Observing Jupiter

Why observe Jupiter? Shows the greatest amount of detail in amateur telescopes of all the planets. Constantly changing. Can be followed for most of the year. Easy to observe even with smaller telescopes. Can be observed even from the city.

Jupiter is like a miniature Solar System Planet is 88,000 miles in diameter (11x Earth). Composed of mainly gas (Hydrogen, Helium, and trace amounts of Methane and Ammonia). Planet rotates once every 10 hours. 50+ moons. 4 large moons (probably formed with Jupiter). Many smaller moons which may be captured asteroids or comets.

Equipment for Observing Jupiter 3 to 12 inch telescope (6 inch + best). Good eyepieces and barlows that will give 40x per inch of aperture (Plossls, Radians, etc.). Good telescope mount (with a drive, optional). Filters: Light Blue or Light Green (optional). Sketch pad and pencils. Watch (accurately set to UT).

Features on Jupiter

What to look for Major Belts (NEB and SEB). Minor Belts (NTB, STB, etc.). Festoons in EZ. Great Red Spot (GRS). Smaller White and Red ovals. Atmospheric Disturbances.

Timing Atmospheric Features Jupiter’s atmosphere actually has more than one rotation rate. The EZ rotates once every 9h 50m. The rest of the planet rotates once every 9h 55m. Other features like the GRS and smaller ovals drift slowly relative to the rest of the planet

To time a feature, watch Jupiter until the feature appears centered on a North-South line bisecting the planet. Note the time to the nearest minute. For large features like the GRS, the leading and following edges can also be timed. Use a planetarium program or almanac to find the longitude at the times you measured (be sure to use System I longitudes for EZ features and System II longitudes for the rest of the planet). Changes in longitude can be seen in as little as a couple of weeks for some features.

Sketching Jupiter On a good night, Jupiter can show a wealth of detail. At such times, making a drawing of the planet is a simple and satisfying way to make a record of what you see. Using a small sketch pad and pencil quickly draw a rough sketch just showing the outlines of features. Label each feature with a number from 0 to 10 to describe how light or dark a feature is (0 = black, 10 = white). Also note any obvious colors or other interesting details.

Write down the time (to the nearest minute), the telescope and magnification, plus any other relevant information. Later or the next day indoors, the rough sketch can be filled in and shaded using the intensity estimates that were made at the eyepiece. For the best results, use pencils with soft lead. An eraser can also be used to gently blend the lead.

Observing Jupiter’s Moons The four largest moons of Jupiter (the Galilean moons) can be seen even with binoculars. These moons all lie in the plane of Jupiter’s equator which means they pass in front of and behind Jupiter as seen from Earth. When the moons pass in front of Jupiter, they cast their shadows on the planet. The shadows look like bullet holes and take about an hour or so to cross the planet.

When the moons pass behind Jupiter, they are occulted by the planet. These occultations usually take 5 to 10 minutes for the moon to completely disappear or reappear. The moons can also be eclipsed by Jupiter’s shadow. When watching one of these, the moon will start to fade (or brighten) very gradually then more rapidly until the moon is completely faded from view (or fully brightened in the case of a reappearance). The disappearances (or reappearances) last about 5 to 10 minutes.

Resources Sky & Telescope magazine’s Jupiter Observing Guide: s/planets/ html?page=1&c=y s/planets/ html?page=1&c=y Association of Lunar and Planetary Observers: Christopher Go’s Jupiter images: