Spring Equinox 2021

Happy Spring Equinox!

Every year, we have two equinoxes, one in September and the other in March, when the Sun shines directly on the equator. At this time, the lengths of day and night are almost equal. For Australia, the Equinox will be at 5:21 am on Thursday, September 23rd 2021. In the Southern Hemisphere, it is the Spring Equinox, and in the Northern Hemisphere, it is their Autumnal Equinox.

Figure 1: Earth has seasons because its axis is tilted. Earth rotates on its axis as it orbits the Sun, but the axis always points in the same direction.

So the September Equinox occurs when the Sun crosses the celestial equator. The time for us in Eastern Australia will be at 5:21 am AEST. The celestial equator (ecliptic) is an imaginary line in the sky by the Earths Equator from North to South. The Earth’s axis is tilted at an angle of 23.4 degrees to the ecliptic on any other day of the year. The Southern Hemisphere or the Northern Hemisphere tilts a little more towards the Sun. Still, on both Equinoxes, the Earth’s axis’s tilt is perpendicular to the Sun’s Rays.

So why do we call it Equinox?

It means the nights are the same length  (about 12 hours) all over the world. Equinox is derived from the Latin ‘aequi’ and ‘nox’ meaning “Equal Night”.  However, even though this is widely accepted, it’s not entirely accurate. The night and day lengths are only nearly equal on the September and the March Equinoxes. Most Earth locations tend to enjoy more daylight hours than nighttime hours on those two nights of the year.

Thanks to the Earth’s atmosphere and our definition of Sunrise and Sunset, Now in Australia, we celebrate the beginning of Spring on September 1st. Our Northern neighbours commemorate the start of Spring on the date of the Equinox.

In Sydney, on the day of the Equinox, our day will be 12 hours and 6 minutes long; mind you, if we are at the South Pole, our day would be up for 24hours, a perfect opportunity to see the midnight Sun.

Whereas if we lived on the equator, the days are always a little longer than 12 hours all year round. So, the reason that most locations on Earth do not enjoy precisely 12 hours of the day and 12 hours of nighttime on the Equinox is how we define Sunrise and Sunset. Suppose Sunrise and Sunset were defined as the moment the geometric centre of the Sun passes the horizon. In that case, this is the time it takes the Sun to set fully, which can take several minutes, making the day just a little longer than the night on Equinoxes.

Another reason the day is a little bit longer on the Equinox is that the Earth’s Atmosphere refracts the Sunlight. This refraction or the bending of the light causes the Sun’s upper edge to be visible from the Earth several minutes before the edge reaches the horizon. The same thing can happen at Sunset, where you can see the Sun for several minutes after it has dipped below the horizon. This causes every day on Earth, including the days of the Equinox, to be at least 6 minutes longer than it would have been without this refraction.

Now the extent of the refraction depends on atmospheric pressure and temperature. When the calculations are done, the standard day at sea level of 1013.25 kPa and 15 degrees Celsius is used.  Another thing of interest is that even if day and night aren’t exactly equal on the day of the Equinoxes. There are days when sunlight and nighttime hours are very, very close to 12 hours. This day depends on the locations latitude and can never occur several days or even weeks before or after an Equinox.

In Sydney or Coonabarabran or in that general vicinity of about 35 degrees south latitude, the Equilux will eventually occur on September 19th. If we are in the Northern Hemisphere are around the same thing, it will be on September 26th.

So even though we often celebrate the Equinox as an all-day event, it is only a minute. So if you want to be up at 5:21 am on Thursday, September 23rd, because it’s at this instant that the Earths rotational axis is neither tilted away from nor towards the Sun. It is all to do with the tilt of the Earth.

The September Moon is astronomically special. They call it the Harvest Moon in the Northern Hemisphere, but we call it the September full moon. It is so astronomically special because the time of the Moonrise to the subsequent Moonrise is longer in the Southern Hemisphere. Shorter in the Northern Hemisphere, meaning here in the South Hemisphere, the Moon rises more than 50 minutes later on the previous day. So this means that it is just interesting.

