# Other Moons

By Riddle, Bob | Science Scope, February 2007 | Go to article overview

# Other Moons

Riddle, Bob, Science Scope

Byline: Bob Riddle

Recent observations of the largest planet in our solar system, Jupiter, have led to the discoveries of additional moons, or satellites, orbiting the planet. Jupiter, at least so far, has the greatest number of orbiting satellites of the eight planets, with 63. These satellites include both regular and irregular satellites, with some having prograde revolutions and others orbiting Jupiter in retrograde paths. Regular satellites are those that formed with Jupiter, while irregular satellites are those objects believed to have had orbits around the Sun and were subsequently captured by Jupiter's gravitational attraction.

Questions for Students

As Galileo observed Jupiter, he noticed that there were times when not all of the Galilean satellites were visible. Explain how this might happen. (One or more of the Galilean satellites may not be visible as there will be times when they are either in front of or behind the disk of the planet.)

From your data, determine the approximate orbital periods for each of the four Galilean satellites. (Io orbits Jupiter in about 1.8 days; Europa orbits Jupiter in about 3.5 days; Ganymede orbits Jupiter in about 7.2 days; Callisto orbits Jupiter in about 16.8 days.)

From your data, determine which of the Galilean satellites is closest to Jupiter. Which is the farthest? (The moon with the shortest orbital period will be the moon that is the closest to Jupiter. Io is the closest to Jupiter of the Galilean satellites, while Callisto is the farthest away.)

Nearly 400 years ago, on January 7, 1610, Galileo Galilei turned his new telescope toward the planet Jupiter. As he observed the planet, he commented on three bright stars arranged in a straight line near Jupiter (see Figure 1).

The following evening, he observed that these same three stars were now arranged in a different pattern. Over the course of the next several evenings, Galileo observed a fourth star. He determined that the change in the positions of these stars was not a result of Jupiter's proper motion against the fixed stars in the background, and concluded that these four bodies were not fixed at all, but rather were objects in orbit around Jupiter.

In a tribute to the Grand Duke Cosimo II de Medici and his very influential family, Galileo named these four stars the Medicean stars. Galileo had tutored the Grand Duke in mathematics, and over the years the two stayed in touch. We now know these moons as Io, Europa, Callisto, and Ganymede-the Galilean satellites.

In his notes and later in a publication titled Sidereus Nuncius, or The Sidereal Messenger, Galileo described his observations of Jupiter and its four moons, as well as his observations of our Moon and the planet Venus. A translation of this book was completed in 1800, and an online copy is available for your students to read courtesy of the Linda Hall Library (see Resources).

"On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view. . . . I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude. …

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