Jupiter: An Observing Guide

Rotating Jupiter
One complete rotation of Jupiter can be seen in this CCD sequence obtained from December 1999 through February 2000. The series advances in rotation by approximately 25° of Jovian longitude per image. South is up.
Donald C. Parker

Jupiter has been called "the amateur's planet," because it offers a wealth of opportunities for amateur observers to make contributions to planetary astronomy. All it takes are determination and the effective use of equipment you may already have.

Equipment Matters

For planetary observations, what telescope is best? The answer is simple: the one best capable of giving sharp, high-contrast views. First and foremost, this means large aperture. Right up there too is top-notch optical quality. Next comes telescope type; the best planetary scopes have traditionally been apochromatic or long-focus achromatic refractors and long-focal-length Newtonian reflectors. Telescopes with contrast-robbing large secondary mirrors, such as Schmidt-Cassegrains or Maksutov-Cassegrains, have been considered less desirable, but their central obstructions can be more than made up for by large aperture (assuming the optical quality is high), and many of these scopes have produced impressive results in recent years.

But no observer should put off observing Jupiter for lack of the perfect telescope. The truth is, the "best" telescope is one that you use rather than an ideal one that you don't use. Regardless of telescope type, the optics should be perfectly collimated. A well-made 5-inch refractor or 6-inch reflector on a sturdy tracking mount is really about the minimum for serious Jupiter observing. Larger instruments will allow scrutiny of fine detail and subtle low-contrast markings.

John McAnally & his 8-inch Schmidt-Cassegrain
John W. McAnally is seen here with his 8-inch Schmidt-Cassegrain telescope, the instrument he uses for most of his Jupiter observing. Says McAnally, 'I pay very close attention to the telescope¹s collimation, which really makes a difference in being able to see subtle detail on Jupiter.'

Although Jupiter is big and bright, it doesn't tolerate high magnification well — the image tends to go soft quickly. Consequently, you will rarely use more than 40x per inch of aperture. I find that my 8-inch is limited to about 200x on nights of steady seeing. As with the telescope itself, the eyepiece too must deliver sharp, high-contrast views. Many serious planetary observers prefer high-quality Plössl or orthoscopic eyepieces to complex models designed for ultrawide-field views.

Color filters that screw into eyepiece barrels can improve the contrast of certain Jovian features and assist in identifying them. As a general rule, choose a filter with a color opposite that of the feature you want to observe. For example, the Great Red Spot (GRS) and reddish brown belts are best seen with blue filters such as Wratten 82A (light blue), 80A (medium blue), or 38A (blue). Red filters such as Wratten 21 (orange-red), 23 (light red), and 25 (red) can be used to enhance bluish features, such as the projections and festoons often found on the southern edge of the North Equatorial Belt. I like to use yellow filters such as Wratten 12 (medium yellow) and 8 (light yellow) to enhance the contrast of the polar regions. The Wratten 8 filter is especially effective as a general-purpose contrast enhancer.

Experimentation is the best way to discover which filter works best with a given Jovian feature. For example, I've found yellow filters especially effective for viewing the low-contrast south temperate ovals. Depending on the viewing conditions, observing without a filter sometimes proves to be the best strategy.

Observing Jupiter

Sketch showing Jovian belts and zones
Almost any kind of Jupiter observation requires familiarity with the correct names for the various belts and zones. Here north is up; in an inverting telescope such as a Newtonian reflector, or a refractor, Schmidt-Cassegrain, or Maksutov used without a star diagonal, north will be down and east to the right. Telescopes used with a star diagonal will have north up but east and west reversed. The planet's rotation causes features to move from east (following) to west (preceding).
Sky & Telescope illustration

Of course, even the best telescope fitted with the proper filter is still at the mercy of the churning atmosphere above us. The Association of Lunar and Planetary Observers (ALPO) uses a scale of 0 to 10 to describe seeing conditions, with 0 being the worst and 10 the best. Unless the seeing is better than 5, you will most likely have to wait for another time to do high-power observing.

Jupiter is thrilling to view in just about any telescope. Even a small department-store refractor will reveal several cloud belts and its four brightest moons. Our mobile app JupiterMoons can help you find your way. It shows the locations of the Great Red Spot and the four largest moons at any day and time, and it also includes a detailed chart of Jupiter's atmospheric bands.

Jupiter is one of the most dynamic telescopic sights — you never get the same view twice. This is partly the result of its rapid rotation — gas-giant planets like Jupiter exhibit differential rotation; that is, they rotate more rapidly at the equator than they do at the poles. Jupiter's observable "surface" has two general systems of rotation that differ by approximately 5 minutes: System I (9 hours 50.5 minutes) and System II (9 hours 55.7 minutes). Most of the planet falls under the System II rotation rate, while System I rotation applies to the Equatorial Zone.