There is nothing magical about the Moon currently. It occurs due to the low angle of the Moon’s orbit around the Earth makes with the horizon during this time of year.

Indigenous traditions

Indigenous Australians have remarkable traditions going back to 65-70 thousand years, conferring the local honour of being the oldest continuous culture on the planet and certainly often considered the original astronomers.

For thousands of years, they have marked and celebrated the Spring Equinox as a time of birth and renewal. We know that the accent Australians were keenly aware of the astronomical movements. Numerous stone arrangements have been built by many different Aboriginal cultures.  While the exact use of these arrangements is not known with certainty, it is believed that several may have been involved with charting the position of the stars.  One side, in particular, seems to suggest the culture that built it was explicitly aiming to chart the position of the Sun to recognize the solstice and the Equinoxes.

Wurdi Youang in Victoria was noted by European settlers two centuries ago and charted by archaeologists in 1977is located near the town of little river in Victoria is known by the Wathaurgang people. The site consists of about 100 basalt stones arranged in an egg-shaped ring approximately 30-50 metres in diameter along the central axis aligned East-west. Studies of the position of a stone in the outer area has revealed that 7 of the alignments are astronomical. This strongly suggests its builders deliberately intended the stone arrangement to point to the setting Sun at both solstices and Equinox.

The site could be anywhere from 200-20,000 years old. Still, research seems to point to a deep entrenchment of the stones to support a theory that it has been there for thousands rather than hundred’s of years. The central axis indicates directly towards the Equinox, so it is possible that the accent Aboriginal people gathered at this spot for thousands of years anticipating the two times of the year when the length of the day equals that of the night.

Wurdi Youang is the first objective evidence that an Aboriginal culture used the stone arrangements as an astronomical guide. Indigenous songs, dance and stories, along with their songlines, show a clear understanding of the movement of the Sun, the Moon and the stars. They use these as a Calendar, particularly when we talk about the Emu in the Sky, to allow them to anticipate the changing of the season.  When it would be an excellent time to seek out a new food supply, others use stories about constellations and the heavens to explain traditions, such as why catching a particular fish was forbidden or how to communicate with deceased loved ones during the rising of Venus.  Many of these traditions have been passed down through generations and are still used in Indigenous Australian Cultures today.

Interestingly Australia’s indigenous people were not unique in these associations with the cosmic displays. What is particularly intriguing is how the circle arrangement is found in thousands of ancient sites worldwide. Most famously, of course, is being the Stone Henge and got them in Near Olympic Villages off the coast of Israel, in Poland, in India and Ireland. There is also a place called Van lor in Hong Kong.

To those and other early settlers the mysteries of the heavens were embodied in that ancient circle, its boundaries protecting those inside the configuration symbolizing the light giving Sun, and the various seasons of the movements of the Earth. Both modern and ancient Australians were aware of the coming of the Spring Equinox of the time of growth and new life.

Figure 2: an egg shaped ring of standing stones in Australia could prove to be older than Britain’s Stonehenge, and it may show that ancient Aboriginal cultures had a deep understanding of the movement of the stars.

If you go back into the 19th century in a study of the Golden Bough, James Frasier talks about the native indigenous Australians regularly practising magical ceremonies for awakening the energies of nature or what may be called the approach of the Australian Spring.

Nowhere apparently are the alterations of the seasons more striking than in the deserts of central Australia. Here at the end of long periods of drought, the sandy, stony wilderness over which silence and desolation of death appeared to prove it suddenly after a few days into a landscape smiling with verdure and people were teaming multiples of insects and lizards and frogs and birds. Sadly, of course, many of the traditions have been lost along with their languages and culture. However, there are still some areas where it continues to thrive throughout Australia and groups dedicated and preserving and reviving the culture.

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Donna

Donna the Astronomer

I am a keen astronomer lucky enough to live and work in Coonabarabran the Astronomy Capital of Australia! I am a ‘Drover’s Brat’ and discoverer of a couple of comets and asteroids. I operate Milroy Observatory and can show you how to best integrate dark sky experiences into your tourism, farm stay or AirBnB business.

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