An Observer’s Guide to the Planet Jupiter

The planet Jupiter is always one of the brightest objects in the night sky. It’s brighter than any star, and is only outshone by the planet Venus and the Moon, and, very rarely, by Mars and Mercury. Jupiter reaches a position for optimum viewing in a telescope once every 13 months, roughly, and as you are about to discover, the visible face of Jupiter reveals so many interesting features in a small telescope that the planet is a favorite target for new and experienced stargazers.

Jupiter is the largest planet and, by far, the largest object in the solar system save for the Sun. The planet is 2.5x as massive as all other planets combined. Like Saturn, Jupiter is a “gas giant”, a massive planet made up almost entirely of cold hydrogen and helium gas along with traces of other gases like ammonia and methane.  (Uranus and Neptune are also sometimes called gas giants, or, because they are much colder, “ice giants”).  Gas and ice giants do not have well defined rocky surfaces like the Earth.  Their outer layers are made entirely of clouds; further down the gas turns to compressed liquid, and at the very center of these planets may be a rocky carbon core.

 

Layout of the belts and zones of Jupiter (University of Wisconsin)

Layout of the belts and zones of Jupiter (University of Wisconsin)

What To See On the Face of  Jupiter

As backyard observational astronomers, we are most interested in the part of Jupiter we can see: the outer layers of the atmosphere.  To see any detail on the face of Jupiter, a telescope is required. But even with the smallest telescope, you can see structure in the atmosphere, usually two dark bands surrounded by lighter regions. The dark bands on the face of Jupiter are called belts and the lighter bands between them are called zones.  The two prominent belts you see in the image at top are the north equatorial belt (NEB) and the south equatorial belt (SEB) since they each lie just north and south of the equator, respectively.  These are the most prominent belts, and they are visible in nearly any small telescope, but there are many more visible in steady sky with larger telescopes at higher magnification.

The belts and zones are caused by churning layers of gas in the atmosphere.  The rotating atmosphere has segmented itself into layers that are confined to certain latitudes on Jupiter, much like the trade winds on Earth blow in the same place and same direction throughout the year.  The zones are caused by convection regions wherein warm gas rises, cools, and sinks repeatedly in the upper atmosphere.  The belts are regions of cooler gas that have sunk lower in the atmosphere.  The belts are dark and colorful, with tones of red, salmon, tan and brown.  The color comes from phosphorus compounds that are ionized by radiation and lightning in the fast-moving winds.  The zones, which are regions of warmer rising gas, are usually pale-yellow or white caused by ammonia gas crystallizing and blocking the view of the more colorful gases below.

The interplay between the planet’s rotation, high winds, and convective rise and fall of gases have resulted in a complex structure of belts and zones, the structure of which is more or less permanent, at least over decades and perhaps centuries.  The structure of the observable atmosphere of Jupiter along with the names of the major belts and zones are shown below.

Names and positions of the belts and zones of Jupiter.

Names and positions of the belts and zones of Jupiter.

 

Not all the belts and zones in the images above are easily distinguished, but the most prominent regions visible in small telescopes are the equatorial belts and zones, the temperate belts and zones, and the darker polar regions.

Jet streams of opposite direction on each edge of the belts and zones confine each structure, and as the gases whirl across the face of the planet at speeds exceeding 300 km/h, turbulence occurs at the boundaries.  This turbulence leads to intricate and exceedingly beautiful structures called garlands and festoons between and within each belt and zone (again, see image at the top of this page).  On nights of steady seeing, you can get glimpses of these structures in a small telescope.

