Visibility
All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted from Earth without a telescope. (They are, however, easily visible with even low-powered binoculars.) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and are about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter since they are obscured by its brightness. The maximum angular separations of the moons are between 2 and 10 minutes of arc from Jupiter, close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to "cover" Jupiter with an object, e.g., a tree limb or a utility pole that is perpendicular to the plane of the moons' orbits.
Read more about this topic: Galilean Moons
Famous quotes containing the word visibility:
“My children have taught me things. Things I thought I knew. The most profound wisdom they have given me is a respect for human vulnerability. I have known that people are resilient, but I didn’t appreciate how fragile they are. Until children learn to hide their feelings, you read them in their faces, gestures, and postures. The sheer visibility of shyness, pain, and rejection let me recognize and remember them.”
—Shirley Nelson Garner (20th century)