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Observing Jupiter -- What You Need, What You Can See

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Jupiter, The Giant Of Our Solar System

Jupiter is without a doubt one of the favorite targets for amateur astronomers. It is big -- really big. The table below presents some of the physical aspects of Giant Jupiter. Similar data for all of our solar system's planets is available at Planetary Data.

Jupiter
Dia (mi) 88850
Rel Dia (to Earth) 11.21
Rel Mass (to Earth) 317.923
Rel Mass (to All Planets) 71%
Rel Gravity (to Earth) 2.53
Min Dist to Earth (mi*e6) 366
Max Dist to Earth (mi*e6) 602
Rel Dist from Sun 5.2
Max Angular Size (arc-sec) 50.12
Min Angular Size (arc-sec) 30.45
Average Magnitude -2.94
Number of Moons 67

The table shows some of Jupiter's parameters as relationships to Earth rather than absolute unit values. I think it helps one get a sense of Jupiter's real characteristics, in that we have a sense of Earth's characteristics.

For example, to answer the question: "What's the diameter of Jupiter?", the table shows Jupiter's diameter as 11.21 times Earth's diameter, rather than 89,000 miles. 89,000 miles seems a useless number without some kind of reference, but 11.21 times Earth's diameter immediately conveys information.

So here are some popular questions, and the answers, mostly in relative to Earth measurements:

What is Jupiter's diameter? It's about 143,000 km, or nearly 89,000 miles. But compared to Earth, it's about 11.21 times the diameter of Earth. Big, in other words. That's it's apparent size as measured from Earth. It turns out, we're actually looking at the diameter of Jupiter's thick atmosphere. Is there a rocky planet in there somewhere? Probably not, at least nothing like the rocky planet we call Earth. Some models suggest that Jupiter's atmosphere just becomes thicker and denser as one goes toward the center of the planet, turning to a liquid at some pressure, then perhaps to a slush, the finally a solid core. Hard to say what it's really like at the bottom of Jupiter's atmosphere.

How far is Jupiter from the Sun? It's around 500 million miles, or 778 million kilometers, but perhaps easier to visualize is it's about 5.2 times further from the sun than is Earth. So it gets about 1/25 the amount of sunlight per square meter. Then again, since it's 11.21 times the diameter of Earth, it has about 125 times the surface area of Earth. So the entire planet gets about 5 times the solar energy of Earth, scattered over it's entire sun-facing surface.

All amateur astronomers know that the planet Saturn has rings around it, making it one of the most beautiful objects in our solar system. But -- does Jupiter have rings? Or is Saturn unique in that respect? It turns out that Jupiter does have rings, probably from some of it's many moons colliding at some time in the past. But don't expect to see Jupiter's rings with an amateur telescope, they are far too small and tenuous.

Some ask Who discovered Jupiter? That's a question that doesn't make direct sense, in that Jupiter is a naked eye object. In fact, other than the Sun and Moon, one of the brightest objects in the sky. So even cavemen saw Jupiter. The ancients after cavemen noticed that like the other planets, something was strange about Jupiter. It's position, like the other planets, changed with respect to the star background. Of course, the ancients didn't know what Jupiter and the other planets were, just that they moved with respect to the stars.

Copernicus was one of the first to likely grasp what a planet was, in that he figured out that planets appear to move with respect to stars because, like Earth, they orbit the Sun. Not a popular idea in his time, but it turned out to be true none-the-less.

Galileo was the first to point a telescope at Jupiter. His instrument was quite poor by today's standards, and didn't magnify enough or provide clear enough details to show features of Jupiter that today's amateur astronomers are familiar with. But with his early instrument, crude as it was, he saw the four biggest moons of Jupiter. They are known as the Galilean moons in his honor. Galileo figured out that the moons he observed were orbiting Jupiter, as our Moon orbits Earth. You can see the Galilean moons yourself with just a pair of binoculars.

