As Comet NEOWISE continues to dominate our northern skies, we should remember that in the southern skies two of the Solar System’s largest planets are now nicely on view.
Jupiter and Saturn lie fairly close to each other in the night sky, and now is a great time to observe them. For owners of small to medium sized telescopes, there’s quite a bit to see.
For info on how to find Jupiter and Saturn, read our guide on how to see the planets in the night sky this month.
And if you’re really interested in seeing how many planets you can spot, read our guide on how to find the planets in the night sky.
Gas giants Jupiter and Saturn (not to scale!), sketched by Dr Paul G Abel. Credit: Paul Abel
What to look out for when observing Jupiter
Jupiter currently resides in the constellation of Sagittarius and it is quite unmistakable as a bright yellowish object, low down in the south east after 22:00 BST.
Binoculars will reveal the four largest Galilean moons Io, Europa, Ganymede and Callisto. In a small telescope however, Jupiter is transformed into a flattened yellowish disk crossed by darker bands.
Telescopes of 90mm or larger will show the two main equatorial belts, which are brownish in colour.
Larger telescopes will reveal other fainter belts and brighter zones.
Telescopes of 150mm aperture or more reveal some fascinating details inside the belts: particularly the North Equatorial Belt, which is quite active at the moment. I had a fine view of the activity recently using my 203mm Newtonian.
A computer-generated view of Jupiter and Saturn as they will appear from southwest England, 18 July 2020, 23:00 BST. Credit: Stellarium
A good target for small telescopes is the Great Red Spot: a vast hurricane that sits in the south equatorial belt.
I have seen it in a 90mm refractor, and you’ll find a light blue filter will help increase the contrast making it easier to see.
A good exercise is to see whether you can notice it shrinking. For more on this, read our guide on how to measure Jupiter’s Great Red Spot.
The spot drifts in longitude (although not latitude) and the side of Jupiter facing Earth need not always contain the GRS, so you’ll need to plan when to observe it.
You can use any free planetarium software like Stellarium or WINJUPOS to find out dates and times when the spot is visible.
Finally, there is a nice phenomenon involving the Galilean moons and their shadows called a transit.
A view of Jupiter’s moon Callisto casting its shadow on the gas giant. Credit: Pete Lawrence
A transit occurs when a satellite passes in front of Jupiter. Io and Europa do this quite frequently, but transits of Ganymede and Callisto are much rarer as they are much further away from Jupiter.
Transits of Ganymede are particularly impressive as the satellite is so large and quite dark, making it quite unmistakable when pitched against the colourful clouds of Jupiter.
Again, planetarium software will show you when these transits are due to occur.
What to look out for when observing Saturn
Just to the east of Jupiter (practically next door) lies the planet Saturn. It is not a brilliant as Jupiter, but it is reasonably bright and shines with a yellowish tint.
The planet is a wonderful sight in telescopes at the moment. Saturn’s rings are wide open providing excellent views of the planet’s northern hemisphere.
Telescopically, (apart from the rings) Saturn looks similar to Jupiter, its atmosphere crossed by dark belts and lighter zones. You’ll find these features are more muted however due to a chemical smog situated above the atmosphere of the planet.
A view of the Milky Way over Gran Sasso, Italy. The bright red ‘star’ on the left is Mars, while Saturn is at the middle of the image. The bright ‘star’ on the right is Jupiter. Credit: Dneutral Han / Getty Images
Saturn reaches opposition on 20 July 2020 so if you have a telescope, now is the time to watch out for the ‘Seeliger effect’.
A few days before opposition, Saturn’s rings will start to brighten and then after opposition they will fade back to normal. This is the Seeliger effect and it’s due to the fact that at the time of opposition, the Sun is shining directly onto the rings and so the shadows cast by the particles in the ring system are temporarily out of view.
The effect can be quite striking and is easily visible in small telescopes.
Saturn’s rings can appear to brighten significantly at opposition. Credit: Pete Lawrence
Like Jupiter, Saturn has a large family of satellites. The largest, Titan, is visible in binoculars.
An interesting satellite to follow is Iapetus: when on the western side of Saturn it is is around magnitude 10.2, however as it passes towards the eastern side of the planet it starts to dim and can become as faint as magnitude 11.9, making it quite hard to see in a small telescope.
We now know why this is the case. Iapetus is a sort of ‘two-toned’ world with one hemisphere covered in ice, while the other side is covered in dark red organic compounds.
If you’ve not seen Jupiter or Saturn through binoculars or a telescope before, now’s the perfect time to take a closer look.
Dr Paul G. Abel is a theoretical physicist and Director of the British Astronomical Association’s Mercury and Venus section.