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

The Autumnal Equinox is Near

Happy equinox, Earthlings! Sept. 22 marks the fall equinox, when day and night are nearly equal.

“However, that day/night length depends on where you are on Earth,” said NASA solar scientist Mitzi Adams. “For example, at the North and South Poles, the length of the day and night is six months!”

At the North Pole, the Sun will sink below the horizon for a kind of twilight from now until sometime in October when it will be completely dark, explained Adams. Spring twilight begins a few weeks before the vernal, or spring, equinox in March, when the Sun rises above the horizon again.

Autumnal Equinox
Credit: NASA/JPL-Caltech

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 equinox – the Sun shines equally on both north and south.

Here in the Northern Hemisphere, it’s the first day of astronomical fall. From now until the beginning of spring, nighttime hours will last longer than daylight as the Sun travels a shorter arc across the sky each day. The Sun has its shortest path of the year at the time of the winter solstice — the shortest day and longest night of the year — when sunrise and sunset are as far south as they can go (at any one location). It’s just the opposite in the Southern Hemisphere, where September 22 kicks off astronomical spring.

The equinox—meaning “equal night” in Latin—occurs at 8:31 a.m. CDT.

Here Comes the Sun… and Moon!

Happy equinox, Earthlings! March 20 marks the spring equinox, one of two seasonal markers in Earth’s year-long orbit when the Sun appears to shine directly over the equator, and daytime and nighttime are nearly equal lengths–12 hours–everywhere on the planet.

During the equinoxes, both hemispheres receive equal amounts of daylight. (Image not to scale.)
During the equinoxes, both hemispheres receive equal amounts of daylight. (Image not to scale.) (NASA/GSFC/Genna Duberstein)

It’s the start of astronomical spring in the Northern Hemisphere, meaning more sunlight and longer days. From here until the beginning of fall, daytime will be longer than nighttime as the Sun travels a longer, higher arc across the sky each day, reaching a peak at the start of summer. It’s just the opposite in the Southern Hemisphere, where March 20 marks the fall equinox.

What’s more? The first full Moon of spring will rise tonight, lighting the skies on the equinox. Usually, a full Moon arrives a few days to weeks before or after the equinox. It’s close, but not a perfect match. Tonight’s full Moon, however, reaches maximum illumination less than four hours after the equinox. There hasn’t been a comparable coincidence since the spring equinox in 2000.

When the Moon, on its orbit around Earth, reaches the point farthest from the Sun, we see a full Moon.
When the Moon, on its orbit around Earth, reaches the point farthest from the Sun, we see a full Moon. (NASA/GSFC/Genna Duberstein)

And because the Moon is near perigee, it qualifies as a supermoon–the third and final of 2019. It’s not a big supermoon, so you won’t really be able to see the difference between this full Moon and any other one with your eyes. But keep an keep an eye on the Moon as it rises and creeps above the eastern skyline. A low-hanging Moon can appear strangely inflated. This is the Moon illusion at work.

Super or seemingly not, it’s a rare celestial coincidence to usher in springtime.