Planetary Orbits

Current Earth, Jupiter, and Saturn Relationship

How far is Jupiter from Earth? Even though the orbits of both Earth and Jupiter are near circular, since they both orbit the Sun their distances vary. When on opposite sides of the sun (call conjunction because from Earth Jupiter appears to be near the Sun), the distance from Earth to Jupiter is some 602 million miles. But when Earth and Jupiter are closer to one another, when on the same side of the Sun (called opposition because Jupiter appears to be opposite the Sun), they can get as near as 366 million miles. The image above shows the current orbital relationship between Earth and Jupiter.

Those distances answer the next question, what's the angular size of Jupiter when viewed from Earth? When Jupiter is furthest away, it appears to be only about 30 arc-seconds in size, only a bit bigger that Mars when it's at its best. But when Earth and Jupiter are at their closest, Jupiter appears to be about 50 arc-seconds in size, twice the size of the best Mars appearances. That's about the angular size of the big Moon crater Copernicus.

Look through the table to see other interesting facts, like how many moons does Jupiter have? A lot, over 60 at present count. All but the the Galilean moons (Callisto, Ganymede, Io, and Europa) are invisible to amateur sized telescopes, however. But the Galilean moons are certainly visible, and provide much of the delight in observing Jupiter. They often eclipse the planet (called a transit), and their shadows are easily visible as they move across the clouds of the planet. I've watched moon transits with my DIY 60mm refractor, so the experience can certainly be enjoyed with even a small telescope.

One interesting fact I think is the gravity one would experience at the visible extremity of Jupiter, as if one could stand on a platform suspended at the top of Jupiter's clouds. As the table shows, Jupiter is much more massive than is Earth, nearly 318 times as massive. So you might think that standing on a platform at the top of Jupiter's clouds would crush you. But, Jupiter's diameter is also big, putting any such astronaut much further from the center of Jupiter than we on Earth are from Earth's center. So when all is taken into account, one would only weigh about 2.5 times what they weigh on Earth. Certainly noticeable, but not deadly. Still, at it's mass, Jupiter has alone over 70% of the sum total mass of all the planets in our solar system.

Tips For Observing Jupiter

The Public Domain Jupiter Image below was Supplied by NASA/NSSDC

This Site Not Endorsed by NASA

NASA Public Domain Jupiter Image

The Jupiter image above is typical of what an amateur astronomer might see on a good evening with a 10 inch or so telescope. Actually, the colors here are more vivid than what might actually seen through a telescope. But the details and Jovian features are about what a large telescope can observe.

The Jupiter View Through A Smaller Telescope

Jupiter - Typical Small Telescope View

At left is an impression of the planet Jupiter that a smaller telescope might give. First you'll notice that the image appears in mostly shades of gray, and the finer details are lost. The image also has much less contrast. Still an enticing image because you would be observing it live, but not like you see in text books and magazines.

The image shows fairly well that even with a small telescope, some amazing aspects can be seen. To get a good idea of what more aperture brings, check out the Simulated Views page.

Just keep in mind that even with a small telescope, such as the commonly used 60mm refractor, you can still make out the major cloud bands of Jupiter and occasionally some additional details. You can also see the 4 Galilean moons of Jupiter. In fact, the big moons of Jupiter, as mentioned earlier, are even visible as pin points of light through nothing more than a pair of binoculars. Refractors in general tend to give superior planet views, partly because of their steadier images, and partly because of their superior contrast. I use a long focus 60mm telescope to good purpose, but something like the Orion Observer 70mm Equatorial Refractor Telescope would show more than my 60mm.



You can get more out of Jupiter observing sessions if you plan a bit before going out to view. By that I mean it's nice to know what you are likely to see at different times of the evening. You may want to find out what nights of the week something interesting is happening that would be fun to view. There are a number of ways to do that planning. One of the best ways to plan a Jupiter observing evening is with a computer planetarium program, such as xephem, kstars, or stellarium. If you really want to get into Jupiter observing, maybe a book specifically about Jupiter, like Jupiter: and How to Observe It (Astronomers' Observing Guides) would help you out. Jupiter offers more variety to the observer than any of the other planets, with its Great Red Spot, moon transits, and other transient features.

You can also use this website to see what tonight's Jupiter view will be.

