Jupiter is often called the king of the planets because it is the largest of the eight planets. Jupiter’s diameter is approximately eleven times that of the Earth. In fact, you could take the seven other planets and put them all inside Jupiter and there would be room to spare. Jupiter is huge! And it is easily observed even with a small telescope.
We will learn a little about Jupiter, then we will discuss observing Jupiter with your binoculars and with a telescope. We will consider what can be seen and what to look for as you observe. We will also discuss tools to enhance the experience and challenges we face when observing Jupiter.
First, let’s take a look at where Jupiter lies in the scheme of the 8 planets. In their order from the Sun:
So, Jupiter is the fifth planet from the Sun. Relative to Earth it is the next planet out after Mars.
Jupiter is called a gas giant because it is composed mostly of hydrogen and helium. As far as we know there is no solid surface beneath the cloud tops of Jupiter. So when we observe Jupiter we are looking at the gaseous outer layer of Jupiter. But Jupiter’s clouds present a very interesting show.
Why isn’t Jupiter always visible in the night sky?
All the planets orbit the Sun, however, their orbit periods differ. Earth’s orbit is defined as 1 year. Jupiter’s orbit takes 11.88 Earth years.
Jupiter and the Earth travel in elliptical orbits, meaning they are not perfect circles. And since Earth circles the Sun faster, the distance between Jupiter and the earth is constantly changing. At our closest, we are a little less than 400 million miles apart. At the farthest, we are over 600 million miles apart, which would be when Jupiter is on the other side of the Sun from the Earth. So, there are better and worse times to view Jupiter. And there are times when we can’t see Jupiter at all.
How Do We Find Jupiter?
A simple internet search will find the dates when Jupiter will be visible in the evening sky. Or you can use a planetarium program, like Stellarium that displays the sky and the movement of the planets and stars. The picture shown here is a screen capture from Stellarium.
The view shown is facing directly south from approximately 40 degrees north latitude. The red line that goes from left to right in an arc is called the ecliptic. It is an imaginary line that goes from East to West along which the Sun, Moon, and planets appear to travel.
To locate the ecliptic, simply watch the path of the Sun as it goes from sunrise to sunset. If the Moon is visible at this time, it will travel along a similar path. The Moon is shown in the picture.
When Jupiter is in our night sky it will follow a similar path. Just like the Moon, Jupiter will rise in the east, travel the ecliptic and set in the west. So, when Jupiter is in the sky it is easy to find.
Jupiter is very bright. If we look at the brightest objects in the sky they are the Sun, Moon, Venus and then Jupiter. So, when Jupiter is in the sky it is brighter than any star in the sky.
You might mistake Jupiter for a bright star but when you notice it is on the ecliptic you will also notice that it looks larger than a star. And if you look at Jupiter with binoculars you can see that it is a disk, not a point of light.
What is Interesting to See When Observing Jupiter?
First, let’s look at Jupiter with binoculars. Even modest 7X35 binoculars are enough to see the four bright moons of Jupiter. These are called the Galilean moons as they were first observed by Galileo in 1610.
When using binoculars or the low power eyepiece of a telescope, Jupiter and its moons will look something like this picture. The moons move through their orbits pretty quickly. As a result, their positions will be different every night. Some nights you may only see two or three as the other moons may be behind Jupiter. They can form interesting combinations.
The four moons are named Io, Europa, Ganymede, and Callisto. Io being the closest to Jupiter has the shortest orbit period, only 42 hours. If you were to observe Jupiter for a couple of hours you might actually see Io move behind Jupiter or come out from behind Jupiter. And, on occasion, you can watch one of the moons travel across the face of Jupiter and you might be able to see its shadow crossing as well.
Europa circles Jupiter every 85 hours. Ganymede orbits in about 7 days. Callisto is the farthest out and takes 17 days to circle Jupiter. Compare this to our moon which takes about 27 days to circle the earth. These moons, especially IO, are really moving.
A fun thing to do would be to take an observation every clear night and make a note of the location of the moons. See if you can predict where they will be on the next clear night.
There are lots of programs and web sites, including the aforementioned Stellarium, that can show you which moon is in which location. An app for your phone is called Jupiter Simulator.
Jupiter has more than 70 moons, but most cannot be seen with typical hobby telescopes. However, if you have a large enough telescope, say 6” or larger, you may be able to see more than these four brightest moons.
The moons are small, a and the distance great, so don’t expect to see any surface details. Just enjoy the show as they move around Jupiter.
As mentioned earlier, what we see of Jupiter is the top of the clouds. However, Jupiter’s clouds tend to form into bands and these can be seen with our telescopes. Even at 60X in a 70 mm telescope, you can begin to see the two main cloud bands which are shown at the right in a darker tan shade.
Depending on the aperture of your telescope and the atmospheric conditions you may be able to magnify Jupiter enough to see shadings at the poles. Apply more magnification and you may start to see more regions of bands and even swirls within the bands called festoons.
The Great Red Spot
There has been a monster cyclone raging on Jupiter for hundreds of years called the Great Red Spot. This storm is larger than the Earth and can be seen traveling across the face of Jupiter. Jupiter rotates once in approximately 10 hours so that is fast enough for you to watch the Great Red Spot move even during a 1-hour observing session.
If you do an internet search you can find programs and websites devoted to predicting when the Great Red Spot will be visible. The phone app, Jupiter Simulator, which I mentioned earlier, also tells you when the GRS will be visible.
Most of the time you will likely observe Jupiter without filters. However, using colored filters that attach to the eyepiece can help bring up details that you might not be able to see otherwise. Filters may not work well on telescopes of less than 100 mm in aperture, but go ahead and try them.
Colored filters are often numbered based on the Wratten numbering system. Wratten is a system also used for photographic filters.
Light yellow, #8 and yellow, #12, can help enhance certain features in the cloud bands of Jupiter. Red and orange features will be emphasized. Deep yellow, #15 can sometimes bring up the festoons in the cloud bands.
The #80A filter is often called the Jupiter filter. This is a blue filter and many say if you only have one color filter this is the one to have. The 82A filter is also a blue filter and also works well for Jupiter.
Don’t expect these filters to cause things to explode into view. They enhance subtle shadings and help bring up details, but they won’t radically change what you see. Think of this as teasing out details. However, if your aperture is too small they may darken the image too much to be useful.
While Jupiter is very large, compared to the other planets, it is still quite distant from us. You are looking at something that is 400 million miles away.
The larger the aperture of your telescope, the more light it gathers, the more detail you are likely to be able to see and the more magnification you will be able to apply to the image. An 80 mm telescope may be limited to 120X under most conditions. A 200 mm telescope may be able to apply over 250X and bring out more details.
Seeing and Transparency
Remember that you are viewing Jupiter through miles and miles of air that will distort the image and refract the light as it comes from Jupiter to your telescope. The condition of the atmosphere, the air, will vary from night to night and so will the quality of the image you get and the amount of magnification you can apply.
Air pollution, high humidity, smoke and other things that scatter light will make things look less sharp. Turbulence in the air, the thing that causes stars to twinkle, will cause the image to drift in and out of focus. These disturbances become more evident as you go up in magnification.
So, if the image of Jupiter tonight doesn’t seem as good as it was a couple of days ago, it just may be the atmospheric conditions that are causing the difference.
Jupiter is one of the most exciting things you will see with your telescope. It is easy to find when it is visible in the night sky. The more you observe Jupiter the more you will be able to see. And don’t hesitate to give filters a try.