Jupiter’s Moons Lab
The purpose of this lab was to determine the period and semi-major axis for the orbit of each moon around Jupiter. We then apply Kepler’s Laws to calculate the mass of Jupiter from out observations of each moon.
Choose a moon per lab partner.
Open the Moons of Jupiter program by clicking on START → Programs → Academic
Department → CLEA Exercises → Jupiter Moons.
The Moons of Jupiter program simulates the operation of an automatically controlled telescope with a Charge-Coupled Device (CCD) camera that provides a video image to a computer screen. It is a sophisticated computer program that allows convenient measurements to be made and the ...view middle of the document...
You control the observing session from this Main Telescope Screen. Notice that Jupiter is displayed in the center of your computer screen. To either side are the small point-like Galilean satellites. Sometimes a moon is behind Jupiter, so it cannot be seen. Even at high magnifications, the moons are very small compared to Jupiter. The current telescope magnification is shown in the upper left corner. The date, the UT (Universal Time – the time in Greenwich, England), and the Julian Day (JD) are all displayed in the lower left. You can display the screen at four scales of magnification by clicking on the 100X, 200X, 300X, and 400X buttons at the bottom of the screen. Try them now. To improve the accuracy of your measurement of a moon, you should always use the largest possible magnification which leaves the moon visible on the screen.
Be careful NOT to click the button marked next just yet! Now, position the mouse cursor over the screen and hold down the button. The measurement system turns on and displays the apparent perpendicular distance X (in JDs) that the cursor is away from the center of Jupiter. Notice that each edge of Jupiter is X = 0.5 JD. In order to measure the perpendicular distance of each moon from Jupiter, move the cursor until the moon is centered in the cross hairs and then hold down the mouse button. When the satellite is carefully centered, release the mouse button and information about the moon will appear at the lower right corner of the screen. This information includes the name of the selected moon, the X and Y pixel location on the screen, and the perpendicular distance X (in units of Jupiter's Diameter) from the Earth-Jupiter line-of-sight for the selected moon, as well as E or W to indicate whether it is east or west of Jupiter. If the moon's name does not appear, you did not center the moon in the cross hairs exactly and you should try again!
To measure a moon, always switch to the highest magnification that still leaves the moon on the screen. It is important to use the highest magnification possible for each moon for best accuracy. The data are for an imaginary moon, Clea. Alternatively, if you recorded the data on the computer, you can analyze them by clicking File → Data → Analyze and Select a moon from the menu. All of that moon’s data are plotted and you can now determine its orbital period and its distance from Jupiter. Use the Plot → Fit Sine Curve → Set Initial Parameters to help you do this. For some moons, you may not get enough observations for a full period, so these points may be of use to you in determining the period, even though the moon has not gone through a complete orbit. On the other hand, if you have enough observations for several cycles, you can find a more accurate period by taking the time it takes for a moon to complete, say, 4 cycles, and then dividing it by 4.
Once you find P, it must then be converted to units of years by dividing by 365.25 days. Remember, it is important to stay...