphic
Click to view larger. Credit: NASA/Space Place

In the Northern hemisphere, the September equinox marks the start of a period bringing us later sunrises and earlier sunsets. We will also feel cooler days with chillier winds, and dry, falling leaves.

The people of ancient cultures used the sky as a clock and calendar. They knew that the Sun’s path across the sky, length of daylight, and location of sunrise and sunset all shifted in a regular way throughout the year. Additionally, earlier civilizations built the first observatories, like Stonehenge in Wiltshire, England, and the Intihuatana stone in Machu Picchu, Peru, to follow the Sun’s annual progress.

Today, we celebrate the equinox as an astronomical event caused by Earth’s tilt on its axis and its motion in orbit around the Sun.

Enjoy the new season – whichever side of the globe you’re on!

by Lance D. Davis