Observing Projects
You can easily find the apparent positions of the Galilean satellites for any date and time using the Starry Night™ software if you have access. For even more detailed information about satellite positions, consult the “Satellites of Jupiter” section in the Astronomical Almanac for the current year. If your goal is to view Saturn’s satellites, consult the section entitled “Satellites of Saturn” in the Astronomical Almanac for the current year. This includes a diagram showing the orbits of Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Hyperion. Plan your observing session by looking up the dates and times of the most recent greatest eastern elongations of the various satellites. You will have to convert from universal time (UT), also known as Greenwich Mean Time, to your local time zone. Then, using the tick marks along the orbits in the diagram, estimate the positions of the satellites relative to Saturn at the time you will be at the telescope. Another useful resource is the “Celestial Calendar” section of Sky & Telescope. During months when Saturn is visible in the night sky, this section of the magazine includes a chart of Saturn’s satellites.
Observe Jupiter through a pair of binoculars. Can you see all four Galilean satellites? Make a drawing of what you observe. If you look again after an hour or two, can you see any changes?
Observe Jupiter through a small telescope on three or four consecutive nights. Make a drawing each night showing the positions of the Galilean satellites relative to Jupiter. Record the time and date of each observation. Consult the sources listed above in the “Observing tips and tools” to see if you can identify the satellites by name.
If you have access to a moderately large telescope, make arrangements to observe several of Saturn’s satellites. At the telescope, you should have no trouble identifying Titan. Tethys, Dione, and Rhea are about one-sixth as bright as Titan and should be the next easiest satellites to find. Can you confidently identify any of the other satellites?
Use Starry Night™ to observe the moons of Jupiter. Select Favourites > Explorations > Galilean Moons from the menu. (a) With the Time Flow Rate set to 2 hours, use the Step time forward button (just to the right of the Play button) to observe and draw the positions of the moons relative to Jupiter at 2-hour intervals. From your drawings, which moon orbits closest to Jupiter and which orbits farthest away? Explain your reasoning. (b) Determine the periods of orbits of these moons (change the Time Flow Rate if necessary). (c) Are there times when one or more of the satellites are not visible? What happens to the moons at those times?
Use Starry Night™ to examine some of the satellites of Saturn. Open Favourites > Explorations > Saturn’s Moons. The view shows Saturn from a position in space hovering above the planet’s equator in the plane of the rings. Many of Saturn’s moons are labeled and their orbits are also shown. You can use the elevation controls in the toolbar (the buttons to the left of the Home button) to move closer to and further away from Saturn. This allows you to identify the satellites of Saturn, since they will appear to move whereas the distant stars will remain stationary as you move with respect to the planet. Alternately, you can reduce the confusion in identifying these moons by clicking on View > Stars > Stars to remove the stars from the view. You can also use the location scroller to obtain a different perspective and help you to identify more of Saturn’s moons. Finally, you can position the cursor near to the planet precisely in the plane of the rings and when one of the inner moons, which are dimmer and more difficult to see, passes the cursor, the HUD will briefly display its name. You should be able to find at least eight satellites. (a) Which satellites are these? (b) Saturn’s rings are in the same plane as Saturn’s equator. Which satellite’s orbit appears to be the farthest from this plane?
Use the Starry Night™ program to view Saturn from the surface of its satellite Enceladus. Open Favourites > Explorations > Enceladus from the menu. The view shows Saturn as it might appear if you were observing it from the crater named Ahmad on the surface of Enceladus. With the Time Flow Rate at 1 minute, click the Play button and observe the position of Saturn in the sky as seen from Enceladus. (a) How do the stars appear to move? (b) How does Saturn appear to move in the sky? (c) What do these observations suggest about the orbital and rotational periods of Enceladus?
Use Starry Night™ to explore the orbits of some of Saturn’s smaller outer satellites. Open the view Favourites > Explorations > Small Outer Saturn Moons from the menu. This view from a location in space hovering above Saturn in the plane of the rings shows the orbits of a number of the smaller, outer moons of the planet. For comparison, the orbit of Titan is also shown in the view. (a) What are some of the striking differences between the orbits of these smaller outer moons and that of Titan? (b) What hypothesis might account for these differences?
Collaborative Exercises
Imagine that scientists are proposing to send a robotic lander to Jupiter’s satellite Callisto. Create a 100-word written proposal describing a robotic lander mission to another of the Galilean satellites, explaining why your group found it to be the most interesting and why the government should allocate the money for your alternative project. In your proposal, be sure to demonstrate your knowledge of Callisto and the other satellite.
From the data and the accompanying images for Table 13-1: Jupiter’s Galilean Satellites Compared with the Moon, Mercury, and Mars, use someone’s shoe to represent the 150,000-km diameter of Jupiter and determine about how many “shoes” away would each of the Galilean satellites be from Jupiter.
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