Galilean Satellites: Jupiter's Largest Moons

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Author: Janice VanCleave

The four largest satellites of Jupiter are called Galilean satellites because they were discovered in 1610 by Galileo. Their names are 10, Europa, Ganymede, and Callisto, from closest to farthest from Jupiter.

In this project, you will discover the motion of each Galilean satellite during an Earth day (24 hours). You will plot the positions of the satellites as they move around Jupiter and determine their locations relative to Jupiter from a bird's-eye view above Jupiter's north pole, as well as from an observer's view from Earth. From actual observations, you will identify the Galilean satellites by name and learn more about each one.

Getting Started

Purpose: To determine the motion of 10 during an Earth day (24 hours).


    • sheet of white copy paper
    • drawing compass
    • metric ruler
    • pencil
    • protractor


Rounding the number to the closest millimeter, the diameter of the scale model of 10 would be 25 mm.

  1. The radius of Io's orbit is about 1.42 × 105 km. Using a scale of 6 mm = 1 × 105 km, calculate the radius of the orbit for a scale model as follows:
    • actual radius of Io's orbit/model radius of Io's orbit
    • 1.42 × 105 km ÷ 1 × 105 km/6 mm = 25.2 mm
  2. In the center of the paper, use the compass to draw a circle with a radius of 25 mm. The circle represents the orbit of Io.
  3. In the center of the circle, make a small circle with a diameter of about 8 mm. This circle represents Jupiter.
  4. Label the directions "East" and "West" on the paper as shown, in Figure 21.1 with "Facing South" at the bottom of the paper.
  5. Make a dot on the east side of the circle. Number the dot 1. This dot represents the first observation location of Io.
  6. To find the angular distance of Io's movement in one Earth day (24 hours), divide the angular distance of one revolution (360°) by Io's orbital period (1.77 days). Round the answer to the nearest whole number of degrees.
    • d = 360° ÷ 1.77 days = 203°
  7. Use the protractor to find a point on the circle 203° from the first dot. Make a dot on the circle at this point and number the dot 2.
  8. Use the protractor to find a point on the circle 203° from dot 2. Make a dot on the circle at this point and number the dot 3. Repeat to find the location of dot 4.
  9. Use the compass to draw 203° arcs as shown in Figure 21.1 to indicate the angular distance between the dots.

Galilean Satellites: Jupiter's Largest Moons


The dots represent Io's position after 0, 1, 2, and 3 Earth days, with observation beginning at position 1.


A satellite is a celestial or man-made body revolving around another celestial body, such as the moon 10 around Jupiter. Jupiter's four largest moons are called Galilean satellites. 10 is the Galilean satellite closest to Jupiter. The orbital radius of 10 is about 4.2 × 105 km (2.6 × 105 miles). In this project, you used a scale of 0.6 cm = 1 × 105 km. At this scale, the small center circle approximates Jupiter's diameter of 1.4 × 105 km (0.875 × 105 miles). The distance of 1 inch (2.5 cm) from the center of the circle approximates the orbital distance of 10 from Jupiter's center.

The locations of dots 1, 2, 3, and 4 indicate the movement of 10 over 3 Earth days (72 hours). During the first Earth day, 10 moved counterclockwise from dot 1 to dot 2, moving 203° around Jupiter. During the second Earth day, 10 moved from dot 2 to dot 3, another 203°. Then during the third day, 10 moved from dot 3 to dot 4, another 203°

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