Enjoying Jupiter

Jupiter photo by NASA/JPL/University of ArizonaJupiter offers great observing opportunities for urban astronomers. It is very bright, easy to find and shows interesting details even in small scopes. A view of Jupiter through a telescope will never fail to impress your family, friends, and neighbors. It’s relatively large size, cloud detail, and the ballet of its moons make Jupiter my favorite planet to observe. But after you’ve stared at Jupiter for the 20th time, then what? Well, here are few ideas of things to look for that should keep you busy for many nights to come...

Photo courtesy of Jupiter photo by NASA/JPL/University of Arizona.

What follows is a quick round-up of observing ideas for Jupiter. All of the features I mention can be seen with modest telescopes from light-polluted areas, although expect the ones I call out as “challenges” to indeed be challenging. It is true that a larger aperture will indeed show you more detail and make it easier to see the “challenge” items. But the really important factor is atmospheric seeing: the detail you can see on Jupiter is very sensitive to seeing conditions. When seeing is 9/10 or 10/10 (rock steady), even a 90mm telescope will show you tremendous detail in Jupiter’s cloud belts, with all kinds of spots, festoons, and swirls. If the seeing is 5/10 or less, don’t expect to see anything beyond the major belts, even with a 10 or 12 inch aperture. In seeing conditions between these, if you patiently wait for brief moments of steadiness you’ll be rewarded for a few seconds with an amazingly detailed view.

Jupiter’s Belts

You should begin your exploration of Jupiter by getting the know the major belts, which are labelled in the photo and diagram below (click on them for larger versions). It helps to understand the terminology: the bright bands are called “zones” and the dark bands are called “belts”, and the polar regions are called “regions.” The North and South Equatorial Belts (NEB & SEB) are very distinct even in small (70mm) apertures and are generally visible even when the seeing conditions aren’t great. Currently (September 2010) the SEB has faded almost completely, so don’t be surprised if you can’t see it and be sure to watch for its return. The North and South Polar Regions are also quite distinct in small telescopes. The North Temperate Belt is narrow and intensely dark, making it pretty easy to pick out when the seeing conditions are good. But the remaining belts are harder to see with smaller telescopes and require steady seeing. Some challenges to look for:

  • Can you see the South Temperate Belt distinct from the South-South Temperate Belt and the South Polar Region?
  • Can you see the North-North Temperate Belt distinct from the North Polar Region?
  • Can you see the large eddies in the Equatorial Zone where it runs along the NEB and SEB? Seeing these in particular will require very steady seeing, but on nights of 9/10 seeing, I can see them with my 4.25 inch (100mm) Newtonian using about 175x. A blue (#80A) color filter does help to make these eddies somewhat more distinctive.

Jupiter photo with belts labelledNomenclature for Jupiter's belts, zones, and regions.


The most famous of Jupiter's spots is the Great Red Spot (GRS). I remember it much more distinct in the mid-1970s when I found it easy to see with 4.25 inch (100mm) Newtonian (but my eyes were also much younger then). The GRS is rather pale these days. I can still see it when I look for it with the 4.25 inch Newt, but it doesn’t really jump out—neighbors and friends generally can’t see it even when I point it out. With my 10 inch (250mm) Dobsonian, the GRS does jump out quite clearly if the seeing is steady: visitors to the eyepiece usually have no trouble seeing it when I point it out, and some see it without my prompting. A blue (#80A) color filter does help to bring out the GRS more distinctly, but while the filter is helpful the difference isn’t night-and-day.

You can find out when the GRS is out using the convenient GRS Tool at Sky & Telescope (you’ll need to register to access the tool, but registration is free). Just enter a time and the tool will tell you the next few times that the GRS will cross Jupiter’s central meridian (i.e., when the GRS will be in the middle of Jupiter’s visible face).

As a challenge, try to also find the other major spot on Jupiter, the Oval BA  (also know as Red Spot Jr.) which is located in the South Temperate Belt. It appears to the left of the GRS  in the photo below, courtesy of NASA, ESA, I. de Pater, and M. Wong (UC Berkeley).  I have seen Oval BA in my 10” Dobsonian, but I have not yet been able to see it in my smaller telescopes. Again, a blue (#80A) filter will help to bring out Oval BA.

You should always be on the look out for other smaller spots, which are generally light in color and set in the South and North Temperate Belts. You'll need very steady seeing to observe them. Again, a blue (#80A) filter can help to bring them out, and I've also found a light-green (#56) filter to be helpful.

The Galilean Moons

The moons of Jupiter (Io, Europa, Ganymede, and Callisto) offer a fascinating orbital ballet performance. Before the invention of ship-borne chronometers, the scientists of the period developed a means of telling time (for the purpose of determining longitude for maritime navigation) by measuring the positions and eclipses of Jupiter’s moons. Of course, making such observations from the deck of a pitching sailing ship proved impractical. However this interst in the precise timing of eclipses of Jovian moons led Ole Christensen Rømer to notice that the timing of eclipses of Io differed slightly from the predictions in a way that depended on whether Jupiter was closest or farthest from Earth.  He used this descrepancy to prove that the speed of light was finite and derive the first estimate of the speed of light. The photo below of Jupiter and the Gallilean moons is by Jan Sandberg.

Jupiter and the Galilean Moons by Jan Sandberg

Begin your exploration of Jupiter’s moons by learning to identify them. Sky & Telescope has a great Jupiter's Moons tool to help you identify the moons (you have to register to gain access, but registration is free). Just enter the time you plan to observe and select the proper eyepiece orientation and it will produce a little chart showing how the four moons will be appear arranged around Jupiter. After regularly observing Jupiter’s moons for some time, I found that I can generally identify Io and Europa just by the way they look: Io has a “creamy”, off-white color, is close in, and moves fastest, while Europa appears “smallest” and is very white compared to the other moons. I cannot, however, tell Ganymede and Callisto apart just by appearance. When seeing is steady, as a challenge try to see the moons as disks.  In my 10 inch (250mm) Dobsonian with around 200x Ganymede and Callisto clearly show small disks when seeing conditions are steady; at about 275x all four moon show small disks and are clearly non-stellar.

A particularly interesting aspect of the Jovian moons is when they and/or their shadows transit across the face of Jupiter. The Jupiter's Moons tool will not only identify which moon is which, but also alert you to any transits of Jupiter by moons or their shadows. I find the shadow transits relatively easy to see even if the seeing isn’t very good: they are usually visible in my 4.25 inch Newt (see the photo below of Europa's shadow in transit, courtesy of NASA/JPL/University of Arizona).  Transiting moons themselves are harder and require near-perfect seeing to pick out.  With my 10 inch Dobsonian and about 275x I can generally identify the moons themselves while they are transiting, but only if the seeing is pretty steady. A light-red (#23A) color filter can help to highlight transiting moons. I have only rarely been able to see the moons in transit using my 4.25 inch Newtonian, and it always was on nights of extraordinarily steady seeing. Also watch for the rarer events of one moon occulting or eclipsing one another. These generally happen only during the portions of Jupiter's orbit in which the plane of the Gallilean moons aligns with the sun; during these periods Sky & Telescope will post a list of all the interaction events.

Photo of the shadow of Europa transiting Jupiter by NASA/JPL/University of Arizona