As on Earth, the dynamic atmosphere of Jupiter and the so-called Coriolis force lead to circulating cells of gas that result in cyclonic storms.  On Earth, these churning cells are about as high as these are tall, but Jupiter’s faster rotation flattens its cells and stretches the cells longitudinally into flattened ovals.   Some of these oval cells coalesce into visible and long-lasting vortices, the most famous of which is the Great Red Spot (GRS).  This spot straddles the south equatorial belt and the lighter south tropical zone, impinging on each.  The GRS, which is larger than Earth, has been observed for centuries and may be a permanent feature in the Jovian atmosphere. Which is not to say it never changes.  The GRS and the SEB both change color at unpredictable intervals.  When the SEB turns lighter, the GRS darkens and vice versa. During the past several decades, the GRS was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93.  No one knows why it changes, or indeed what causes its red color.

Best Times to See Jupiter

At its closest approach to Earth, the planet’s disc spans about 50” (arcseconds) which makes it easy to observe.  Only Venus, which is much closer, occasionally appears larger from Earth.  But unlike featureless Venus, Jupiter reveals an enormous amount of detail to observers with a small telescope.

Some telescopes are better than others for seeing Jupiter and the other planets.  But the best telescope for you is the one you have right now.  Here’s how to get the most out of your optics and your situation when looking at Jupiter…

First, you need to pick the right time to see Jupiter, namely, when Earth is closest to the planet.  That time is when Jupiter is at opposition, when it is opposite the Sun in the sky.  At opposition, the planet rises in the east as the Sun sets in the west. But Jupiter is large enough to look good for a few months on either side of opposition, so there’s still plenty to see. The next oppositions of Jupiter fall on April 7, 2017 and May 9, 2018.

Next, you need a night with good steady seeing.  You’ll learn more about this later in the course. But you can tell if the air is steady and the “seeing” is good if stars exhibit very little twinkling.  If the air is turbulent, even a big telescope will show Jupiter as a boiling featureless blob.

Tips for Observing Jupiter in a Telescope

What’s the best telescope to see Jupiter?  We haven’t discussed telescopes yet, but here are a few tips.  Generally, you want a long focal length which gives you a larger image for a given eyepiece. Refracting telescopes are often best because they have an unobstructed view and so provide the best image contrast.  Larger aperture gives more resolution of fine detail, assuming your sky conditions allow it.  And if you use a Newtonian reflector telescope, make sure it is well collimated.

With whatever telescope you presently have at your disposal, use as much magnification as it will handle when you look at the planet.  This depends on the quality of your optics and on the seeing.  Generally a magnification of 30-50x the aperture of your telescope (in inches) works well on nights of average seeing.  So if you have a 4-inch telescope, try 120x to 200x.  If you have razor sharp optics and steady sky, you can get away with even more magnification.  But you should experiment.  Start at low magnification and work your way up until you get the best combination of image size and contrast and clarity.  The best magnification may change from night to night because of changing sky conditions.

Another tip—make sure you give your telescope time to come to equilibrium with the outside temperature. If you bring a telescope from a warm house to a much cooler outdoor environment, for example, it will take 15-30 minutes for the scope to cool down. During this time, the lenses or mirrors will be somewhat warped from their ideal shape, and there may be moving air currents inside the tube. These effects will cause image distortion until things settle down. A big telescope generally takes longer to come to equilibrium.

With all planets in amateur telescopes, the images, even at high magnification, will be quite small.  You may find this disappointing at first.  But even small images can present nearly as much detail and color as you see in magazine-grade images of Jupiter (see image below).  Wait for moments of steady air and you will see tiny details snap into view, including the smaller belts and zones, festoons, the GRS (if visible) and possibly other smaller spots and shadows cast by the moons. Patience is critical: you might need to watch Jupiter through your telescope for half an hour to get just a few flashes of detail when the air steadies.  But these fleeting moments make the effort worthwhile.

Jupiter as it might appear at 200x in a small telescope in steady seeing

Jupiter as it might appear at 200x in a small telescope in steady seeing

In time, as you build your astronomical toolkit, you will acquire color filters which fasten onto the back of your eyepieces.  These filters help bring out more detail on the planets.  A quick suggestion: try a green filter (#56 or #58) or a dark blue filter (#38A) with Jupiter to bring out the contrast of the red belts and spots.

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