Northern Illinois University
Department of Physics
College of Liberal Arts and Sciences

Jupiter is the 5th planet in the solar system. It is typically one of the brightest objects in the night sky, and is readily visible through the telescopes of the Davis Hall Observatory or a telescope you can borrow. On a good night, with the reflecting telescope, the banding of the planet is readily visible, as well as the four Galilean Satellites; Io, Europa, Callisto, and Ganymede.

Since the moons are so readily visible around Jupiter, they frequently become the center of projects focusing on the Jovian system. With the camera mount, it is extremely easy to take pictures of its moons. If one compares the pictures from night to night, the movement of the satellites will become evident. With time, the alert observer will be able to pick out which ones are which by the orbital period of the moons, which can be calculated based on the observations. Both the astronomy magazine Sky and Telescope, and many astronomy textbooks have information on the moons which can help identify which ones you are observing.

The moon's are also observable using a portable telescope which the Department can loan you. It is of a higher quality than what Galileo used. If you want to make observations of Jupiter's moons over a number of nights, it may be easier to just use the portable telescope.

The precise experimental procedure is outlined below. Depending on your ambition, you can utilize Kepler's laws of planetary motion, or use the simple geometric trick, both described below. With the use of the camera, the three earth diameter wide red spot, signifying the center of a great storm in the atmosphere of the planet, may become visible. If you can capture images of the storm regularly, then an attempt to measure the rate at which the storm moves around the planet could be attempted, though we would recommend this as a bonus rather than as the main focus of the project, due to the elusive nature of the target and the precise timing required.

Procedure - with Davis Hall Observatory

1. Locate Jupiter in the telescope at Davis Hall.
2. With the observatory manager's assistance, capture at least one image, or more if you choose.
3. Try to take a picture on a different night and note how the moons have moved. ALWAYS RECORD THE TIME between when picture sets are taken.
4. Keep the images organized in order. Try to pick out which moons are which by size, brightness and position. You don't have to identify them yet, only keep track of which ones are the same ones from image to image.
5. It would be useful, once you have acquired several images, to begin construction of a data table. Lable the entries "Moon1, Moon2, etc." and record the times and positions achieved in relation to Jupiter in those times. You can measure in relative terms with a millimeter ruler. This is where consistent timing on the part of the observer comes into play.
6. Keep in mind that we are looking at Jupiter edge on, and so the orbits of the moons will look more like oscillations back and forth rather than elliptical orbits. If a moon appears to be moving slowly, or not at all, it is more than likely that it is moving more toward or away from us rather than across our field of view.
7. Once you have collected all your observations, you can compare data collected with the expected results which can be found in a book or calculated, using Kepler's laws below. Then you can attempt to distinguish which moon is which by name.

Procedure - with portable telescope

1. go to as dark a spot as you know bringing along a flashlight.
2. Locate Jupiter by eye and then center it in the telescope using the wide angle eyepiece. You can then switch to the other eyepiece to see a more magnified image.
3. Sketch a picture of what you see and record the time.
4. Reperat on different nights and attempt to identify the moons as discussed above and below.

Kepler's First Law

Each moon will orbit Jupiter in an ellipse, with Jupiter at one of the foci. The semi-major axis, which is the average of the maximum distance from Jupiter (apogee) and the closest approach (perigee), is related directly to the period of the moon's revolution about Jupiter. Measuring the period (P) in years and the distance in astronomical units (AU), the relation can be given most simply in mathematical terms as P = sqrt( AU^3 ). This is Kepler's first law. Attempt to estimate motion with time to estimate the period of the moon. (Since you will have only three months worth of data, this will be a difficult task, but possible.) Based on these estimated periods, try to reverse engineer the distances of each of the moons and determine which ones they are. Cross reference with the quick method below as a check.

Quick Easy method

With photographs of Jupiter, one can use facts such as the width of Jupiter to measure the apparent distance form the planet to the moon. For example, you could say that since an observed moon in the photo is measured as having a distance from the planet greater than the maximum possible for Io and Europa, it must be either Callisto or Ganymede. This method is highly unreliable because you are only seeing the moons edge on, and moons that may seem close to the planet are actually pretty far away. However, it may be convenient and helpful for verification of the moons. If you make observations over an extended time, you may see the moons' maximal sepration from Jupiter, or be able to measure the time it takes to make a quarter orbit.