**Discover the unique motion of Jupiter’s moon Io during an Earth day experiment. Learn about the Galilean satellites, explore their orbits, and observe their positions through a creative scale model. Explore the dynamics of celestial bodies and engage in hands-on astronomy with step-by-step instructions and insightful insights. Dive into the intriguing world of space science and planetary motion while uncovering the fascinating secrets of Jupiter’s moons.**

**SCIENCE** – The four largest satellites of Jupiter are called the Galilean satellites because these satellites were discovered in 1610 by Galileo. The names of these satellites are Io, Europa, Ganymede, and Callisto, from the closest to the farthest from Jupiter.

The four satellites you mentioned are natural satellites orbiting the planet Jupiter. These four satellites are known as the Galilean satellites or Galilean moons, named after the scientist Galileo Galilei who first observed them in the 17th century.

Io is the innermost satellite of the four Galilean satellites and has a highly unique surface across the entire Solar System. Io is one of the hottest and most geologically active objects in the Solar System. Its surface is adorned with various types of geological activities, including volcanoes, lava plains, and relatively large craters.

In this experiment, you will discover the motion of each Galilean satellite during one Earth day (24 hours). You will plot the positions of these satellites as they orbit Jupiter and determine their locations relative to Jupiter as seen from above Jupiter’s north pole and from an observer’s viewpoint on Earth. Based on actual observations, you will identify the Galilean satellites by their names and further study each of them.

**Learning Objectives**

Determine the motion of Io during one Earth day (24 hours).

Materials

sheet of A4 paper

ruler

protractor

pencil

compass

**Procedure**

- The actual radius of Io’s orbit is approximately 1.42 x 10^5 km. Using a scale of 6 mm = 1 x 10^5 km, calculate the radius of the scaled model of Io’s orbit as follows: Actual radius of Io’s orbit / Scaled radius of Io’s orbit = 1.42 x 10^5 km / 6 mm = 25.2 mm Rounded to the nearest millimeter, the diameter of the scaled model of Io’s orbit becomes 25 mm.
- Use a compass to draw a circle with a radius of 25 mm. This circle represents Io’s orbit.
- Create a small circle in the center with a diameter of approximately 8 mm. This circle represents Jupiter.
- Label the directions “East” and “West” on the paper as shown in Figure, with “Facing South” at the bottom of the paper.

- Mark a point on the eastern side of the circle and label it as “1.” This point represents the initial observation location of Io.
- To find the angular distance traveled by Io during one Earth day (24 hours), divide the total angular distance (360°) by Io’s orbital period (1.77 days). Round the answer to the nearest degree. Angular distance = 360° / 1.77 days = 203°
- Use the protractor to locate a point on the circle at 203° from point 1, and label it as “2.”
- Repeat step 7 to locate a point at 203° from point 2, and label it as “3.” Repeat the process again to find the location of point 4.
- Use a ruler to draw arcs at 203° as shown in Figure 21.1 to represent the angular distances between the points.

**Results**

The points represent Io’s positions after day 0, 1, 2, and 3 on Earth, with observations starting from position 1.

**Why?**

Satellites are celestial bodies or human-made objects that orbit other celestial bodies, such as Io orbiting Jupiter. The four largest moons of Jupiter are called the Galilean satellites. Io is the closest Galilean satellite to Jupiter. The actual radius of Io’s orbit is approximately 4.2 x 10^5 km (2.6 x 10^5 miles). In this experiment, you used a scale of 0.6 cm = 1 x 10^5 km. On this scale, the small circle at the center approximates Jupiter’s diameter as 1.4 x 10^5 km (0.875 x 10^5 miles). A distance of 2.5 cm from the center of the circle approximates Io’s orbital distance from the center of Jupiter. The locations of points 1, 2, 3, and 4 depict Io’s motion over 3 Earth days (72 hours). On the first day, Io moves counterclockwise from point 1 to point 2, covering 203° around Jupiter. On the second day, Io moves from point 2 to point 3, also covering 203°. Then, on the third day, Io moves from point 3 to point 4, again covering 203°.

**Try a New Approach**

Assuming all of Jupiter’s moons begin their motion from the same point on the first day of observation, how will their actual distances from Jupiter appear at the end of two Earth days? Repeat the experiment using data from Table to determine the angular distances for each moon’s motion during one Earth day.

Draw the orbits of these moons with their accurate radii from Jupiter’s core using the same scale (0.6 cm = 1 x 10^5 km). Use different colored markers to represent each satellite. One by one, draw the positions of each satellite over two days.