September Equinox marks the arrival of the first day of spring in the southern hemisphere. That’s why in the earth’s southern areas, the September Equinox is known as the Spring Equinox. In 2021, the official first day of spring in Australia (southern hemisphere) will occur on Thursday, September 23, at sharp 5:21 Australian Eastern Standard Time (AEST).
On the other hand, for the people living in the northern hemisphere, this Equinox marks the arrival of the official first day of fall in the northern hemisphere. 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 Newyork (northern hemisphere) or Sydney (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 spring in the southern hemisphere is actually the official first day of spring or not? Therefore, 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.
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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 south and north. To put it differently, why when there is a spring equinox in the southern hemisphere, there is a fall equinox in the northern hemisphere? Let’s find out!!!
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Thanks to Earth’s Tilt, We See Equinoxes
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 southern and northern 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 Comparision||Southern Hemisphere||Northern Hemisphere|
|March Equinox||In the southern hemisphere, it marks the first day of fall, therefore, Fall or Autumn Equinox.||In the northern hemisphere, it marks the first day of spring, therefore, Spring or Vernal Equinox.|
|June Solstice||It marks the first day of winter, therefore, Winter Solstice.||It marks the first day of summer, therefore, Summer Solstice.|
|September Equinox||Marks the first day of spring, therefore, Spring Equinox.||Marks the first day of fall, therefore, Fall Equinox.|
|December Solstice||It marks the first day of summer, therefore, Summer Solstice.||It marks the first day of winter, therefore, Winter Solstice.|
September Equinox – First Day of Spring in Southern Hemisphere
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.
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So, how to define March Equinox or September equinox. OKAY, let me rephrase my question for you!!! What’s the difference between the march and September equinox? Any Guess???
Well, the primary difference between the March and September equinox is that during 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
Moving ahead, as the sun crosses over the imaginary celestial line from the north to the south direction. The earth’s southern areas start to warm up rapidly. Refer to the above image for proper understanding.
Hence, marking the end of the winters and the arrival of the first day of spring in the southern hemisphere. That’s why the September Equinox aka Southward Equinox is officially known as the Vernal or Spring Equinox 2021 in the southern hemisphere.
Equal Day and Night
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.
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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.
Longer Days and Shorter Nights in Southern Hemisphere
There are two ways to practically observe the first day of Spring. First, you will feel that the weather starts to turn warmer, trees begin to grow their leaves, plants start to flower, etc. Second, You can notice that the days are getting longer and the nights are getting shorter.
In other words, after September Equinox, the sun will start to move southward. Therefore, in the southern hemisphere, the days will become longer and longer with each passing day. In fact, this process of longer days shorter nights will go on until the arrival of the December solstice.
Spring Equinox – First Day of Spring or Mid-spring?
Now is the time that we should talk about the meteorological definition of the spring equinox. Well, technically speaking, if we go by the meteorological definition of the spring equinox, it marks the end of the half of the vernal or spring season i.e mid-spring.
However, on the other hand, if we go by the astronomical definition of the Spring Equinox, it marks the arrival of the spring season. 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 and observed with the naked eyes, which is in fact in the modern notation is known as the meteorological definition of Spring Equinox or simply Meteorological Spring.
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On the other hand, what our astronomers and scientists calculate with their super-sophisticated technology is defined as the astronomical definition of Spring Equinox or simply Astronomical Spring.
Therefore, to conclude, I would say that both definitions are correct in their own sense. Hence, as a result, some people define the Spring Equinox in the southern hemisphere as the first day of the spring season.
On the contrary, some people define it as the end of the half of the vernal or spring season, hence mid-spring. 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
|Seasons||Meteorological Season Dates||Astronimocal Season Dates|
|Fall Equinox||Monday, 1 March 2021||Saturday, 20 March 2021|
|Winter Solstice||Tuesday, 1 June 2021||Monday, 21 June 2021|
|Spring Equinox||Wednesday, 1 September 2021||Thursday, 23 September 2021|
|Summer Solstice||Wednesday, 1 December 2021||Tuesday, 21 December 2021|
When is the First Day of Spring 2021 in the Southern Hemisphere?
This year’s September Equinox i.e the Spring equinox in Australia (southern hemisphere) will happen on Thursday, September 23, at sharp 5:21 Australian Eastern Standard Time (AEST). Not to mention, this astronomical event will occur at the same moment for all of us.
I mean, whether you are living in New Zealand or in Australia, or in Newyork (i.e in the northern hemisphere), it would be a moment of joy for all of us at the same instant. Well, yeah, for American’s, it will be the Fall or Autumnal Equinox. I hope you know the exact reason behind it.
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- Difference Between Summer Solstice and Winter Solstice in Tabular Form