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# Galilean Satellites: Jupiter's Largest Moons (page 2)

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Author: Janice VanCleave

### Try New Approaches

Assuming all of Jupiter's moons start at the same point on the first day of observation, how would their apparent distances from Jupiter appear at the end of 2 Earth days? Repeat the project using a data table like Table 21.1 to determine the angular distance each moon moves during 1 Earth day. Draw orbits at the correct radius from Jupiter's core using the same scale (0.6 cm = 1 × 105 km). Use different colored markers to represent each satellite. One at a time, plot the positions of each of the satellites for 2 days.

1. The gravity (a force of attraction between all objects in the universe) of Jupiter has pulled the satellites into its equatorial plane, meaning the moons are basically in line with Jupiter's equator. At times the satellites with larger orbital radii appear closer than those with smaller radii. In the previous experiments, you modeled a face-on view of Jupiter's moons, but that's not how we see them from Earth. Find a way to model the edge-on locations of the moons as we do see them from Earth. Since we on Earth are almost perfectly aligned with the orbital planes of the moons, they seem to line up in a straight line across Jupiter's equator. One way to show this is to lay a sheet of clear plastic over the diagram of the positions of the satellites in the previous experiment. Make a red grape-size ball of modeling clay to represent Jupiter. Place this clay ball on the part of the plastic covering Jupiter. Using four different colors of clay, make four pea-size clay balls representing Jupiter's moons and place them on dot 1. Slip a piece of stiff cardboard or a book under the paper with the plastic on top, and raise the cardboard so the plane of the plastic is perpendicular to your face and Jupiter is directly in front of your eyes. Observe the positions of each of the satellites in relation to Jupiter. Two diagrams can be made as in Figure 21.2, one of the location of the satellites as seen from above Jupiter's north pole (above the plastic) and the other from the face-on view (as seen by an observer on Earth). Repeat this procedure, moving the satellites to their location after 1 Earth day and then again after 2 and 3 Earth days.
2. Observe Jupiter through a telescope for a period of 17 or more days. During this time, most of the satellites will have made many revolutions around Jupiter, but Callisto will have made only one. Design an experiment to plot the position of the satellites from day to day. Make four or more observations at least 1 hour apart to identify the satellites. They can be identified by the rate at which they move: from fastest to slowest, they are 10, Europa, Ganymede, and Callisto. For information on identifying the satellites on any specific day, see astronomy magazines, such as Sky and Telescope and Astronomy, for the month of the observation.

### Get the Facts

At the time of the publication of this book, astronomers know of 16 satellites (moons) orbiting Jupiter. Others may be discovered as our instruments continue to improve. What are the names of these 16 moons? What are their orbital paths and periods? For information, see Brian Jones, The Practical Astronomer (New York: Simon & Schuster, 1990), p.23.

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