Jupiter has often been referred to as the amateur's planet, and it offers a wealth of opportunities for amateur observers to make substantial contributions to the science of astronomy. All it takes are determination and the effective use of equipment you might already have.
Equipment Matters
When the topic of making scientific-quality Jupiter observations is discussed, one of the first questions to arise is, what telescope is best? The answer is simple: one capable of producing high-contrast views. This generally narrows the field down to color-corrected refractors and long-focal-length Newtonian reflectors. Telescopes with contrast-robbing large secondary mirrors, such as Schmidt-Cassegrains or Maksutov-Cassegrains, are less desirable.
Having said that, no observer should put off observing Jupiter for lack of the "perfect" telescope. Many valuable observations have been made by amateurs using ordinary Schmidt-Cassegrains. The truth is, the "best" telescope is one the observer uses well. Regardless of telescope type, its optics should be of high quality and perfectly collimated. Size is also important — a well-made 5-inch refractor or 6-inch reflector on a sturdy tracking mount is the minimum for serious Jupiter observing. Larger instruments are even better, since they will allow scrutiny of fine detail and subtle low-contrast markings.
Although Jupiter is large 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's optics, those of the eyepieces 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 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.
The Observations
Of course, even the best telescope fitted with the proper filter is still at the mercy of the churning atmosphere above us (Sky & Telescope: January 2000, page 125). 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. Jupiter is also 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
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.
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.
Today many amateurs have put aside pencil and paper in favor of CCD cameras. In the hands of skilled amateurs, CCDs 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 with CCD cameras. During the most recent Jupiter apparition, CCD images provided crucial observations of features that may otherwise have been missed.
A Tale of Two Ovals
Jupiter watching can be both exciting and scientifically rewarding, as illustrated by the case of the South Temperate Belt (STeB) ovals. For 60 years, three large bright ovals, designated BC, DE, and FA, had been observed in the STeB. However, when Jupiter emerged from the Sun's glare at the beginning of the 1998-99 apparition only two remained! Apparently BC and DE had merged while Jupiter was out of view. This was startling news to many astronomers who had thought the dynamics of these ovals would simply cause them to bounce off each other if they ever came into contact. The new merged oval (renamed BE) appeared larger but a little less bright than the original unmerged pair. While their drift rates and positions changed, the two surviving ovals remained separate.
During the 1999-2000 apparition, observers continued to closely watch the dramatic movements of BE and FA. In April 1999 their centers were separated by 20° of longitude. But as the apparition progressed, amateur CM-transit timings and CCD images showed the gap between the ovals was closing. Organizations such as ALPO and the British Astronomical Association kept the professional community supplied with updates and position measurements based on data acquired by amateur astronomers. These data were hard won — the contrast between the ovals and the background zone is very low.
As the apparition was nearing its end, the gap between BE and FA continued to close, prompting ALPO's Jupiter Section to issue numerous alert messages over its J-Net e-mail network. In February 2000, measurements indicated that the centers of the ovals were only 12° apart. Perhaps more important, the following edge of BE and the preceding edge of FA were separated by only 5°! Would the ovals merge before solar conjunction? Anticipation was running high.
On March 4, 2000, Glenn Orton of NASA's Jet Propulsion Laboratory announced that infrared observations showed the ovals' outer rings touching. However, CCD images by Florida amateur Maurizio Di Sciullo showed them as separate but very close. Images taken by António Cidadăo in Portugal on March 15th and 20th and by Tim Parker in California on March 19th indicated BE had shifted north and FA appeared to be overtaking it!
A Million Stories
The story of Jupiter's ovals is only one of many that play out from year to year. 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. During the last two apparitions, the position of the GRS had been fairly constant: 60° to 62° System II longitude in 1997 and 64° to 66° in 1998-99. But toward the end of 1999 all that began to change. In early October the spot's position was reported to be 68° to 70°. In late December and early January 2000, transit timings and CCD images indicated the GRS had shifted to 74°, where it remained through the end of the apparition. As of late 2003, the Red Spot is at Jovian System II longitude 84°. Changes in the feature's longitude are not unusual, but what made this episode noteworthy was that it had suddenly moved so far after being stationary for so long.
© 2005 Reprinted with permission from Sky Publishing Corp.
