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 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

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.