Amateur observing patrols of Jupiter were begun over one hundred years ago by the BAA’s A. Stanley Williams during the 1886-87 apparition. This patrol work is still conducted by dedicated amateurs in the United States, England, Japan, Europe, and Australia. Notwithstanding the current Galileo mission, professional astronomers do not now patrol the planet as actively as they did in years past. Long-term monitoring of Jupiter's cloud patterns is firmly in the hands of enthusiastic observers who contribute to the one-hundred-year-old Jovian record that has been amassed by amateur astronomers before them.

Jupiter can be profitably observed for about ten months each year. The very best time to view it is at opposition, when the planet is visible all night long and its apparent diameter may be as large as 49.8”. Its apparent magnitude is -2.5, outclassed only by the Sun, Moon and Venus. Jupiter is always large enough to permit serious study with small telescopes.

Even the smallest instruments will record the dark belts, the yellowish zones, and the four large satellites. With increasing aperture the observer will be able to see the Red Spot and other fine details. With a magnification of 40X, the planet's apparent diameter is the same as that of the Moon when viewed with the naked eye. A 4” refractor or a 6” reflector is recommended for serious work. But larger apertures will more. The rule is “the bigger the better”!

You should observe the planet as often as your schedule permits and the weather allows. By persistent patient observation you will become a first-rate observer. At first, you may have difficulty seeing the structural detail of the belts and zones. After a few weeks of continued observing, you will begin to see the dark spots, projections, bright features and faint contrasts on the planet.

While observing you should be comfortable. Your optical system should be acclimated to the night air. Concentrate upon small portions of the planet’s disc and use averted vision to detect faint details. Don’t hesitate to increase magnification if necessary. However, bear in mind that Jupiter does not take the highest powers well. Anything over 300X may result in a washed out or fuzzy disc. Observers with larger telescopes usually use 195 to 350X. The best rule is to try for 30X to 40X per inch of aperture.

Drawings are very important to the observer of Jupiter. Some observers are gifted in this technique and perform well on their own. However, some basic guidelines may help the serious student to become a better draftsman of the giant planet. 

• ·  Try to complete your sketch in less than ten minutes. If a longer interval is used, the markings will appear to be distorted due to the planet’s rotation.

• ·  Do not make a sketch unless seeing is good and adequate detail is resolved.

• ·  Sketch the larger, more prominent features first; then position the remaining ones relative to them.

• ·  Be careful to position the belts accurately on your drawing.

• ·  Draw the markings in their proper scale relative to each other.

• ·  Shadings and subtle contrasts may be more adequately done with an artist's stump and pencils of varying softness. Erase areas with a pointed eraser to represent bright spots.

• ·  Complete your sketch at the telescope and check it out before closing up.

• ·  Experiment with different powers and use the magnification that gives the clearest definition that conditions allow.

Since Jupiter is a gas giant, markings at different latitudes rotate at different rates. Observations have determined that in effect, there are two main rotation rates. They are known as System I (equatorial) and System II (polar). Calculate the two CM longitudes for the time of your sketch using the tables found in the Astronomical Almanac, the BAA Handbook or Sky & Telescope magazine.

It is informative to make strip sketches of the belts and zones, whenever they are very active. Include all markings in proper scale, of course. You may also make detailed sectional drawings of disturbances and the GRS region.

Timing central meridian transits is another important scientific tool. To do this you will need a good watch in addition to the telescopic tools described above. This technique provides a very accurate idea of a marking’s exact position, allowing its drift rate in longitude to be determined.

Recording colors is important. Jupiter exhibits a rich palette of colors with tones of red, brown, yellow, white, gray, and blue. For color estimates a reflecting telescope is absolutely necessary because of its achromatic quality. The power of magnification should always be the same for these estimates and the planet should ideally be at least 40° above the horizon. Detailed instructions on observing Jovian colors are available from the coordinator.

Intensity estimates are also of value. The observer estimates the intensity of the dark belts and bright zones, or selected individual features, by using a numerical scale. The ALPO scale is 0 to 10, where 0 is blackest and 10 is brightest. This scale is much more accurate than vague word descriptions and with practice it can become a very reliable tool. These observations permit us to keep track of changes in intensity of Jovian markings over periods of many years, and they may even form a guide for determining correlations with solar activity.

Timing the eclipses of Jupiter’s Galilean satellites is also a favorite activity for ALPO observers.         The four major satellites of Jupiter form a miniature Solar System, where "years" are measured in days. The motions of these worlds have interested astronomers since Galileo and are still of interest for two reasons: improving the "models" of their orbits and investigating the effects of tides on the movements of Io and Europa. In order to do this, we need accurately to measure the positions of these moons. From Earth the two best means of doing this are by photographic astrometry with large telescopes and by timing when these bodies enter or leave the shadow of Jupiter. This last approach is surprisingly accurate, and is well within the means of most amateur astronomers..

The ALPO began a program of visually timing Galilean satellite eclipses in 1975, and to date we have gathered over 10,000 such timings by observers throughout the world.

The appearance of a satellite eclipse is as follows. An eclipse begins with the satellite's ingress (disappearance) and ends with its egress (reappearance). Note that an eclipse ingress or egress may be invisible because the disk of the planet gets in the way. For example, only disappearances of Io are visible before opposition, and only reappearances after that date. Except for brief periods the same is true for Europa. On the other hand, both disappearance and reappearance can often be seen within a few hours of each other for Ganymede and Callisto. Callisto has eclipses only half the time ¾ for three-year periods separated by three years.

The first step in timing a satellite eclipse is to know when and where it will occur. If the event is a disappearance, you need to be able to identify the correct satellite before it disappears. If you are observing a reappearance you need to know where, in relation to the planet, the satellite will reappear. This information is available in the Astronomical Almanac, the BAA Handbook and Sky & Telescope magazine.

Besides satellite eclipses by the shadow of Jupiter, satellites are occulted when Jupiter blocks their view from Earth. A satellite can also pass "in front" of the planet. This is called a transit. When its shadow crosses Jupiter, it is called a shadow transit. We have found that, in general, these events cannot be timed as accurately as can eclipses. However they are spectacular to watch!

Due to their larger diameters and slower velocities, the eclipses of the outer two Galilean satellites are much more leisurely affairs than those of the inner two. The difference in the ingress or egress duration is even more extreme, due to the added factor of the greater width of Jupiter's shadow when it is located farther from the planet. Callisto can even undergo partial eclipses, with a maximum duration over two hours. Glare from Jupiter is less of a hindrance when timing the outer satellite eclipses than is the case with the inner satellites.

Making visual timings of these eclipses is very simple. In the case of ingress, time the moment when the "last speck" of the satellite disappears. For egress, time the reappearance of the "first speck." You can do this with a telescope over about 2 inches in aperture. We recommend fairly high magnifications (the minimum is perhaps 60-80X) in order to clearly separate the satellite from Jupiter's glare.

The Jupiter Section coordinators welcome any observational contributions we receive. Please feel free to ask questions whenever they arise.

And remember ¾ every observation of Jupiter should include the following information: date, time (in UT), name and address of the observer, telescope used (with aperture and type), magnification, the seeing (on a scale of 1-10), and the sky transparency (on a scale of 0-6). It is also important to include CM values (both systems) for the time of your observation, as well as any filters you may have used.