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Jupiter

Summary: In this lab you will observe Jupiter, identify its main features, determine the length of a day on Jupiter (rotation period), and observe the motion of its four brightest moons. Parts 2 and 3 should be done more than once.

Note: Clear skies are relatively rare in Nashville, so you should make the very best of your time at the telescope and use it to OBSERVE. Use it to make sketches, make notes and write down measurements and discuss the questions asked in the lab. Leave out all calculations, writing down your answers to the questions, etc., for later, and complete the lab in the comfort of a heated, well-lit room.

Background: Jupiter is the largest planet of the solar system and the second brightest in the sky after Venus. It is a gaseous planet with a composition similar to that of the Sun, that is, mostly hydrogen and helium. Jupiter has no hard surface like the Earth: the atmosphere just gets thicker and denser at greater depth. This planet is radically different from our familiar Earth! Nevertheless, the outer layers are cold enough for some gases to form ice crystals, mainly ammonia ice, ammonium hydrosulfide ice and water ice. These clouds are very reflective and cover the entire planet. The top of the clouds is what you see through the telescope. Some sulfur or phosphorus compounds (we are not sure yet) add some coloration to the otherwise bright white clouds. The rapid rotation of the planet causes the formation of alternating dark and bright clouds bands. While the general appearance of Jupiter has been very much the same for centuries, the detailed cloud structure is ever changing. Jupiter is one of the most dynamic objects that can be observed in a small telescope.

The famous great red spot of Jupiter is an enormous hurricane system about twice as large as the Earth. This hurricane has been visible on Jupiter since its discovery by Galileo almost 400 years ago! The ``red'' color of the spot is actually quite subtle. The intensity of its color varies through the years and it has been very pale for more than a decade. The red spot is usually identifiable as an oval, white area surrounded by a darker outline, looking somewhat like a pale outline of an eye. Jupiter's rotation carries the spot to the back side once every rotation, so it is not always visible. Your TA can tell you whether the spot is visible during your lab night or not.

When Galileo pointed his crude telescope to Jupiter in 1610, he immediately noticed that it had four companions or moons. These are the four brightest (and by far the largest) satellites of Jupiter and are now known as the Galilean moons. They also are visible in binoculars. In order of increasing distance from Jupiter, they are Io, Europa, Ganymede and Callisto. They have roughly the same size as our Moon and each is a unique world in itself.

In more modern times, NASA has explored Jupiter through the Voyager I and Voyager II missions (in 1979 and 1981) and now through the active Galileo mission. For the latest information and cool pictures, consult the Galileo spacecraft home page.

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Procedures

1. Notes: Make notes describing the atmospheric conditions tonight. Is the sky clear, partly cloudy, high cirrus clouds? Is the air moist? Is it windy? How does this night compare with other nights on which you have observed (better? worse? in which way?). Think about how the weather may affect your view of Jupiter.

2. First locate Jupiter in the sky (it is brighter than all stars). Using the 25 mm eyepiece, and with the help of the finder scope, locate Jupiter in the telescope and then center it in the field of view. Make a sketch of the field of view, showing the planet, stars and moons (use the generic template for this sketch, provided by the TA). Try to make an accurate record of the positions of the objects in the field of view and reproduce carefully the relative scale of the separations between the moons and the size of Jupiter itself.. At this magnification, the moons appear just like stars, except that they are rather bright.

   Indicate the orientation of your sketch (direction of North and West in the field of view). Remember, the sky rotates from East to West, so when you turn off the motor drive of the telescope, objects drift westward in the eyepiece. If you move the telescope in declination toward the South (lower declinations), objects will drift North in the field of view.

   Using the corkscrew chart provided by your TA, identify the four Galilean moons on your sketch (do this when you are done with your observations of Jupiter for the night) . Keep in mind the orientation of the image as seen through the telescope! Also, the chart uses Universal Time. This is the time in the Greenwich (England) time zone. Universal time is UT = CST + 6 hours = CDT + 5 hours. Remember that this usually means that the date changes too! Does any moon appear to be missing? What could explain this curious observation?

