Jupiter
Goals of the Lab
To identify and observe the moons of Jupiter and their movements;
To identify and observe the features of Jupiter through careful observation.
Requirements: the telescope, your logbook, pencils, a calculator, a watch, printouts of several Observation Templates.
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 in the 1990s through the Galileo mission. For the latest information and cool pictures, consult the Galileo spacecraft home page.
Part I: The Moons of Jupiter
Make sure you have available several of the Observation Templates, either the large version or the small version.
Find Jupiter in the telescope, with the 25mm eyepiece. You should be able to see Jupiter and up to four moons. When you find Jupiter, pan the telescope east and west, moving Jupiter to either side of the field of view, to make sure that you've seen all the moons.
Make some notes in your observing log what you see. Remember, as always, to include the date and time, and the observing conditions.
Draw the field of view of the 25mm eyepiece on an observation template. Include Jupiter and all of the moons, as well as any stars in the field of view. (You may not be sure which are moons and which are stars; draw them all!) Draw the positions of the moons as accurately as possible. Make sure to include the date and time on your observation. Also make sure to indicate North and East on your drawing.
Repeat step 3 four more times during the night, with each drawing separated by half an hour. This means that you will need to come back during your work in Part II (below) to complete this. On each drawing, make sure you indicate the date and time. Also include in your logbook any notes or thoughts you have to go with each drawing. Naturally, each logbook entry should also include the date and time.
Use the Method of Tranist Times to measure the angular separation between the center of Jupiter and the moon which is closest to it. To determine this angular separation, you will use the same principle as the Method of Transit times. In this case, you want the distance between the center of Jupiter and its closest moon.
If Jupiter is to the West of the closest moon: use the fine adjust knobs to position the center of Juipter at the edge of the field of view, i.e. half of Jupiter appears in your field of view. Turn off the drive, and measure the time it takes the closest moon to hit the edge of your field of view.
If Jupiter is to the East of the closest moon: use the fine adjust knobs to position the moon right at the edge of your field of view. Turn off the drive, and measure the time it takes for the center of Jupiter to leave the field of view, i.e. the time until Jupiter is just half visible.
Report the time you measure (in seconds), and perform the calculations necessary to determine the angular separation in arcseconds or arcminutes.
Part II: Jupiter
Note: You will need to do this part of the lab interspersed with the repeated drawings in Question 4 of Part I.
Find Jupiter in the telescope with the 25mm eyepiece. Center it and switch to the 10mm eyepiece. Focus the telescope as well as you can. Make some notes in your logbook as to what you see. As always, include the date and time in your logbook entry.
Make a sketch of Jupiter on a large observation template. Draw the full field of view; use the large template so that your drawing of Jupiter will be large enough that you can include some detail. Draw everything you can see. It should be fairly easy to see two east-west bands on Jupiter. In addition, ask yourself the following questions, and look for the features described in the questions below. You won't see all of them, but you may see some of them. Include any you see in the drawing. With your drawing, make notes in your logbook describing what you see and don't see, and addressing the following questions. (Make sure to write sentences in your logbook. Just saying "no" or "yes" is not helpful, as the TAs grading your lab will have no idea which of the questions you are responding to. Instead, say something along the lines of, "both prominent equatorial bands appear equally dark and wide", or "The northern equatorial band is thinner but darker than the souther one", depending on what you see.)
How dark are the two most prominent east-west bands? Is one darker than the other? Is one wider than the other?
Are the bands uniform, or can you see any evidence of "clumpiness"? That is, are there spots on the bands which are slightly darker than the rest of the band?
Can you see any other east-west bands which are either darker or lighter than the rest of Jupiter?
Do the regions around the north and south poles of Jupiter look similar to the rest of Jupiter, or are they noticably a different color?
Can you see any features oriented north-south?
Can you see any coloring right at the limbs (edges) of Jupiter? If so, what colors do you see, and where do you see them?
Can you see the Great Red Spot? It has in recent years been very pale, and it is not always facing us, so there is a very good chance you won't be able to see it. If you do, indicate it on your drawing, and describe what it looks like.
Are there any moons of Jupiter in the 10mm field of view? If so, draw them.
Does Jupiter itself appear perfectly circular, or is it squashed in one direction? If it is squashed, be sure to indicate that in your drawing. Is it wider east-west or north-south? How apparent is this, if you can see it at all?
Make sure to include the date and time on both the drawing and the logbook entry. Also remember to indicate North and East in the field of view on your drawing.
Notes and suggestions: Take your time with this part of the lab. You are going to be here for a couple of hours finishing question 4 in Part I, so there's no reason to rush through this part. The inevitable shaking of the field of view of the telescope will require some patience. Watch Jupiter, and make sure to take full advantage of the moments you get when the telescope is not shaking. Also take the time to carefully look and see what features you can see, and to answer all of the questions above.
Measure the angular diameter of Jupiter using the Method of Transit Times. Report your observations (the time of the transit), and perform the calculations to provide an angular diameter in arcminutes or arcseconds.
As always, indicate the sky and air conditions in your logbook entry. Include the date and time in both the logbook entry and on your drawing.
Part III: Questions and Calculations
Answer all of the following in your logbook.
Did you see any of the moons of Jupiter move? Given how much they moved in the time you watched, comment qualitatively on how the period of at least some of Jupiter's moons compare to the period of Earth's moon (which is about a month).
Use the corkscrew diagrams (provided by the TAs) to determine which moon is which in your first drawing from part I. Label the names of the moons on the drawing.
Jupiter is between 4 and 6 AU away from the Earth. Typically when it is visible in lab, it will be roughly 4.5 AU (=6.7×108 km) away. Calculate the physical diameter of Jupiter in km using the small angle formula:
A / 206265 = d / Dwhere A is the angular diameter in arcseconds, d is the physical diameter of Jupiter, and D is the distance to Jupiter. 206,265 is the number of arcseconds in a radian; the more familiar small-angle formula A=d/D works when A is in radians. The formula above includes the conversion.
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)?
Use the small angle formula to calculate the projected physical distance between Jupiter and the closest moon. In this case, use the angular separation you measured in question 5 of Part I. Use the small angle formula (above), only now d is the projected distance. How does this compare to the average Earth-Moon distance?