December Solstice 2021 – First Day of Summer in Southern Hemisphere

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December Solstice marks the arrival of the first day of summer in the southern hemisphere. That’s why in the earth’s southern areas, let’s say for Australian people, December Solstice is known as the Summer Solstice or Estival Solstice.

In 2021, the summer solstice in Australia (southern hemisphere) will occur on Wednesday, December 22, at sharp 02:59 Australian Eastern Daylight Time (AEDT). Not to mention, this astronomical event will also mark the brightest as well as the longest day of the year in the southern hemisphere.

On the other hand, for people living in the northern hemisphere, this solstice marks the arrival of the first day of winter. That’s why in the earth’s northern areas, December Solstice is known as the Winter or Hibernal Solstice. Again, for the people living in the northern half of the earth, this astronomical event will also mark the darkest as well as the shortest day of the year.

Simply speaking, one can say that the meaning of solstice changes depending on the fact that at which side of the hemisphere you live upon. Moving ahead, in this exclusive article, we will talk about the fact that why December Solstice i.e summer solstice in the southern hemisphere is the winter solstice in the northern hemisphere.

Or, the questions like that December Solstice i.e the first day of summer in the southern hemisphere is actually the official first day of summer or mid-summer? Therefore, in order to get the answer to questions like these, I would suggest you stick with me till the end. Let’s dive right in!!!

 

What is Solstice?

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

So, moving ahead, according to the astronomical definition of solstices, this is a natural event during which one of the hemispheres is at the maximum tilt away from or towards the sun. And, when this happens, the sun appears to stand still and reverses its direction. In layman, Solstice is a day that occurs due to the sun’s changing path.

 

Four Seasons

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 summer and winter solstice is responsible for the arrival of summer and winter seasons on earth. In a similar way, the spring & fall equinox is responsible for the arrival of spring and autumn seasons on earth.

types-of-seasons-on-earth
Four types of seasons occur on earth: (above) Winter, Spring, (below) Summer, Autumn/Fall. Image source: Wikimedia Commons

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 the summer solstice in the southern hemisphere. At the same time, there is the winter solstice in the northern hemisphere. Let’s Find out!!!

 

Thanks to Earth’s Tilt, We See Solstices

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

Well, there is a small correction here. Not only do we see solstices because of the earth’s tilt. We see equinoxes 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 Solstice.It marks the first day of summer, therefore, Summer Solstice.
From the above comparison chart, it is clear that the solstice that occurs in December in the southern hemisphere is the summer solstice. On the other hand, due to the eath’s tilt, the solstice that occurs in December in the northern hemisphere is the winter solstice.

 

December Solstice – First Day of Summer in Southern Hemisphere

december-solstice-first day-of-summer-in-southern-hemisphere

As you already know that a solstice is a natural event during which one of the hemispheres is at the maximum tilt away from or towards the sun. Moreover, you also know that there are two types of Solstices.

These are June Solstice and December Solstice. So, how to define June solstice or December Solstice? OKAY, let me rephrase my question for you!!! What’s the difference between June and December solstice? ANY GUESS?

 

See, the only difference between them is that during the June solstice, the north pole of the earth is at its maximum tilt towards the sun. That’s why the June solstice is also known as the Northern Solstice.

On the other hand, during the December solstice, the south pole is at its maximum tilt towards the sun. That’s why the December solstice is also known as the southern solstice.

December Solstice – Southern Solstice

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

During December Solstice, the south pole of the earth is at its maximum tilt towards the sun. Therefore, half of the earth’s surface (southern hemisphere) receives maximum sunlight.

Hence, marking the brightest as well as the longest day of the year in the southern hemisphere. That’s why the December Solstice aka Southern solstice is officially known as the summer solstice in the southern hemisphere.

In addition, not only the sun’s orbital path across the sky will be as high as it can be. The sun will also enter the Tropic of Capricorn on the eve of December Solstice.

And with that, days get shorter and shorter after the occurrence of the summer solstice. That’s why the first day of summer in the southern hemisphere is also known as the mid-summer.

Summer Solstice – First Day of Summer or Midsummer?

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

However, on the other hand, if we go by the astronomical definition of the summer solstice, it marks the arrival of the first day of summer. 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 summer solstice or simply meteorological summer.

 

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

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

On the contrary, some people define it as the end of the half of the summer season, hence, mid-summer. Not to mention, the meteorological seasons always arrive approx 20 or 21 days before the arrival of astronomical seasons. To understand it more clearly, refer to the table given below.

 

Meteorological vs Astronomical Seasons

SeasonsMeteorological Season DatesAstronomical Season Dates
Autumn EquinoxMonday, 1 March 2021Saturday, 20 March 2021
Winter SolsticeTuesday, 1 June 2021Monday, 21 June 2021
Spring EquinoxWednesday, 1 September 2021Thursday, 23 September 2021
Summer SolsticeWednesday, 1 December 2021Wednesday, 22 December 2021
All the seasons (meteorological and astronomical) dates are strictly in accordance with the southern hemisphere of the earth.

 

How Long is the Longest Day of the Year in the Southern Hemisphere?

solstice-to-solstice-solar-graph
Summer solstice to winter solstice solar graph showing the maximum and minimum elevation of the sun’s orbital path. Image Source: Ian Hennes

How long is the longest day of the year in the southern hemisphere simply depends on the fact that how close you are residing to the south pole. In other words, the closer you reside to the south of the antarctic circle towards the south pole, the maximum hours of daylight you will receive.

According to the data released by the Finnish Meteorological Institute, during the 2015 summer solstice, the exact south pole received 24 hours of daylight. Similarly, Johannesburg received approximately 13 hours and 47 minutes of daylight.

Capetown received 14 hours and 25 minutes of daylight. The city of Invercargill of New Zealand received 15 hours and 48 minutes of daylight. The city of Ushuaia of Argentina received 17 hours and 19 minutes of daylight.

To summarize, if we talk about the whole of the southern hemisphere as one, the longest, as well as the brightest day of the year in the southern hemisphere, received an average of 15 hours of daylight. Don’t forget, the closer you are to the south pole, the more hours of daylight you will receive.

 

When is Summer Solstice in Southern Hemisphere?

In 2021, the December Solstice i.e summer solstice in the southern hemisphere will occur on Wednesday, December 22, at sharp 02:59 Australian Eastern Daylight Time (AEDT). Therefore, as a result, marking the longest as well as the brightest day of the year in the earth’s southern areas.

Not to mention, this astronomical event will occur at the same moment for all of us. In other words, whether you are living in Australia (i.e in the southern hemisphere), or in London (i.e in the northern hemisphere), it would be a moment of joy for all of us at the same instant.

Why does the summer solstice date vary?

Well, it’s not rocket science to understand, just some basic physics. See, after every four years, there is a leap year. Therefore, in order to adjust the Gregorian Calendar with the eve of the summer solstice in the southern hemisphere, the December solstice date varies.

For example, in 2020, the December solstice occurred on the 21st of the month. But this year i.e 2021, it will occur on the 22nd of the month. Not to mention, there are some other factors too, that contribute to the variation in the dates of the Summer solstice. But, just to make things easy, I am simply neglecting them.

 

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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.

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