   Corkscrew diagrams for Jupiter's Moons

   • February, 2003
   • March, 2003
   • April, 2003

   If you observe Jupiter more than once, notice how the moons have moved along their orbits. Comment on their orbits, based on your observations of the moons' positions.

   Note: If you are lucky, you will witness eclipses and transits of Jupiter satellites. Your TA will bring them to your attention if they occur during lab periods.

3. Sketch Jupiter: Switch to the 10 mm eyepiece. Focus as precisely as you can. You should be able to see cloud bands, at least the two most prominent ones crossing the disk near the middle (see figure). Indicate the orientation of you sketch (N and W). Does Jupiter appear perfectly round? If not, is the short dimension parallel or perpendicular to the equator of the planet? What could be the cause of this? Do you think that this oblate appearance is real or possibly just an optical illusion due to the bands crossing the disk? Can you convince yourself, either way? How?

   

   Use the planet template provided by the TA to draw the cloud features of Jupiter. An example of such a sketch is shown above, more can be found in the binder of examples kept in the telescope shed.. You don't have to be an artist to do this! Try to be accurate, showing the features at the right place on the disk, and only features you can see with certainty. Remember that before the invention of photography, this is how scientific observations of the planets and all discoveries were made. Details are usually fairly subtle and quite small. Make an effort to locate the red spot (if visible that evening) and to see structure in the equatorial region. How much you see will depend on the telescope, the atmosphere, how good of an observer you are and how patient you are. Keep comparing what you sketch with what you see in the eyepiece to ensure a faithful rendition. This can take 15-20 minutes. Always note the date, time, magnification, the orientation of the sketch (indicate N and W), etc., of your observation . Add personal comments about your observations: impressions, intriguing features, questions you may have, etc.

4. Measuring Jupiter's rotation period: You will need to sketch Jupiter again, about 1-2 hour after the first sketch (toward the end of the lab period). Again, pay attention to features visible in the equatorial region. The shift of features should be obvious. In which direction is the planet rotating (E to W or W to E)? How does this compare with the direction of rotation of the Earth? Using your two sketches to track the motion of one (or more) feature, estimate how long it would take for the planet to make one complete turn. Use the grid template provided by your TA to see how far the features have shifted in longitude. A copy of that template is shown above. Show your calculations and how you proceed. Compare to the rotation period of the Earth. Can you think of why Jupiter appears flattened?

5. Diameter of Jupiter: Using the Method of Transit Times, determine the apparent diameter of Jupiter. During the Spring 2002 semester, the distance of Jupiter will be about 4.13 AU from Earth (1 AU = 1.496 X 10^8 km). You can find the declination of Jupiter using the dial on the declination axis of the telescope. Calibrate the dial by checking its reading with the declination of a nearby star of known declination (see the list of stars in "Star hopping"). Alternately, you can plot the position of Jupiter on the SC001 chart (e.g. Jupiter's motion lab) and read its declination on the chart to within 1^o.

   • Transit Time for Jupiter [seconds]:
   • Diameter of Jupiter [seconds of arc]:
   • Diameter of Jupiter [km]:

6. Follow-up work: Answer the following questions:

   Question: How does the diameter of Jupiter compare with Earth's diameter (13,000 km)? In other words, what is the ratio of Jupiter's diameter relative to that of Earth? How does Jupiter's diameter compare with the average Earth-Moon distance (384,000 km)?

   Question: Recall that density rho is a measure of the amount of matter contained in a certain volume and is given by the formula rho = m / V, where m is the mass and V is the volume. Given that the mass of Jupiter is about 318 times the mass of the Earth, calculate the average density of Jupiter relative to that of the Earth. Assume a spherical volume for both the Earth and Jupiter.

   Question: List some of the possible sources of error that may be significant in the procedure you used to determine the diameter of Jupiter. Which is most important?