The Lake County Astronomical Society

Jupiter and its Moons

Jack Kramer

From one night to the next, you never quite know what new sight Jupiter might present. It's one of the most dynamic planets, with intricate and changing details in the cloud belts. There's also the Great Red Spot (plus the recent Little Red Spot), which hasn't quite returned to its former noticeably ruddy color. The following diagram will help identify in which hemisphere or zone a feature is located and provide the correct terminology for your observing notes.

A major draw is the dance of the four bright Galilean moons, which are visible in binoculars. In fact, some people have claimed naked eye visibility for a couple of satellites when they are farthest from the planet. A good 4-inch telescope sometimes shows them as distinct disks, rather than as star-like objects. It's possible to determine which satellite is which based on size and coloration. To do this you must have exceptionally fine optics, an aperture of at least 6-inches, and the seeing must be very steady. The unique aspect of each satellite comes into play here. For example, Callisto tends to appear smaller than you'd expect due to its low reflectivity - you're really seeing just the central part of the disk. Ganymede will appear as the largest of the moons.

As Jupiter's satellites cross the disk of the planet, their shadows are often visible even in relatively small telescopes. The sizes of the shadows are always quite a bit smaller than the satellites themselves, since we're mainly seeing the darker umbra; if you were able to also see the penumbra, that would make the shadows appear larger. This becomes more pronounced the farther out the moon lies from Jupiter.

Here's a table of the angular diameters taken from a Terry Dickinson article in the RASC Observer's Handbook:

As you can tell, the shadow will generally appear as little more than a black dot on the surface of Jupiter.

While it's fairly easy to see a shadow, it's not quite so easy to see the satellite itself when in transit. The reason for this has to do with the percentage of light reflected by each object (its albedo) expressed as a decimal. A shadow would have an albedo of about 0 (zero). Jupiter's average albedo is .52, while those of the Galilean satellites are as follows:

So it follows that Ganymede and Callisto will appear darker than the surface of Jupiter, and Io and Europa will appear brighter. Of course, Jupiter's albedo is not quite uniform, the limb being darker than the center of the disk, and the belts and zones having various albedos.

Because of their smaller orbits, Io and Europa transit Jupiter quite often. More distant Ganymede and Callisto seldom transit. This is explained by Jupiter's maximum axial tilt of only 3o, but that's enough to shift the orbital plane of the most distant satellites above or below Jupiter, as we view it from Earth. In fact, Callisto's orbit takes it so far out that for several years at a time it never transits Jupiter. Currently, Jupiter's polar inclination (axial tilt with respect to the plane of the Solar System) is about -2.7o. When the sun is shining directly on Jupiter's equator, we'd be seeing Jupiter, and the orbits of its satellites, with no apparent tilt with respect to the plane of the Solar System. This means we would have a better chance to see transits of Jupiter's farthest-flung satellites and mutual events where one satellite occults another. Jupiter's polar inclination won't reach 0o ("straight-on") until April 2009.

There are many resources for determining when a satellite will transit or be in eclipse. Probably the oldest is Sky & Telescope's monthly chart of Jupiter's moons. Many computer programs also show satellite and Great Red Spot positions. Two such freeware programs are Cartes du Ciel http://www.stargazing.net/astropc/index.html and Meridian http://www.merid.cam.org/meridian/index.html