How to Track Jupiter's Moons

When you've gained some experience in astronomy, you might want to repeat Galileo's classic observation of Jupiter's moons. Galileo plotted their positions every night and tracked them by making a simple graph. Now you can repeat his classic observation without worrying about being accused of heresy!


  1. Image titled Track Jupiter's Moons Step 1
    Know where Jupiter is in the sky. You'll need to point a telescope at it later. Use an online sky chart to find the best time for observing Jupiter.
  2. Image titled Track Jupiter's Moons Step 2
    Find a good time to observe. You have to observe at the same time (within a 30 minute window) every day, or else the results won't be as accurate.
  3. Image titled Track Jupiter's Moons Step 3
    Aim your telescope at Jupiter. Focus on it and center it in the eyepiece. You should see from one to four moons, all of which should be in a straight line.
  4. Image titled Track Jupiter's Moons Step 4
    Graph the moons. On a piece of lined paper, draw a vertical line (perpendicular to the lines) representing the position of Jupiter. Gregg ruled tablet paper has a vertical line in the center. For every other line, make a large dot representing Jupiter at the intersection of the lines. Plot the moons relative to the vertical line. It's a good idea to number them by brightness, but only do this if all four moons are visible. Remember to place the date on the side!
  5. Image titled Track Jupiter's Moons Step 5
    Repeat this observation for at least a week. A month is even better, as you'll increase the accuracy of your measurements. If you cannot observe on a certain day, skip the graph for that day, and fill in data for another day.
  6. Image titled Track Jupiter's Moons Step 6
    Connect the observations with a line. When you do this correctly, you'll see a wave pattern being traced out. Now you have a graph of the moon's positions over time. With a bit of trigonometry and calculus, you can even develop an algorithm to predict the moons' positions over several years!


  • A dark sky site is not needed. However, you need a telescope.
  • Note the vertical displacement of the moon's positions. When you overlay all of the observations, you'll get an ellipse.
  • If your telescope is large enough, you may be able to catch Jupiter during the day.
  • Jupiter is better placed for Northern Hemisphere viewers now.
  • You can do this with Saturn as well. But you'll need darker skies and a larger telescope.
  • If your telescope is large enough, you can also trace out details on Jupiter itself.
  • You might see a moon crossing in front of Jupiter. Don't log brightness of the moons if this happens.


  • Remember to watch your surroundings when going out to observe!

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Categories: Astronomy