If you want to seriously study Jupiter, you should observe it as often as possible; the more time you spend at the eyepiece, the more adept you will become at seeing the planet's most subtle features.

Sketching Jupiter

Strip sketch of Jupiter.
A long-time ALPO observer, Ohio amateur Claus Benninghoven used a 12-inch f/15 Clark refractor running at 220x and Wratten 82A (light blue) and 58 (green) color filters to record Jupiter¹s appearance on November 15, 1999, in this strip sketch. Note the series of festoons stretching out into the Equatorial Zone from the southern edge of the North Equatorial Belt. Click on the image to see the entire sketch.
One way to get to know Jupiter is to make full-disk drawings of its ever-changing cloudtops. Usually this involves sketching the entire planet in a single session on a preprinted form. Be sure to note the date and time (in Universal Time) you began and ended your drawing, as well as the seeing conditions and the type of telescope, magnification, and filters used, if any.

A variation on the disk drawing is the strip sketch. To make a strip sketch you normally concentrate on only one or two belts or zones at a time. By focusing attention on a smaller portion of the planet, more detail can be recorded. Because of this, a strip sketch is often more valuable than a full-disk drawing.

Sketch of Jupiter
How much can you see? This beautiful and highly accurate disk drawing was made on November 25, 1999, by Ohio amateur Phil Plante using a 6-inch f/5 Newtonian reflector at 228x. In moments of steady seeing a scope of this size is capable of showing even greater detail. South is up.
Because of the planet's rapid rotation, full-disk drawings should be completed in 20 minutes or less to ensure that features are accurately plotted with respect to one another. A strip sketch, by contrast, may be continuous, recording features as they cross the planet's central meridian (CM), the imaginary north-south line that crosses the center of the planet's disk. Observing forms for both types of drawing can be found at ALPO's Web site.

While drawings are useful, timings of the central-meridian transits of Jovian features are the most scientifically valuable data an amateur can produce. According to Phillip Budine, the former assistant coordinator for transit timings of ALPO's Jupiter Section, "Visual CM transit observations have provided almost all that is known about the rotational characteristics of Jupiter."

Patient observers can produce a wealth of data. The procedure couldn't be simpler: using a watch accurate to within 30 seconds, note the time (in UT) a feature appears on the central meridian. For large features, such as the GRS, note the CM transit times for the preceding edge, middle, and following edge, and take the average. Later, you can find the Jovian longitude of the feature by simply checking the time noted against a published ephemeris or one of the many computerized charting programs that calculate Jovian longitude. If you observe a particular feature long enough, you may notice its position changing. By plotting the feature's longitude against the date of the observation, you can find the feature's drift rate and therefore the planet's rate of rotation at that particular latitude.

Painting of Jupiter
An accomplished artist, Greg Mort made this painting of Jupiter and its moon Europa based on observations from October 1999 with his 12½-inch tri-schiefspiegler telescope. Mort¹s goal was to accurately capture the planet's subtle colors. South is up.

Today many amateurs have put aside pencil and paper in favor of CCD cameras or, especially, small, cheap, lightweight webcams. These can produce incredible images that can yield the same type of data as the methods described above. In addition, many features with contrast too subtle for visual observation can be captured this way. During recent Jupiter apparitions, webcam images have provided crucial observations of features that might otherwise have been missed.

A Million Stories

Jupiter and the Great Red Spot
Jupiter with the Great 'Red' Spot transiting (crossing) the central meridian. Amateur Ed Grafton took this beautifully detailed CCD image at 2:19 Universal Time Feb. 15, 2002, through a 14-inch Schmidt-Cassegrain telescope.

Perhaps the best-known character in this ongoing drama is the Great Red Spot. This immense oval-shaped anticyclone has been observed for at least 300 years, and it wanders east and west on the planet unpredictably. From 1997 to 1999 its position was fairly constant: 60° to 62° System II longitude in 1997 and 64° to 70° in 1998-99. But toward the end of 1999 it started taking off. By early January 2000, transit timings and CCD images indicated the GRS had shifted to 74° longitude, and it kept going. As of 2008, the Red Spot was at Jovian System II longitude 104°. Changes in its longitude are not unusual, but what made this episode noteworthy was that it suddenly moved so far after being stationary for so long.

And Jupiter's dynamic atmosphere continues to keep astronomers on the lookout. In 2006 amateur astronomers spotted a new spot on Jupiter. Dubbed Red Spot Jr., the storm developed throughout early 2006. What might happen next? Keep your eye on the King of Planets to find out.

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