How and when to see the Great Conjunction of Jupiter and Saturn – the 'Christmas Star'
Extraordinary sight hasn’t been seen for nearly 800 years
This month, look to the skies for an extraordinary sight – one that hasn’t been seen for nearly 800 years.
Jupiter and Saturn will appear extraordinary close to one another on December 21 – just 0.06 degrees apart, in fact, which is less than a 10th of the size of the full Moon in the sky.
Some people believe such a ‘Great Conjunction’ was responsible for the star of Bethlehem followed by the Three Wise Men in the Bible story, hence its nickname, the Christmas Star.
The last time these planets appeared this close together was back in 1623 – but they were too close to the Sun to see from the UK or other middle latitudes at that point. The last time they could actually be observed so close together was on March 4, 1226.
And at the next Great Conjunction, on October 31, 2040, the planets will be separated by a greater distance – 1.1 degrees.
That means this year’s event is truly a once-in-a-lifetime astronomical occasion, not to be missed. But it won’t happen for long, and it would be easy to miss.
So here we tell you everything you need to know to enjoy it…
What is the Great Conjunction?
In astronomical terms, a conjunction is the apparent meeting of two celestial bodies.
It is caused by our line of sight to the objects, making them appear close together in the sky.
A Great Conjunction is the apparent coming together of two giants of our solar system, Jupiter and Saturn. They occur every 19.6 years, but they are not all equal – this year’s Great Conjunction will be the closest since 1623.
When is the 2020 Great Conjunction – and what direction should I look in?
A skychart showing how to find the Great Conjunction of Jupiter and Saturn on December 21. Image: NASA/JPL-Caltech
Jupiter and Saturn will appear closest together shortly after sunset on Monday December 21, 2020. This happens to be the winter solstice – the shortest day of the year.
They will appear low in the sky, to the south-west.
You can practice finding them now – as the planets are already appearing close together.
The sunset will be just before 5pm on December 21. Twilight begins at about 4.30pm, meaning by this stage you should be able to pick them out.
But you will have no time to lose. The planets will be sinking towards the horizon and will be just 11 degrees above it by this stage.
By 6pm, full darkness will have descended – meaning the planets will be at their brightest, but you will need a clear view to the horizon, as by now they will be only two degrees above it.
What will I see with the naked eye, with binoculars and with a telescope?
Jupiter and Saturn appearing together will be a spectacular sight – assuming the weather allows us to see them, of course.
Since they haven’t been seen appearing so close together for nearly 800 years, we cannot be sure how it will look. They could look like one great big star, but astronomers think it is more likely that those with good eyesight will be able to separate the two.
Jupiter is the larger, nearer planet and will be the brighter of the two, as you can see if you observe them now.
Binoculars will be a great way to observe them, and will enable both in your field of view at once.
You should certainly be able to separate the two using them. If you have a larger, astronomical pair of binoculars – like 20x80s – you should be able to detect the shape of Saturn, see some of Jupiter’s markings and the four Galilean moons of Jupiter.
What if the weather is bad?
You’ll have to wait 20 years for the next Great Conjunction – and it won’t be as good as this one. But don’t forget you can see Jupiter and Saturn close together throughout the month.
And if the weather is bad in the UK on December 21, you could still see the spectacle thanks to overseas observers.
There will be a livestream from Lowell Observatory in Arizona, US, which you can view.
How do I photograph the Great Conjunction?
It is worthwhile practising this in advance of December 21, as you won’t have long to get it right on the night – and, of course, we’re at the mercy of the weather.
In particular, make sure you’ve got a clear view to the horizon – scope out a location in advance.
Henry Throop, at NASA, says: “Think about composition. Jupiter and Saturn will just appear as points of light. To make your photo more interesting, try to frame the planets with something – the silhouette of a tree, an outdoor landscape, the arch of a building, or even a neon sign.
“Experiment with both wide-angle and telephoto shots. In early December, the two planets will be about two degrees apart, and will get progressively close toward December 21. In order to show them clearly in your photos, you might use a wide-angle composition early in December, and zoom in later in the month as they get closer.”
Here are some useful tips:
Use a tripod – if you don’t have one, keep your camera steady by leaning on something solid, like a fence or car.
Straight after sunset, you’ll get a different type of view than when darkness has descended – try staying out for an hour or so and capture a different look.
The crescent moon will also pass close by a few days before, offering another opportunity for a great composition.
Using a mobile phone camera
It will be possible to capture these bright planets with mobile phone cameras, although you won’t capture any detail so you’ll need to think about how to frame your image, perhaps with something in the foreground
Mr Throop suggests: “Some recent cell phones have a ‘night mode,’ which will automatically stabilise a long-exposure, even without using a tripod. This can be great for capturing the dark foreground of your photo. Some phones will let you use ‘night mode’ on exposures up to 30 seconds, if you also use a tripod.”
Phone pictures may not separate Jupiter and Saturn on December 21 – but in the days ahead of that, you should be able to capture both distinctly.
Using a DSLR camera
Manually focus on something at distance, and set your aperture so that it is wide open (a low F number).
Longer exposures can be taken with a tripod. Try a shutter speed of less than a quarter of a second if you’re not using a tripod and check for blurriness.
Mr Throop says: “If you use a 200 mm telephoto lens, you should be able to see Jupiter's four bright moons in a short exposure. Saturn’s rings will usually need a longer lens or a telescope in order to resolve clearly.
“To capture Jupiter and Saturn as sharp ‘points’ while using a tripod, use a shutter speed of up to a few seconds. More than this and the Earth's rotation will smear out the planets and stars. If you are using a wide-angle lens, you can use a longer exposure.”
And of course if you have a telescope, you may be able to capture the planets in even more detail.