September Equinox 2021 – First Day of Fall in Northern Hemisphere

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September Equinox marks the arrival of the first day of the fall in the northern hemisphere. That’s why in the earth’s northern areas, the September Equinox is known as the Fall Equinox. In 2021, the official first day of fall in the northern hemisphere will occur on Wednesday, September 22, at sharp 19:21 Universal Time Coordinate (UTC).

On the other hand, for the people living in the southern hemisphere, this Equinox marks the arrival of the official first day of the spring. In other words, the meaning of the Equinox changes depending on the fact that at which side of the earth you live upon.

Regardless of the fact whether you are living in London (northern hemisphere) or Canberra (southern hemisphere), the length of the day and night are almost exactly equal on the eve of the Equinox. CONFUSED??? Don’t worry, we will talk about it in the later section.

Moving ahead, in this article, we will talk about some of the exclusive facts like why there is a mismatch between the north and south equinoxes? Or, the first day of fall in the northern hemisphere is actually the official first day of fall or not? Therefore, in order to know that in detail, I would suggest you stick with me till the end. Let’s dive right in!!!

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What is Equinox?

There are two ways to define the meaning of equinox. These are astronomical and meteorological definitions of Equinox. Right now, I am going to explain it on the basis of astronomy. That doesn’t mean I won’t be explaining the meteorological definition of equinoxes. We will talk about it in the later section.

So, moving ahead, according to astronomy, this is a naturally occurring event during which the center of the sun appears to cross over the celestial equatorial line of the earth. In layman, one can say that an equinox occurs when the sun switches sides from one hemisphere of the earth to the other.

Not to mention, there are two types of equinoxes. Fall equinox, also known as the autumnal equinox that marks the first day of fall. And, the Spring Equinox, also known as Vernal equinox that marks the first day of spring.

Four Seasons

types-of-seasons-on-earth
Four types of seasons that occurs on earth: (above) Winter, Spring, (below) Summer, Autumn/Fall

In case if you don’t know, there are generally four seasons that occur on earth. These are summer, winter, autumn, and spring. Just like the fall and spring equinox is responsible for the arrival of fall and spring seasons on earth.

In a similar way, the winter and summer solstice is responsible for the arrival of winter and summer season on earth. To summarise, each year earth experiences 2 solstices (summer and winter) and 2 equinoxes (spring and fall) that help us to understand the patterns related to the changing seasons.

Now, the question arises that why there is a mismatch between the north and south. To put it differently, why when there is a fall equinox in the northern hemisphere, there is a spring equinox in the southern hemisphere? Let’s find out!!!

Thanks to Earth’s Tilt, We See Equinoxes

earth's-tilt-reason-for-seasons
Earth’s axial tilt (obliquity) is currently about 23.4°/Credit: Wikimedia Commons

Well, there is a small correction here. Not only we see equinoxes because of the earth’s tilt. We see solstices too. Moving ahead, just because the earth has an axial tilt of 23.4 degrees, there is a mismatch between the northern and southern hemispheres.

In fact, Kepler’s law also contributes to the formation of the different seasons across the globe. YES, YES, the same Kepler’s law that states that all the planets in the solar system revolve around the sun in an elliptical orbit.

However, Kepler’s law contribution can’t be compared to the role played by the earth’s tilt. I mean, if I say in terms of approximated ratios. Then it will be like 90:10.

Basis of ComparisionNorthern HemisphereSouthern Hemisphere
March EquinoxIn the northern hemisphere, it marks the first day of spring, therefore, Spring or Vernal Equinox.In the southern hemisphere, it marks the first day of fall, therefore, Fall or Autumn Equinox.
June SolsticeIt marks the first day of summer, therefore, Summer Solstice.It marks the first day of winter, therefore, Winter Solstice.
September EquinoxMarks the first day of fall, therefore, Fall Equinox.Marks the first day of spring, therefore, Spring Equinox.
December SolsticeIt marks the first day of winter, therefore, Winter SolsticeIt marks the first day of summer, therefore, Summer Solstice.
From the above comparison chart, it is clear that the equinox that occurs in September in the northern hemisphere is the Fall or Autumn Equinox. On the other hand, due to the earth’s tilt, the equinox that occurs in September in the southern hemisphere is the Vernal or Spring Equinox.

September Equinox – First Day of Fall in Northern Hemisphere

first-day-of-fall-in-northern-hemisphere
Autumn season in Indian Union Territory Of Jammu and Kashmir/Credit: The Statesman

As you already know that an equinox occurs when the center of the sun appears to cross over the celestial equatorial line of the earth. Moreover, you also know that there are two types of Equinox. These are march equinox and September equinox.

So, how to define March Equinox or even September equinox. OKAY, let me rephrase my question for you!!! What’s the difference between the march and September equinox? Any Guess???

Well, the only difference between the march and September equinox is that during the march equinox, the sun switches sides from the southern hemisphere to the northern hemisphere of the earth. That’s why the march equinox is also known as the Northward equinox.

On the other hand, during the September equinox, the sun switches sides from the northern hemisphere to the southern hemisphere of the earth. That’s why the September equinox is also known as the Southward Equinox.

September Equinox – Southward Equinox

earth's-illumination-during-different-season-in-different-hemisphere
Image Credit: Wikimedia Commons

Moving ahead, as the sun crosses over the imaginary celestial line from the north to the south direction. The earth’s northern areas start to cool down rapidly. Refer to the above image for proper understanding.

