The Satellites of Jupiter

Jack Kramer

With Jupiter well up in the evening sky now (January 2001), it takes its place as one of our favorite telescopic targets. Beyond the intricate detail of the cloud belts and Great Red (presently tan) Spot, there is also the dance of the four bright Galilean moons.

The satellites are visible in binoculars; in fact, some people have claimed naked eye visibility for satellites that are farthest from the planet. A good 4-inch telescope should show the satellites as distinct disks, rather than as star-like objects. Some observers have been able to determine which satellite is which based on the size and coloration. With my 4-inch refractor I can generally identify Ganymede just by its larger size. But to identify by color you must have very good optics and an aperture of at least 6-inches. The unique aspect of each satellite comes into play here. For example, Callisto often appears smaller than it really is because of its low albedo (reflectivity) - 0.2 as opposed to 0.4 for Ganymede. You're really seeing just the central part of the disk.

As Jupiter's satellites cross the disk of the planet, their shadows are usually visible in telescopes of far smaller apertures than one would expect, based on the shadow diameters. Apparently the Dawes limit applies only to point sources; with extended sources the rules seem to change. The sizes of the shadows are always quite a bit smaller than the satellites themselves, since we're mainly seeing the umbra, though the penumbra could make them appear somewhat larger. This becomes more pronounced the farther out the moon is from Jupiter. Here's a table of the angular diameters taken from Terry Dickinson's article in the Jupiter section of the RASC Observer's Handbook:

Moon Disk Shadow
Io 1.2" 0.9"
Europa 1.0" 0.6"
Ganymede 1.7" 1.1"
Callisto 1.6" 0.5"

With a large enough telescope, you should also be able to see the satellite itself crossing above the surface of Jupiter. It will appear as a speck that is just a bit brighter than the background of the planet. Again, the albedo of the satellite affects its visibility, and it also helps if the satellite is superimposed over a darker feature on the surface of Jupiter.

Published in the January 2001 issue of the NightTimes