Jupiter

In this lesson you’ll learn where to find Jupiter over the coming months and what features to look for when you’ve got it in your telescope.

Jupiter

Jupiter is a massive planet.

Measure the width of Jupiter’s disc and you’ll get somewhere between 30 and 50 arcseconds across, depending on how close it is to us. Mars, which is significantly closer to us, is just 3.5 to 25 arcseconds across. So, even at its furthest from us, Jupiter appears larger than Mars at its closest.

We astronomers benefit from that massive size because it reflects such a lot of sunlight back in our direction, making it the brightest planet in the night sky after Venus. Depending on its distance from us, the apparent magnitude of Jupiter ranges from -1.6 to -2.9. It’s large disc and interesting surface patterns also mean there is always something to look at.

Where to Find Jupiter in the Night Sky

The following calendar shows you where Jupiter can be found for each month of 2023. The best time of year to observe the massive planet is around opposition in early November. The worst conditions are from March to May as Jupiter travels around the back of the sun.

2023

January: As the year begins, Jupiter is in Pisces, where it will stay until May. The last opposition is several months’ past and the giant is coming to the end of being available for observation in the evening sky.

However, it is still there and readily picked up midway between the southwest horizon and overhead point (zenith) at around 7pm. This giant planet’s disc is never small, and currently spans over 37 arcseconds across, more than 3x the size of the Martian disc. Jupiter shines at magnitude -2.3.

February: An hour after sunset and Jupiter is now only 25° over the western horizon, we are truly coming to the end of evening observations as Jupiter prepares to pass behind our star. Its disc is 35 arcseconds wide and shines at magnitude -2.1.

March: Jupiter leaves the evening sky this month. In the middle of March it is a few degrees above the horizon an hour after sunset but, by the time April comes knocking, Jupiter sets just 40 minutes after the sun, rendering it practically unobservable. It passes behind the sun on 11 April and reappears for morning observations as May gets underway.

What to Look for When Viewing Jupiter

I suspect Jupiter is most famous for its Great Red Spot (GRS), and we’ll get to that below. What’s easier to see are the different cloud bands on its surface, which even the smallest scope for this course will show you in good conditions.

As well as the planet itself, there are its four Galilean moons that are easy to get familiar with. The harder moon challenge is to see a shadow transit crossing. Find out how to tackle that towards the end of this section.

Jupiter’s Cloud Bands

The planet jupiter is famed for being split into many different colored bands.

Other than the GRS, they are its most striking feature and their vibrancy is unique in the solar system. We are all used to seeing the stunning pictures of Jupiter from NASA, like the ones below…

Jupiter

Of course, as you’ve discovered for yourself by now, the bands won’t be anywhere near as detailed as this in your own telescope, but you can clearly see some of them! When you see them for yourself, I guarantee you’ll be mighty impressed with the view.

To see the bands for yourself, get Jupiter centred in your telescope and take some time to just look at it. You’ll know you have the right thing when you see an unmistakable disc and not just a point of light. The other big clue that you’ve found Jupiter is seeing up to 4 points of light – looking light tiny stars – in a line either side of the planet. These are the Galilean moons of Jupiter and we’ll look at them next.

At first look, the planet’s surface might appear to be a uniform white / gray colour. But stare at it for a while, letting your eyes adjust to the view and using averted vision (see tip 10 on this page), and you will soon see bands of different shading. They won’t be in color because your scope doesn’t collect enough light for that, instead they will be in shades of gray, something like this picture, but perhaps not as clear – this is through a 10″ telescope.

After the initial excitement of being able to see the bands, what I found most rewarding was being able to put a name to them, so let’s do that next.

To be technically accurate, ‘bands’ refers to the dark regions on the surface of Jupiter. The light bands are actually called ‘zones’. I know this sounds pedantic, but it’s a point worth knowing when we come to look at their names.

The image below (source) gives the initials for each named band and zones, the names themselves are below the picture.

The abbreviations on the picture above stand for:

• NPR = North Polar Region
• NNTB/Z = North North Temperate Belt / Zone
• NTB/Z = North Temperate Belt / Zone
• NTropZ = North Tropical Zone
• NEB = North Equatorial Belt
• EZ = Equatorial Zone
• SEB = South Equatorial Belt
• STropz = South Tropical Zone
• STB/Z = South Temperate Belt / Zone
• SSTB/Z = South South Temperate Belt / Zone
• SPR = South Polar Region

Whilst it’s unlikely you’ll be able to see these all individually, there are certainly ones (and groups) that stand out more than others and you should be able to pick out. That’s helpful because, personally, I start with the center and work north and south to try and identify the other belts.

The lighter equatorial zone crossing the middle of the planet is easy to see because it is flanked above and below with the south and north equatorial belts, which are also easy to distinguish, even in a smaller scope. Some of the thinner belts are harder to pick out, especially in a smaller scope, or if your seeing is not great, but you will probably be able see the darker north and south polar regions at the top and bottom of the planet.

Using a higher magnification works well with Jupiter. In fact, on nights of great seeing, and if your scope is large enough, don’t be afraid to try 200x or more magnification to tease out more detail in the banding.