Xephem Jupiter Display

The depiction above is the Jupiter View from the planetarium program Xephem. It shows the appearance of Jupiter and it's moon for tonight at about 8:00 PM Denver time. If you check this image each day, you'll have some idea of when something interesting is about to happen. More of tonight's sky presentations are available at Tonight's Sky.

The moons in this image are labeled with Roman numerals, with I = Io, II = Europa, III = Ganymede, and IV = Callisto.

To see whether the Great Red Spot on Jupiter is visible at any given time, you can check out the Sky and Telescope Jupiter Applet.

The Xephem diagram above will show you when the Great Red Spot is visible, and when Jupiter's moons are in interesting configurations, or when one or more of the moons are casting shadows as they move across the planet.



Some Jupiter Nomenclature

Observers of Jupiter use a set of abbreviations to indicate the features of Jupiter. Being familiar with them will help you understand more of what you read about the planet.

Jovian Nomenclature

Above is a drawing with the zones and belts labeled with their common nomenclature. This is a proper aligned image. Many telescope views show an inverted or horizontally flipped image. The light band across the equator is the Equatorial Zone (EZ). The rest of the surface is broken into zones (light colored) and belts (dark colored). This drawing doesn't have all the labels. It only labels what's visible through most small telescopes.

The features drooping from the NEB are called festoons. I've managed to see festoons with a long focus six Newtonian, but not with my smaller telescopes. I've seen a hint of them with some photographs shown on my ETX 90 Astrophotos page.

The Great Red Spot (GRS) appears as a disruption in the SEB. The GRS varies in color and contrast over the years. In the 2009 season I was unable to glimpse it with a long focus 60mm refractor, though I know others have seen the Red Spot with similar instruments in previous years. I have seen the Great Red Spot in previous years with a 6 inch f/5 Newtonian.

In addition to these features, you may be able to see dark bars in the NEB.



What Do You Need To View Jupiter?

Jupiter can be enjoyed with a variety of equipment. You can start with just a pair of binoculars. With these, Jupiter will appear as a very bright but unresolved object with up to 4 pin points of light nearby. These points of light are the Galilean moons: Io, Europa, Ganymede, and Callisto. If you look and count less than 4, the others are either behind Jupiter and blocked from view, or in front of Jupiter and lost in its glare.

Moving up to just a 60mm, like the Orion Observer 60mm Altazimuth Refractor Telescope (Teal) will show the darkening of Jupiter's polar regions, and at least 2 dark bands, the NEB and SEB. At powers above 100x you may well see additional bands, such as the NTB. You might see the Great Red Spot if it's on the visible side of Jupiter, but don't be surprised if even a 3 inch telescope can't show it sometimes. A telescope like this one may be a little wobbly at high power because of the type of mount.

You can see shadow transits on Jupiter with a 60mm telescope, though you may have to stare awhile until you see your first one. The shadows through a 60mm are very tiny dots. Through larger telescopes the shadows are still just dots, but they stand out better.

To get the best views of Jupiter, you might consider a long focus refractor of 70mm or larger,like the Orion AstroView 90mm Equatorial Refractor Telescope . Significant detail can be seen with a good 3 to 4 inch refractor. Refractors do well on Jupiter because the contrast between the zones and belts is not great, and refractors deliver the best contrast for any given aperture.

Maksutov telescopes, such as the Questar, Celestron, and Meade ETX series, do very well on Jupiter. The Celestron NexStar 90SLT Mak Computerized Telescope is a popular model that works well, complete with computer-driven clock drive. The secondary mirrors of the Maksutovs cause them to have a bit less contrast than quality refractors of equal diameter, but Maksutovs are known for their crisp, chroma free images.

Also consider the 5 inch and larger Schmidt Cassegrain Telescopes (SCT), like the Celestron NexStar 6 SE Telescope. Like the Maksutov telescopes, the SCTs are very compact and portable, and operate with f ratios in the f/10 range. The SCT also makes a good general purpose telescope for a wide variety of target types.

