Name: ________________________  DUE 10/16  ID number: ________________________ 
Between January 7^{th} and 15^{th} of 1610, Galileo discovered the four `Galilean Satellites' of Jupiter; listed in order of their distance from Jupiter, these are Io, Europa, Ganymede, and Callisto. On the diagram at left, the vertical axis is the date, and the horizontal axis is the angle from Jupiter, measured in degrees. The filled circles show Galileo's observations, while the dotted lines show the positions of the satellites at other times. Using such observations, Kepler tried to find out if the Galilean Satellites obeyed his third law. This assignment involves reproducing some of Kepler's calculations. To do this you need to measure the semimajor axis a and the period P of each satellite's orbit. To measure the semimajor axis of each satellite's orbit, you must first find its greatest angle from Jupiter; call this angle and write it on the first line of the table below. Try to make your measurement accurate to onehundredth of a degree. To measure the period of each satellite's orbit you must determine the length of time before it returns to its original position. One way to make this measurement is to mark two successive points where the satellite crosses the solid vertical line (which represents Jupiter's position); the period is twice the time interval between two such crossings. Write the period on the second line of the table. Try to make your measurement accurate to onetenth of a day.

It's convenient to convert each greatest angle to a semimajor axis a measured in AU; this can be done using the smallangle formula:
Likewise, it's convenient to convert each period from days to years. Just divide the periods P listed in the second line of the table by 365.26; write the results on the fourth line of the table.
Finally, you're ready to check if Kepler's third law is obeyed by the Galilean Satellites! Using the values of a and P you have on the third and fourth lines of the table, compute the quantity a^{3} ÷ P^{2} and write it on the last line of the table.
Now the question is: do the satellites of Jupiter obey Kepler's third law? Recall that the third law says that a^{3} ÷ P^{2} has the same value for all objects orbiting a given body. But no real measurement is perfectly accurate, so even if the law is obeyed the values of a^{3} ÷ P^{2} will not all be exactly the same for all four satellites. But are they close? In other words, do you think that the differences in your a^{3} ÷ P^{2} values are due to errors in your measurements and calculations, or do they show that Kepler's third law really doesn't apply to the Galilean Satellites? Why or why not?
Extra credit: if a is measured in AU and P is measured in years, the quantity a^{3} ÷ P^{2} is equal to the mass of the central body  in this case Jupiter  in units of the Sun's mass, M_{}. From your results from the last line of the table, what is your best value of Jupiter's mass in units of M_{}?
Last modified: October 7, 2001
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