I don’t do this myself, but many astronomers find the detail they detect is improved by sketching what they can see. Sketching helps concentrate your brain on details and may help you tease out the narrower zones and bands on the planet.

The Galilean Moons of Jupiter

I mentioned it earlier, but one of the ways to know for certain you have Jupiter in your sights is the presence of up to four pinpricks of light in a straight line on one or both sides of the planet. This brief YouTube video shows what they look like through a small telescope (about 6 minutes in).

These are the four largest moons of Jupiter and collectively they are called the Galilean moons, because it was Galileo Galilei who first discovered them and realised what they were back in 1610.

From nearest Jupiter outwards they are called

• Io
• Europa
• Ganymede
• Callisto

Because they are so distinct in a telescope, it should come as no surprise to you that these are massive bodies. Indeed, every one of these four moons is bigger than Pluto.

Their continual orbit of Jupiter makes them a good study to make over the period of a few nights. I use Sky & Telescope Magazine as a ready-reckoner for where each moon will be on any night, but any astronomy software will give you a view of them.

Io fair whizzes around the planet, it completes a full orbit every 42 hours or so. This means you can actually see movement within a single viewing session. Note where the closest moon is in relation to the planet, then return an hour or two later and you will be able to see it has moved.

The effect is easiest to appreciate at times when the moons are close together, As you return every hour to look at them you will see they have moved closer together or further apart.

The further from Jupiter they are, the longer the other moons take to complete an orbit. The orbital periods for the remaining three are: Europa 3.5 days; Ganymede 7.2 days, and; Callisto 16.7 days.

One final note with regards to watching the Galilean moons – don’t assume the one you can see closest to the planet is Io. It could be you are seeing Ganymede reappear from behind the planet so it appears close – there is no 3D perception at this distance. always refer back to a chart to see which one you are looking at.

Later on in this section I’ll be talking about the moons’ ‘shadow transits’ but, for now, it’s time to turn to the one feature you’ve been waiting for…

The Great Red Spot

Everyone who gets a new telescope has an ambition to see the Great Red Spot (GRS).

The GRS is a storm on Jupiter’s surface which has been raging for centuries, and – for all we know – it may rage for centuries to come.

Like everything to do with Jupiter, it is large. Almost unbelievably, it is twice the size of our whole planet!

The winds inside this hurricane swirl at 400mph. The whole storm orbits the planet within the South Equatorial Belt and takes just 10 hours to orbit the planet, which is amazing if you think about the size of Jupiter.

Despite its size, color and reputation, seeing it in a smaller scope is a real challenge.

You need great seeing, a dark night to coincide with Jupiter being nearby to have a decent chance of seeing it. Ideally, you will be well over 100x magnification and over 150x is better. And, finally, the GRS needs to be facing us. With the best setup and conditions in the world, there is no seeing the GRS if it’s on the far side of the planet.

To find transit times for the GRS use either your favorite software (Sky Safari has it, for example) or use this free website. If you use this resource, remember that the times it gives are in UT (universal time), so you need to add or subtract hours from that to get local time. This link will tell you how many hours difference your timezone is from UT.

Finding GRS Transit Times on Sky Safari 5 (orange box)

Image Courtesy of SkySafari Pro – www.SkySafariAstronomy.com.

When you have good viewing and (ideally) when Jupiter is near opposition and so closer to us (as in June 2019) take a look for yourself. Go armed with the GRS transit times for the date you are observing. It might even help to have a colored filter to give the spot contrast. Use #38A Dark Blue, or #80A Blue, the cheapest ones cost only around $20 from Amazon.

Look towards the south of the planet, the GRS straddles the South Equatorial Belt and South Tropical Zone. These are the bands immediately below the lighter equatorial zone which circles the middle of the planet. Be prepared for it to take a long time to find, but have patience because, after you find it for the first time, it will be so much easier the second!

Next, we move to the final Jupiter observing challenge: A shadow transit…

Shadow Transits of the Galilean Moons

A transit is what astronomers call one body moving over the face of another. Mercury and Venus sometimes transit the sun, for example.

The moons of Jupiter frequently transit the face of the planet. Unfortunately for us, they are almost the same (gray) color as Jupiter in our eyepieces, making them impossible to detect in front of the planet with a small to medium sized scope.

However, when a moon transits Jupiter, it casts a shadow on the planet’s surface. That shadow is, of course, black, and we can pick that up in a smaller telescope. This is called a shadow transit.

Shadow transits happen every day. The trick is to spot the ones happening at night, when Jupiter is high in the sky and you have good seeing. To find the next ones happening that you can see, you can use your astronomy software (see the screengrab from Sky Safari 5 above – underneath the orange box you can see it says ‘Io Shadow Transit Begins…’), or use this free website.

There is nothing left for you to do now than take the Jupiter quiz and get your telescope out to have a look.

Summary

Jupiter is the biggest planet in the solar system and appears as the largest one in the night sky too. You can see cloud band/zone details on its surface quite easily, but the Great Red Spot and shadow transits are more of a challenge.

Seeing the Galilean moons do their dance either side of the massive planet is also a simple observation exercise.

Next we’ll visit the last and most visually stunning planet of this course: Saturn.