For the next few months take a look at Jupiter. It will be the brightest light in the evening sky this side of the moon.
Imagine you are Galileo Galilei and it is Jan. 7, 1610. You take your homemade telescope and fix it on a bright "star." You see three round balls, then a day later, four balls alongside a much larger ball. You thought you were just looking at Jupiter. You see that these four objects never leave the vicinity of the larger one. They appear to be carried along under its influence. And, miracle of miracles, they change their position with respect to one another and to the larger planet.
You can't believe your eyes. Earth is supposed to be the center of the universe and everything, including the sun, revolves around it. But here are moons orbiting a planet other than Earth. The world you knew is no longer the world you know.
In honor of Galileo's cosmic discovery, we call these four orbs the Galilean moons. Each roughly the size of our own moon, the four are named after attendants to the mythical god Jupiter: volcano-blotched Io, ice-bound Europa, Mercury-size Ganymede, and meteor-cratered Callisto (at last count Jupiter had 28 moons).
Jupiter orbits the sun once every 12 years. Every six years the sun and Earth pass through the orbital plane of Jupiter's equator, with Earth in between, giving us a full-orbed view of the gas giant. This takes place over the next few months.
Using a modest backyard telescope, you can see the movement of the Galilean moons. With a professional-grade telescope, you can see how they step into each other's shadow, eclipsing or occulting each other, as well as casting shadows on Jupiter.
We know much more about these moons than Galileo could ever have imagined. Their behavior became a model for the way the planets of the inner solar system orbit the Sun.
Consider Io and Europa, two of the more fascinating satellites in the solar system.
Io is closest to Jupiter and our solar system's densest and geologically most active object. Astrophysicists study Io's volcanic activity to understand gravitational tides, the interaction of gravity on bodies in space. Jupiter pulls Io one way, while the other three moons tug it in other directions. Prof. Eric Chaisson of Tufts University likens this to a wire being constantly bent. The bending generates enormous heat, which in turn is released through volcanic eruptions. This explains the relatively smooth, crater-free surface of Io as it repeatedly lathers itself in molten matter.
Astrobiologists are intrigued at the possibility of life forms beneath Europa's ice fields. Because water is considered necessary for life, Europa is a prime candidate for the discovery of life on a planet other than Earth.
In December, Jupiter's visible diameter will grow by almost 10 percent, its size seeming to increase until February, when it sits at the farthest point in its orbit from the sun. Observe it through a telescope over a long, moonless night and you will easily see a 360-degree view of its surface, since Jupiter makes a complete rotation every 10 hours.
The spacecraft Galileo, launched in 1989 to study Jupiter and its moons, is itself moving through this same field of view. In September 2003, the hardy little voyager will speed through Jupiter's powerful magnetic field and deadly radiation to meet a crushing end from the gravitational pressure of the planet.