Next best are probably Dobsonian telescopes of moderate to long focal ratios, like the Orion SkyQuest XT6 Dobsonian Telescope. Long focus Dobsonian telescopes in focal ratios of f/8 or longer make excellent planetary telescopes. And don't assume you need a six inch or larger, as the less expensive 4.5 inch models, like the Orion SkyQuest XT4.5 Dobsonian Telescope work very well also, at a cheaper price. I have a 6 inch f/10 Newtonian that delivers very good planetary images. The longer focal length Newtonian telescopes have flatter fields of view (no coma), less sensitivity to alignment, and smaller secondary mirrors that lead to better contrast. An 8 inch DOB of moderate focal length makes a great all around telescope as well as a good planetary performer.

Even a shorter focus telescope, like the Celestron Omni XLT 150mm Telescope Newtonian Reflector can deliver quality images. My old Discovery Newtonian (no longer available) is almost identical to the Omni XLT, and with it I've seen Ganymede as it traversed the surface of Jupiter, the Great Red Spot, and numerous moon shadow transits with my 6 inch f/5 Newtonian. It must be precisely aligned to perform well, but it can deliver an enjoyable show.

There are things you can do to improve your Jupiter viewing. Let your telescope cool down to ambient temperature before expecting it to deliver steady images. The time this takes is longer for bigger telescopes, and generally longer for reflector type telescopes. Don't observe over the top of nearby buildings. The heat rising from the buildings will cause significant image instability. If possible, observe when the planet is highest in the sky, rather than low to the horizon.

Try using a color filter on your eyepiece. Yellow is a common filter used to enhance the contrast of the Jupiter features. I generally use an apodizing screen on my 6 inch f/5. That apodizing screen helps reduce some of the light scattered by the secondary. That has the effect of slightly enhancing contrast.

If you need to get a telescope or observing accessory, try this custom search engine for your shopping.

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The fun Jovian features are the Great Red Spot, festoons (if you can see them), bars in the NEB, and possibly some white spots in the latitude of the GRS. Certainly the positions of the moons are fun to track. The shadows of the moons can be seen when they move across the surface of Jupiter. The shadows may accompany their respective moon closely if Jupiter is near opposition, or greatly lead or lag the actual moon crossing, depending upon Jupiter's position with respect to the Sun.

On occasion, you can actually see a moon moving across the planet rather than its shadow. Seeing a moon image as it moves across the planet is quite difficult. The moons are bright, and so is much of the surface of Jupiter.

I have been able to see Ganymede move across Jupiter. On the occasion I saw it, Ganymede was moving across the polar area where Jupiter is darker, which made the moon just visible. I was observing on that occasion with a 6 inch f/5 Newtonian.

If you do a little planning and following some of these suggestions, I'm sure that you'll find Jupiter to be one of your most exciting observing targets.

Try Some Jupiter Photography

If you have the patience and the time, you might try taking some photographs of Jupiter. To do that, you'll need a camera, and the simplest setup I've used successfully is a converted webcam and a laptop computer. For a short cut, you can inexpensively obtain the Celestron StarShoot camera. With one of these cameras you can take film strips (avi files) of Jupiter, getting a few dozen images at a time. The you can use a stacking program like Registax to combine the images into a very nice rendition.

To help you get some ideas, you can review my Quickcam Astro Camera howto page, and my Celestron NexImage Review page. The Quickcam page shows how to make your own astro camera from a web cam, and the other shows how the Celestron NexImage camera works as well as how to process and stack images.

Below you see some example of web cam and NexImage photos taken through some of my telescopes.

60mm telescope photo of Jupiter Jupiter. 60mm f/17 telescope with Celestron NexImage camera. In this inverted image, the NEB (brown belt) is easily seen. You can also just see a shaded region where the SEB usually is visible, but not in this 2010 image when the SEB all but disappeared. Also visible is the NTB and the darkened polar caps.
ETX90 photo of Jupiter Jupiter Image, Nov 5, 2010, ETX90 With Celestron NexImage Webcam, 2x Barlow, stack of 82 frames. Note GRS and missing SEB.
6 inch Newtonian photo of Jupiter Jupiter Image, Oct 16, 2000, Modified QuickCam Express Webcam, 6" f/5 Newtonian, 4x Barlow.