Hence, marking the end of the summers and the arrival of the first day of fall in the northern hemisphere. That’s why the September Equinox aka Southward Equinox is officially known as the fall or autumn equinox in the northern hemisphere.

Equal Day and Night

equal-day-and-night-during-first-day-of-fall-or-spring
Illumination of Earth by the Sun on the day of the equinox (fall or spring). Image Credit: Wikimedia Commons

Yup, you heard me right. We have equal day and night during September Equinox (also during March Equinox). WHY? Because when the sun crosses over the imaginary celestial line, the earth’s tilt becomes perpendicular to the sunshine. Or, you can say that on the eve of an equinox, the earth’s axis is neither tilted toward nor away from the sun.

Hence, daylight and nighttime hours are equal. In other words, we will have equal day and night i.e 12 hours. Don’t think that the earth’s axial tilt will become Zero during the equinoxes. In fact, it happens because of the sun’s passing over the Equatorial line.

Moving ahead, in the light of equal day and night phenomena, I just want to clarify that it’s not exactly equal. It’s nearly equal to 12 hours. Therefore, just for approximation, we simply say that during the vernal equinox and autumnal equinox, the length of day and night becomes equal.

Shorter Days Longer Nights in Northern Hemisphere

winter-after-first-day-of-fall-equinox-in-northern-hemisphere
Winter picks up pace after the arrival of fall in the northern hemisphere. Image Credit: Adobe Stock

There are two ways to practically observe the first day of fall equinox in the northern hemisphere. First, you can see the sudden change in the color of the leaves around you. I mean, the way your surrounding will become vibrant, it can’t go unnoticed. Second, You can notice that the days are getting shorter and the nights are getting longer.

In other words, after Fall equinox 2021, the sun will start to move southward. Therefore, in the earth’s northern areas, days will become shorter and shorter with each passing day. Not to mention, this process of shorter days longer nights will go on until the arrival of the December solstice.

Fall Equinox – First Day of Fall or Midfall?

Now is the time that we should talk about the meteorological definition of the fall equinox. Well, technically speaking, if we go by the meteorological definition, it marks the end of the half of the fall or autumn season i.e mid-fall.

However, on the other hand, if we go by the astronomical definition of the fall equinox, it marks the arrival of the first day of the fall. Therefore, the question arises that why we have two different definitions for a single event. More importantly, which one of these is the correct one? Let’s find out.

Let’s Go Back in Time

Before the arrival of the scientific revolution, humans didn’t know how to calculate the change in the season in terms of astronomical calculations. WHY? Because they didn’t have the telescope to do such nasty observations.

Therefore, what they used to do is to calculate the change in season as humanly as possible. With time, early humans understood that the sun seems to cross over the horizon after a fixed interval of time. Additionally, they also understood the pattern of the annual temperature cycle.

In other words, what early humans used to do is to calculate the relative things, like what they felt or observed with the naked eyes, which is in fact in the modern notation is known as the meteorological definition of fall equinox or simply meteorological fall.

On the other hand, what our astronomers and scientists calculate with their super-sophisticated technology is defined as the astronomical definition of fall equinox or simply astronomical fall.

Therefore, to conclude, I would say that both definitions are correct in their own sense. Hence, as a result, some people define the fall equinox in the northern hemisphere as the first day of the fall.

On the contrary, some people define it as the end of the half of the fall or autumn season, hence mid-fall. Not to mention, the meteorological seasons always arrive approx 20 days before the arrival of astronomical seasons. To understand it more clearly, refer to the below table.

 

Meteorological vs Astronomical Seasons

SeasonsMeteorological Season DatesAstronomical Season Dates
Spring EquinoxMonday, 1 March 2021Saturday, 20 March 2021
Summer SolsticeTuesday, 1 June 2021Monday, 21 June 2021
Autumn EquinoxWednesday, 1 September 2021Thursday, 22 September 2021
Winter SolsticeWednesday, 1 December 2021Tuesday, 21 December 2021
All the seasons (meteorological and astronomical) dates are strictly in accordance with the northern hemisphere of the earth.

 

When is the First Day of Fall 2021 in the Northern Hemisphere?

This year’s September Equinox i.e the Fall Equinox in the northern hemisphere will happen on Wednesday, September 22, at sharp 19:21 Universal Time Coordinate (UTC). Not to mention, this astronomical event will occur at the same moment for all of us.

I mean, whether you are living in India or in Australia (in the southern hemisphere), it would be a moment of joy for all of us at the same instant. Well, yeah, for Australian’s, it will be the Vernal or Spring Equinox. I hope you know the exact reason behind it.

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I am a mechanical engineer by profession. Just because of my love for fundamental physics, I switched my career, and therefore I did my postgraduate degree in physics. Right now I am a loner (as ever) and a Physics blogger too. My sole future goal is to do a Ph.D. in theoretical physics, especially in the field of cosmology. Because in my view, every aspect of physics comes within the range of cosmology. And I love traveling, especially the Sole one.

2 thoughts on “September Equinox 2021 – First Day of Fall in Northern Hemisphere”

  1. How can the September Equinox fall on Thursday, 22 September 2021. Please note that 22 September 2021 is a Wednesday !! Please correct your webpage!

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