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Author: Jordan Raddick, Johns Hopkins University
Editor: Beth Hufnagel, Anne Arundel Community College
The goals of this module: After completing this exercise, you should be able to:
Recognize the four Galilean moons (Io, Europa, Ganymede and Callisto in order of increasing distance away from Jupiter) traveling with Jupiter around the Sun.
Compare the apparent speeds of a Jovian moon at different parts of its orbit as seen from Earth, and explain why the variation is mostly an effect of watching the orbit from the side.
Measure the approximate orbital period of Io.
In this module you will explore:
What Galileo observed about Jupiter's moons.
How orbits appear when viewed from the side.
How to recognize different moons from observations
Why you are doing it: Galileo's observations of Jupiter's four largest moons in 1610 were the first definitive proof that Earth was not the center of the universe. They also helped astronomers realize that Copernicus's Sun-centered model of the Solar System was better than an Earth-centered model. (Simon Marius claimed that he had observed them first in 1609, but Galileo published first. In addition, Marius had already been suspected of plagiarizing from Galileo two years before1.)
In the early 1600s, astronomers were still not sure about Earth's place in the Solar System and the Universe. Since ancient times, astronomers believed that the Earth was at the center, and that the Sun, planets, and stars all orbited around Earth. In 1543, Nicolas Copernicus proposed a model in which the Sun was at the center of the Solar System, and all planets including Earth revolved around it. Scientific models are judged by the predictions they make, and Copernicus's model predicted the positions of the planets just as well as the ancient models. There was no way to easily decide between the models, so many astronomers stuck with what they knew.
In 1608, an Italian scientist named Galileo Galilei heard about a new invention called the telescope that allowed people to see things that were very far away. Galileo decided to build his own. The next year, he began making observations of the Moon and stars.
On January 7, 1610, Galileo looked at Jupiter through his telescope. He discovered four bright stars next to Jupiter. He thought that Jupiter would leave them behind as it continued in its orbit, but the next night, he still saw them next to Jupiter!
He watched them for a week, and they stayed next to Jupiter. He checked ancient records; no one had ever seen these "stars" before. Could the mysterious stars be moons orbiting Jupiter, just as our moon orbits Earth?
In this tutorial you will see the moons of Jupiter, just as Galileo saw them in 1610.
Image courtesy NASA/JPL-Caltech.
13.3What Did Galileo See in 1610?
The animation below shows what Galileo would have seen in his telescope. These are the actual positions of Jupiter's moons on January 7, 1610. (False colors have been added to the moons to help you see them better.) Click the play button to play the animation from beginning to end. It shows the positions of the moons once per hour from 1800 Coordinated Universal Time (UTC) to 0700 UTC the next morning, which was about the time Galileo could have seen them. Click the pause button to pause the animation at any point. You can also click the step forward buttun to move forward by one hour or the step backward button to move backward by one hour. You can also click the stop button to reset and start the animation over from the beginning.
Observe the animation carefully, and answer the questions below.
At the very end of the animation, on January 15 at 0700, how many moons were at the right of Jupiter?
A.
B.
C.
D.
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Try again. Watch the animation again, and pay attention to where the moons were on January 15 at 0700 (at the end of the animation). It may help to step the animation forward to that hour.
Correct. All four moons were to the right of Jupiter.
Incorrect. All four moons were to the right of Jupiter.
Question
2.
On January 9 at 2300, there were only three moons visible. Where was the fourth moon?
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B.
C.
D.
E.
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Try again. If you step the animation forward to 0200, it reappears.
Correct. We have an edge-on view of Jupiter's system, so we see the moons moving from side to side. As the moons go around Jupiter, once each orbit they go behind and then in front of Jupiter from our perspective.
Incorrect. We have an edge-on view of Jupiter's system, so we see the moons moving from side to side. As the moons go around Jupiter, once each orbit they go behind and then in front of Jupiter from our perspective.
Question
3.
Watch the moon that is furthest to the left at the beginning of the animation (January 7th at 1800). How did it move during the week Galileo observed it?
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B.
C.
D.
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Try again. Watch the animation again, and see what happens to the moon on the far left.
Correct. You can see that the moon moved slightly to the left, then all the way to the right. It completed about half of an entire orbit around Jupiter.
Incorrect. You can see that the moon moved slightly to the left, then all the way to the right. It completed about half of an entire orbit around Jupiter.
Question
4.
Which of the Galilean moons did you watch in Question 3? How do you know?
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B.
C.
D.
E.
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Try again.
Correct. The moon that you watched in Question 3 was Callisto. You can conclude this in two ways. That moon moves more slowly than the other three moons (gravitational attraction decreases as the distance increases), and moves farther away from Jupiter than any of the others.
Incorrect. The moon that you watched in Question 3 was Callisto. You can conclude this in two ways. That moon moves more slowly than the other three moons (gravitational attraction decreases as the distance increases), and moves farther away from Jupiter than any of the others.
Question
5.
At which point in their orbits do the moons appear to move the fastest?
A.
B.
C.
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Try again. Watch the animation again. You can watch any of the moons you want, but the ones that start closest to Jupiter will be the easiest to see.
Correct. The moons appear to move fastest when they are close to Jupiter.
Incorrect. The moons appear to move fastest when they are close to Jupiter.
Question
6.
