March Equinox Brings 2 Seasons: Spring, Autumn

The March equinox – also called the vernal equinox – is the beginning of the spring season in the Northern Hemisphere and autumn season in the Southern Hemisphere. It arrives on March 20, 2021, at 09:37 UTC (Coordinated Universal Time) or 4:37 a.m. CDT (Central Daylight Time).

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

During this equinox, the Sun will shine directly on the equator with nearly equal amounts of day and night, about 12 hours. Throughout the world, the Northern and Southern hemispheres will get equal amounts of daylight.

Click to view larger. Credit: NASA/Space Place

The equinoxes and solstices are caused by Earth’s tilt on its axis and ceaseless motion in orbit. Think of an equinox as happening on the imaginary dome of our sky, or as an event that happens in Earth’s orbit around the Sun.

In the Northern Hemisphere, the March equinox will bring us earlier sunrises, later sunsets, softer winds, and budding plants. With the opposite season, south of the equator, there will be later sunrises, earlier sunsets, chillier winds, and dry, falling leaves.

If you’re in the Northern Hemisphere, start watching the Sun as it sets just a bit farther north on the horizon each evening until the summer solstice. Also, enjoy the warmer weather and extended daylight!

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.

Look For The Harvest Moon This Weekend

Take a moment to gaze at the beautiful harvest moon this Saturday, September 29th.

(Image credit: NASA)

The harvest moon gets its name from agriculture. In the days before electric lights, farmers depended on bright moonlight to extend the workday beyond sunset. It was the only way they could gather their ripening crops in time for market. The full moon closest to the autumnal equinox became “the harvest moon,” and it was always a welcome sight.

Northern summer changed to fall last Saturday, Sept. 22nd, and is called the autumnal equinox. The word equinox comes from the Latin words for “equal night.” The fall and spring equinoxes are the only days of the year in which the Sun crosses the celestial equator.

Keep an eye on the moon as it creeps above the eastern skyline. The golden sphere may appear inflated. This is the moon illusion at work. This optical illusion is caused by the moon’s proximity to distant objects. A harvest moon inflated by the moon illusion is simply beautiful to us, but even more so to the farmers getting their crops in on those cool autumn evenings.

(Image credit: Stefano De Rosa)