21 December 2020 marks the Summer Solstice or “longest day” here in New Zealand. This year, it will also be the date of a Great Conjunction featuring our solar system’s two largest planets, the gas giants Jupiter and Saturn. A conjunction is the term used to describe an event where two or more celestial bodies appear close together in the night sky as seen by an observer here on Earth. A great conjunction simply indicates that the event features Jupiter and Saturn. Though a great conjunction occurs about every 20 years, this year the two planets will appear particularly close together, and in fact the last time that the planets appeared this close was in 1623 – nearly 400 years ago! Great conjunctions can be lost to the glare of the sun – that is Jupiter and Saturn are too close to the sun to be observable. It is believed that the last observable conjunction where the two planets appeared as close as they will this year was in 1226 making this much rarer than a once in a lifetime event. At their closest, Jupiter and Saturn will appear to be only 0.1° apart – that’s just 1/5th of the diameter of a full moon!
So what will it look like for us here in New Zealand? For kiwi observers the moment of closest approach technically occurs when the planets are below the horizon (about 7:20am, 22 December 2020). However, the conjunction and the days around it will still provide amazing views whether you’re observing the sky with the naked eye or with the aid of optical instruments like a telescope or binoculars. On the evening of 21 December, when the two planets appear closest together, they will look almost merged but you should still be able to distinguish them as two separate points of light. If you’re observing with a telephoto lens, binoculars, or telescope you’ll be able to see both planets in the same field of view – spectacular!
How to find Jupiter and Saturn
Finding the two planets is pretty straight forward. Step outside and look to the west about half an hour after sunset and you will see them shining fairly brightly in the western sky. Jupiter is larger and much closer to Earth than Saturn and is therefore much brighter. Saturn will still be observable, brighter than the surrounding stars, but dimmer than Jupiter. You’ll need reasonably clear views towards the western horizon a little north of where the sun just set. If you hold your clenched fist out at arm’s length level with the horizon, Jupiter and Saturn will be sitting somewhere in line with the top of your fist.
On the evening of 17 December, a waxing crescent moon (9% illuminated with the remaining shadowed surface illuminated with reflected earthshine) will join the pair as they move closer towards each other – provided you have clear skies, this should be quite a beautiful sight against the rich blue background of the twilight sky.
Over the next 5 nights, the distance between Jupiter and Saturn will continue to close. You’ll be able to observe the planets until they set between around 11pm and midnight (as seen in Christchurch, New Zealand). Note that the planets will appear a little lower in the sky each night, setting earlier and that the sun sets a little later making the sky brighter for a while longer.
For youngsters living in 2020, don’t worry if you don’t get to see this one, you will get another chance though you’ll have to wait until the year 2080!
Photography Tips
There are a couple of good options for photography here depending on the equipment you have. For wide, standard and short telephoto lenses, the celestial alignment will occur low enough to the horizon that you could include the conjunction as an element of a landscape photo. Look for an interesting foreground and for even more interest perhaps a smooth, calm reflective surface. Just be aware of mountains or any other obstruction that rises more than 10° above the horizon – they’ll obscure your view of the planets! Make the most of the time before sunset to choose your composition. Use an app like PlanIt! Pro or Photopills to check that the planets will be in your shot as the sky darkens during nautical twilight (which is when the sky map shown above is set for).
Wide-Angle / Landscape Settings
Camera Settings: Once you have your composition framed up, you’ll need to keep adjusting your exposure settings to allow for the darkening sky. I recommend shooting in manual mode with manual focus set to infinity.
Aperture: f/8 is probably a good aperture to start with. You won’t need a particularly fast lens as you might use for usual nightscape/astrophotography work as there’ll still be plenty of light in the sky in the transition through the phases of twilight. If you have close-in foreground objects that you’d like to keep in focus, you may need to use a focus-stacking technique.
Shutter Speed: As the sky darkens, your shutter speed needs to slow down to allow more light through the lens to the sensor. However, your shutter speed also needs to be fast enough to ensure the planets remain as sharp pin-points of light (maximum of approx. 10-15s for a 24mm lens on a full-frame camera system). If including the moon in your composition (on the 17th December) then I’d suggest allowing the illuminated crescent phase to be over-exposed in order to capture the detail visible in the shadowed section of the moon which is being illuminated by reflected light from Earth, a phenomenon called earthshine.
ISO: Set your ISO as necessary to maintain a good exposure. Use your histogram to check that you aren’t crushing your blacks and unnecessarily losing too much detail in the shadows.
A wide-angle landscape image featuring a conjunction between the Moon and Venus in the dawn sky earlier this year. Note the overexposed Moon reveals the ‘earthshine’. Setting focus manually to infinity will ensure your cosmic subjects are captured as sharp points of light and not fuzzy, out-of-focus blurs. Note the reflections of the Moon and Venus in the lake.
📷 Canon 6D Mk II, EF24-105mm f/4L at 24mm, f/8, 6s, ISO 800.
Telephoto Settings
With the crescent moon passing near Jupiter and Saturn on the 17th December, it would be very tempting to swap the wider lenses for something with more reach like a 200mm to 300mm lens allowing you to focus specifically on the celestial event. You can certainly do this but it will be much harder to photograph than a landscape-type scene.
Unless you’re using a tracking mount, your subjects will move rather quickly through your frame – just turn on live view and zoom to 100% while focused on the moon next time you see it to see what I mean. Hopefully this will help you to appreciate that much faster shutter speeds will be needed to capture sharp images that don’t just become a blurry streak of light in your image.
It will also be a challenge to capture the dynamic range of the scene – especially with the Moon in the frame. Do you expose for the crescent phase of the Moon? Doing so would underexpose the rest of the scene, including your two planetary subjects and the deep blues in the sky. Do you expose for the earthshine effect? Doing so will overexpose the sunlit crescent of the moon and possibly create a moontrail unless you are also keeping your shutter speed under control.
A tracking mount would make it a lot easier that’s for sure by allowing you to keep your subjects centred in the frame, avoid blur/trailling issues and allow you to take bracketed exposures that you can blend in post-processing to cover the dynamic range on display.
Conjunction with Jupiter, Saturn and 9% crescent moon on 17 December 2020.
This diagram demonstrates the field of view when using a 300mm lens on a full-frame camera.
Prepared using Stellarium (stellarium.org)
Telescope Viewing
If you’re lucky enough to have a telescope, even a small scope, or even binoculars, make sure you have them trained on the conjunction on the evening of 21 December. Depending on the power of the optics, you’ll see the 4 moons of Jupiter and the bands of cloud encircling the planet then look to the side to see the ringed beauty, Saturn.
A simulated telescope view of the Great Conjunction, 10pm NZDT, 21 December 2020.
The amount of detail that can be observed will depend on the type of optical instrument being used to view.
Prepared using Stellarium (stellarium.org)