On January 8 at 0200, two moons were in front of Jupiter. Look at their relative positions then, and how much space separated them. At 0300, they are still in front of Jupiter. Notice their positions and the space that separates them. At 0400, they have moved away from Jupiter, but are hard to separate from one another. At 0500, what is the relative position of the two moons?
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B.
C.
D.
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Try again. Watch the animation again and notice what happens to the two moons.
Correct. The moons have switched places.
Incorrect. The moons have switched places.
Question
7.
In Question 5, you noticed how the moons move when they are in different parts of their orbits. In Question 6, you noticed the speed of two moons when they are lined up from our viewpoint. Therefore, the moon that passed the other one is moving fastest. Keep watching this moon. You should be able to follow it even though you miss part of its orbit due to the daytime. Based on the way it orbits Jupiter, which moon do you think it is?
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B.
C.
D.
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Try again. Watch that moon carefully. Where does its orbit take it, and how fast does it orbit?
Correct. That moon moves faster than all the others, and stays closer to Jupiter. So it must be the closest of the four Galilean moons, Io.
Incorrect. That moon moves faster than all the others, and stays closer to Jupiter. So it must be the closest of the four Galilean moons, Io.
Question
8.
About how long does it take that moon to orbit Jupiter?
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B.
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D.
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Try again. Watch the animation again, and focus on the moon that you watched in Question 7. It might help to try watching once by playing the animation, and again by stepping forward one hour at a time.
Correct. The moon you watched in Question 7 takes about one or two days to orbit Jupiter (the currently accepted value is 1.76 days).
Incorrect. The moon you watched in Question 7 takes about one or two days to orbit Jupiter (the currently accepted value is 1.76 days).
13.4How Would the Orbits Look from Above?
In the last section, you looked at the four largest moons of Jupiter as Galileo saw them over a period of nine days. You noticed how the moons appear to move faster when they are close to Jupiter, and slower when they are far away from Jupiter. Play the animation again, but this time click on the Top View. This is the inferred explanation for how the Jovian system looks from above the North Pole of Jupiter - confirmed centuries later by NASA's Galileo Mission!
How does what you see in the Top View explain the moons' motions?
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B.
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Try again. Watch both views carefully and see how fast the moons appear to move.
Correct. When the moons appear close to Jupiter in the Side View, their motion is mostly side-to-side from our perspective, so they appear to move faster. For a quantitative analysis of the orbits of the Moons, you may want to do the separate Kepler's Laws activity.
Incorrect. When the moons appear close to Jupiter in the Side View, their motion is mostly side-to-side from our perspective, so they appear to move faster. For a quantitative analysis of the orbits of the Moons, you may want to do the separate Kepler's Laws activity.
13.5Evidence for a Sun-centered Solar System
You have seen the four Galilean moons moving as Galileo saw them. You learned in the Background to this activity that astronomers in Galileo's time did not agree on whether the Earth or the Sun was at the center of the Solar System.
Question
10.
How does the fact that the Galilean moons orbit Jupiter provide evidence that the Earth is not at the center of the Solar System?
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B.
C.
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Try again. Which feature of Jupiter's moons makes them possible in a Sun-centered Solar System but not in an Earth-centered Solar System?
Correct. Copernicus's Sun-centered Solar System theory allowed for moons to orbit other planets, so astronomers slowly started to accept the Sun-centered Solar System theory instead.
Incorrect. Copernicus's Sun-centered Solar System theory allowed for moons to orbit other planets, so astronomers slowly started to accept the Sun-centered Solar System theory instead.
13.6What Did the Galilean Moons Look Like to Modern Probes?
The animations below show simulated views of what the Galilean moons looked like to the Galileo space probe when it flew by the moons as part of its exploration to Jupiter. As you can see, we can get a totally different view from robotic missions than Galileo could with his telescope on Earth. Our view of these moons has certainly changed since 1610!
Galileo's view of Io
Galileo's view of Ganymede
Galileo's view of Europa
Galileo's view of Callisto
13.7Quick Check Quiz
Indepth Activity: Orbits of the Galilean Moons
Question
11.
The first person known to use a telescope to study the sky was:
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B.
C.
D.
E.
Correct.
Incorrect.
Question
12.
Jupiter's four large moons were:
A.
B.
C.
D.
E.
Correct.
Incorrect.
Question
13.
Why aren’t all four large moons of Jupiter always visible?
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B.
C.
D.
E.
Correct.
Incorrect.
Question
14.
____________ orbits faster than any of the other large moons of Jupiter. It takes less than ____________ days to make a complete orbit.
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B.
C.
D.
E.
Correct.
Incorrect.
Question
15.
When we look at Jupiter from Earth, Io:
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B.
C.
D.
E.
Correct.
Incorrect.
Question
16.
Why do the moons appear to move faster when they appear closer to Jupiter?
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B.
C.
D.
E.
Correct.
Incorrect.
Question
17.
How long did Galileo watch the moons before he realized they were orbiting Jupiter?
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B.
C.
D.
Correct.
Incorrect.
Question
18.
Put the four Galilean moons in order of increasing speed of their orbits around Jupiter:
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B.
C.
D.
Correct.
Incorrect.
Question
19.
When Galileo first saw the moons of Jupiter, what did he think they were?
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B.
C.
D.
Correct.
Incorrect.
Question
20.
Which of the following was an important effect that Galileo's observations of the moons of Jupiter had on astronomy?
