Galileo and His "Starry Messenger"

In this Launchpad, you will explore a section of Galileo’s booklet, Starry Messenger, in which he describes his observations of the solar system and stars through a telescope that he made. This telescope allowed him to see the distant objects in the sky in ways that no one had ever seen them before.

Background: Stargazing Before Galileo

Watch the short video,which sets the stage for Galileo’s discoveries by introducing the ideas about the nature of stars before he observed them through a telescope.


Here’s some vocabulary that appears in the video. What do these terms mean? Write a definition in your own words on the student vocabulary worksheet:

  • astronomer
  • telescope
  • constellation
  • Aristotle
  • sphere
  • eclipse
  • Ptolemy/Ptolemaic Model
  • Tycho Brahe
  • Nicholas Copernicus
  • Galileo Galilei
  • Sidereus Nuncius

Let’s answer some questions about the video:

  • What was the general understanding of the relative position of the earth to the sun before Galileo? What important model reflected this understanding?
  • What did early astronomers believe about the position of the stars relative to each other?
  • How did they group them in order to make sense of them?
  • What role did the Greeks play in applying mathematical models to the heavens? Why was this important?
  • What crucial observation did Aristotle make?
  • Which astronomer before Galileo proposed a different model of the solar system? What did he get right? What did he get wrong?
  • In his book, Starry Messenger, what did Galileo write about? What did he do that was different than what had been done before?

Now, step into Galileo’s shoes and do an experiment with the night sky. For this, you will need materials to write down your observations and a device with magnification strong enough to look at the sky, such as a camera or phone with a zoom lens, a pair of binoculars, or other hand lens. (Make sure to remember the magnification of the device (usually noted as 3X, 10X, etc.)

  • Locate a common, bright celestial object (the Moon or Venus, for example) and write down how it appears to your unaided eye
  • Can you see its outline clearly? If the surface is visible, is it rough, smooth? How about color? Is there any other heavenly body near it?
  • Now pick up your device and look at this object again. What do you see? How has the object changed its appearance? Is there more detail? Is it bigger or smaller? Is there anything near it you didn’t see before?
  • Record these observations

Close Reading: Starry Messenger

To better grasp Galileo’s understanding of what both you and he saw using a magnifying tool, you will be reading the first seven sections of an English version of Galileo’s Starry Messenger (Sidereus Nuncius in Latin), which deals with the astronomer’s observations of the stars through a telescope and answer questions about the text.

You may be assigned your first reading of the text as homework or as a group activity led by your teacher in class. Each section in your reading has a column for the text and one for your notes and questions. In Sections 5 and 6 of the reading, you will also need to refer to Galileo’s drawings of two star groups: Orion and the Pleiades (see Resources, below).

Your teacher will guide you in a discussion of the focus questions that accompany this activity.


Here’s some vocabulary you have encountered in the reading. What terms were defined by Galileo in the text? For which ones did you need to infer meaning from your reading?


  • compass
  • disc
  • erratic
  • fixed stars
  • fringe
  • globular
  • interval
  • magnitude
  • Orion
  • Pleiades
  • precinct
  • Sirius
  • Taurus

Wrapping it all up

  • Think back to the state of astronomy before Galileo in Activity 1
  • Now compare that to what Galileo has noted in Starry Messenger about the nature of the stars as seen through a telescope (as opposed, to give one example, to what he notes for the planets)
  • Write down as many examples from the text as you can that point to his overall argument for using such a scientific device
  • Now, review your examples and order them from strongest to weakest
  • Compare your answers with your classmates’ and come to an agreement on the best responses to the questions

Comparing Galileo’s Drawing of the Pleiades to NASA Photographs

How accurate was Galileo’s drawing and description of the Pleiades in the Starry Messenger, compared to what we now can see from our high-powered telescopes? Let’s analyze the accuracy of the early astronomer’s conclusions about these systems.

With your drawing of the Pleiades from the resources for Activity 2 and the text and your notes from Section 6 of the close reading, open up the NASA image of this star group and the graphic organizer for the Pleiades from the resources listed below.


On your graphic organizer, list the similarities you can find betweem these two views of the Pleiades as well as their differences:

  • Try to match the larger stars that Galileo drew to the ones you see in the NASA image (you might need to turn the images around to get them oriented in the same direction—note the arrows on the worksheet)
  • Do you see any similarities in the relative scale (size) of the individual stars between the two images?
  • Do these images share any similarities in the way the star group is configured as a whole?
  • Does one image show more detail than the other? Which one?
  • Are some stars larger relative to the others in one image or the other image? Clearer? Which one(s)?

When you have finished filling out the organizer, tally up your observations to see how accurate Galileo was in comparison to what we now know based on more powerful telescopes. Was he very accurate in his drawing? Accurate in his overall findings? If he was not accurate, where did he make a mistake? Why do you think that was?

  • Write a short paragraph summarizing your observations in clear English about how accurate Galileo was in his conclusions about the placement of the stars; their size; the accuracy of their relationship to one another; and how clearly he depicted them, based upon the NASA photograph.

How Much Have You Learned? To Galileo … and Beyond

How much have you learned about the new understanding Galileo gained about the stars using his telescope? Are you ready to try your hand at comparing his knowledge about measurement and distance and a star’s brightness with what contemporary NASA scientists are learning about these factors as they relate to distant stars when they use the powerful Hubble Space Telescope?

Here goes:

  • You will need your filled-out Star Comparison Organizer of the Pleiades from the “Comparing Galileo’s Drawing of the Pleiades to NASA Photographs,” above
  • You will also need to access the news release from a NASA story about the Pleiades listed below. This story presents new scientific information about measuring distances to distant stars developed as a result of analysis of images from the Hubble Space Telescope


Read the news story, including the “Introduction” and the “Release Text”. Look at the images. You may have to go over the longer text several times. In one to two pages, write an analysis on the progress of astronomical observation. Include the organizing clues, below, to help you.

  • Make a list summary of what this text has to say about our newest understanding of how we understand brightness. How do we measure distance?
  • Using your graphic organizer, gather a list of Galileo’s observations on these factors
  • Compare
  • Write a summary of how astronomical observation has progressed based on your comparison of these two methods
  • Don’t forget to refer to images from Activity 3!


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