11 mins read 09 Dec 2020

The Dance of the Gas Giants

A rare alignment of the Gas Giants from Earth’s perspective this December has the world excited in watching both Jupiter and Saturn come closer and closer together. We had a chat with several Australian space community people and outlined how you can best catch this event.

The distance between Jupiter and Saturn (in degrees and arcminutes) from October to December. Credit. Credit: Pete Lawrence.

Humans across the planet are about to be treated to a special astronomical light show, with the two biggest planets in our Solar system – Jupiter and Saturn – drawing closer and closer together, as they head towards the light of the setting Sun.

On the 21st December (which coincidentally happens to be the day with the longest daylight hours – the summer solstice in the southern hemisphere), the two Gas Giants will be a tiny 0.1 degree apart in the sky, which translates to about 1/5 of the Moon’s diameter.

To give you an idea of how close this is, try this simple demonstration – extend your arm out fully and raise your thumb. The Full Moon is about as big as your thumbnail, so imagine 1/5 of this – that’s how close the two planets are going to be.

Jupiter, as captured by the Juno spacecraft which is currently orbiting the Gas Giant planet. Credit: NASA/JPL.

Saturn, imaged by the Hubble Space Telescope. NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team.

The two planets are already closing in on each other, and sky gazers around the world have been both watching and photographing the event. Both Jupiter and Saturn approach each other (in their respective orbits) every 20 or so years, but the close proximity of the two, and their alignment (from our own Earthly perspective) has not occurred for almost 400 years, with the last event of its kind occurring in the year 1623, not long after one of the first telescopes was invented.

The Australian space community is abuzz with excitement about the rare event, with hopes that everyone across the continent has clear skies to view and be inspired by the celestial dance of planets.

Astrophysicists and Science Communicator Kirsten Banks said she was looking forward to the event without the usual weather interruptions. 

"I'm really hoping to see both Jupiter and Saturn at the same time in my telescope! So long as those clouds stay away!"

“This is a once-in-many-lifetimes opportunity to observe Jupiter and Saturn through the same eyepiece, but more importantly, it’s a chance for everyone to get children, grandchildren, nieces and nephews excited by and engaged in STEM,” said Professor Lisa Harvey-Smith, an astrophysicist and the Australian Government Women in STEM ambassador, encouraging families and young people to head outdoors and look up.

Alan Duffy, Professor of Astrophysics from Swinburne University, also outlines how easy it is to observe this historical conjunction.

“Seeing the planets approach one another over the coming nights until finally overlapping during the conjunction is a vivid reminder that our Solar system is a dynamic and ever-changing place.”

“No technology or special viewing conditions are needed to enjoy this reminder that the planets, including us, are rolling around the Sun in a clockwise fashion,” he said.

“That this is the closest approach, from our point of view, in four centuries and will not come this close again in my lifetime, means I will particularly treasure this reminder.”

What are Conjunctions?

The planets of the Solar system – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, and the Dwarf Planet Pluto. The relative size of the bodies in thus composite image is correct – however, the distances are not. Credit: NASA/LPI.

Humans have been observing the planets for thousands of years. In fact, the word planet is derived from the Greek word planētēs which translates to ‘wanderer’ – as these bright points of light would wander across the sky when compared to the much more distant and relatively fixed stars that twinkled.

From Earth, we can see the five closest and brightest planets in our Solar system with our naked eye. Mercury and Venus are often seen within 3 hours of sunrise or sunset, as they orbit closer to the Sun than we do. This is why Venus is often named the ‘morning’ or ‘evening’ star.

Further out, we can see the bright fiery red hue of Mars. And looking further out, Jupiter shines brightly (sometimes even rivalling Venus), and lastly – the amber hue of Saturn.

Beyond this, the Ice Giants Uranus and Neptune (and smaller bodies like Pluto) require binoculars or a telescope to see, as they are at such great distances.

From our Earthly perspective, when two objects – like planets - approach each other in the night sky, astronomers call this a ‘conjunction’. The objects themselves are not in reality near each other – in fact, they are separated by great distances of millions of kilometres. It is just from our view; they appear close together in our sky.

The Dance of Jupiter and Saturn

The view on Dec 21 through a telescope - Jupiter and Saturn with a few of their respective moons. The bright object near Jupiter is a distant star. Credit: SkyGuide App.

Jupiter takes 11.8 years to orbit the Sun, and Saturn – which is at a much greater distance – takes 29.4 years to orbit the Sun. By knowing these two numbers, we can start to draw out some simple geometry to show that the two planets come close together in our night sky about every 20 or so years.

Firstly, we take Saturn’s orbit (roughly a 360-degree circle) and divide it by the 29.4 years and find that Saturn moves across the sky by about 12-degrees per year. We do the same for Jupiter and find it moves across the sky by about 30-degrees.

So, for each year that passes, Jupiter is moving faster than Saturn across the sky by about 18-degrees (the difference between the two). Now all that is left is to divide a full circle of 360-degrees by the difference of 18-degrees and we find it takes 20 years for both these planets to occupy a close region of the sky from Earth’s perspective.

What makes this year’s conjunction special is that it is happening at a fair distance from the Sun (away from the daylight) so everyone on Earth can observe it in the evening sky. The two planets did come this close together back in 1623, but at the time they were only 13-degrees offset from the bright Sun, so many people would not have been able to view them. Linking to historical context, the 1623 conjunction also occurred only 14 years after Galileo first observed the heavens with his telescope – an event which changed the way humans thought about our place in the Universe (namely, we were not its centre!).

Prior to this, the last time an observable ‘great conjunction’ occurred was on the 4 March 1226, almost 800 years ago, and the next one is set to occur on the 15 March 2080, 60 years from now.

Skywatching Across the Ages

Artist illustration of the ancient Greek astronomer, Hipparchus performing calculations of the heavens. Credit: Ann Ronan Picture Library.

Observations of the planets, stars and events is nothing new – in fact, it is one of the oldest sciences known – stretching back tens of thousands of years, here in Australia when Aboriginal and Torres Strait Island people used celestial objects for navigation, agriculture, timekeeping and more.

"One of my favourite interpretations of the planets in Aboriginal astronomical traditions comes from the Wardaman people in the Northern Territory," said Kirsten Banks.

"They talk about the planets as spirits that walk the path (the ecliptic) both forwards and backwards (retrograde) and think that is an incredible example of detailed astronomical observations made by Aboriginal peoples in this country for thousands of years"

Aboriginal astronomy studies (which include papers by Kirsten) have shown that Aboriginal and Torres Strait Islander people noted that planets moved differently from the background stars, that they lacked the twinkling, and recorded retrograde motion (when the outer planets move backwards in the sky due to Earth-orbiting faster).

Assyrian astronomers in places like Mesopotamia also studied the movements of the planets in great aspect, as far back as 3,000 – 4,000 years ago – developing detailed arithmetical and geometrical models, which predicted the rising and falling of the planets in the sky. Back then it was used for the purposes of astrology, but these early observations and calculations would go on to be source of knowledge that astronomy and space sciences would build upon.

Later, this knowledge would be passed to the ancient Egyptian and Greek astronomers, philosophers and mathematicians who would develop numerical formulae and models to explain why the planets would follow the same path across the sky, or the reason why conjunctions would occur.

All of this sky gazing was prior to the invention of the telescope in the 1600s – but as this new technology came into existence, it reshaped our view of the Universe forever. However, one core component remains – that people (and many animals) still to this day, look up, and gaze at the sky.

What you can do

Credit: NASA/JPL/University of Arizona/Space Science Institute.

The term sky gazer describes someone who looks at the sky with their own eyes, for any purpose. At some point or another, nearly every human being will do this in their lifetime. Some people take this further and become observers – who not only look at the objects and events in the sky but make observations of them.

This is the basis of all astronomy – something that nearly everyone can achieve and be a part of. Outlined below are a number of different astronomical observing opportunities for everyone to get in, with or without any equipment. Whilst these are excellent opportunities to partake in with the upcoming Jupiter/Saturn conjunction – these can be applied all year to other celestial objects and events.

The first is really simple. On clear nights, go outside and look towards the western horizon with your naked eyes. You will see Jupiter and Saturn, fairly close together in the evening light – even before the rest of the stars appear. You don’t need any equipment to do this, just go outside and look. You could even take observations by recording their changes on a daily basis – capturing data like what colour they appear to be, or how many thumbnails apart they are from each other on a nightly basis.

This is a handy idea for the stars as well – take a book and pen out and record the different bright stars, their colours, what constellation they appear in and at what time of year you observe them directly above you. Keep this log, and you’re doing astronomy just as the ancients did!

Watching Mercury and Venus traverse in their orbits over the course of a month. These three images are spaced 14 days apart and captured in October 2019. The arrows in the bottom row indicate the speed and direction of each planet. Images captured with a mobile phone camera. Credit: R. Mandow.

If you want to get a bit more hands-on, try using your phone camera to take a photo of Jupiter and Saturn on a nightly basis. If you place your phone in the same spot and position every night, you could build up a timelapse of the planets getting closer when you stitch all the single images together.

There are also some great apps to use on your phone that will help you identify objects in the sky, and record them using longer exposure times. Try SkyGuide App (Apple / Google) to help navigate what you are looking at, and NightCap App to help take space photos.

If you have access to them, using binoculars or a telescope (even a small telescope) will reveal the beautiful rings of Saturn, the bright large disc of Jupiter and the brightest moons of both these planets.

The moons of both Jupiter and Saturn will be visible using telescope or photographic equipment. Credit: SkyGuide App.

Byron Bay astronomer and astrophotographer, Dylan O’Donnell also offered some handy tips in observing the event.

“At its closest, the two planets will be less about a Moon’s width apart, so if you have a telescope and you can see the entire Moon at once, you’ll be able to see or photograph them in the same field of view”

“If you increase the exposure or gain enough, you’ll also be able to see their moons together which is a rare thing to capture!”

Astronomy is not limited to people who have degrees, and in fact – has been the science of societies stretching back for many thousands of years. Events like the upcoming Jupiter / Saturn conjunction allow nearly everyone on Earth the opportunity to participate in astronomy and learn something about the night sky – which has influenced our civilisations in so many aspects.

This event also serves as a fantastic opportunity to inspire younger generations to learn about the heavens, all those bright points of light and what they all mean. Additionally, it also opens the door to teach young people about the beautiful ancient Aboriginal and Torres Strait Island Astronomy that has occurred across Australia for so long – and how the songs from the past can connect us to the world around us.

In fact, astronomy is just that. By looking out into the heavens, remember that billions of people before you have done the exact same thing. Looked up and wondered – an experience we have all shared even though we are separated by distance and time.

And now in the new age of digital technology in 2020 - we get to find that connection with each other once more, simply by looking up and watching the cosmic dance of the Gas Giants.