This guest post comes from Trevor Owens, Special Curator for the Library of Congress Science Literacy Initiative.
When we think about difficult concepts, it helps to think with models, smaller versions of systems that we can hold in our minds or sketch on paper. Science teachers, as well as the Next Generation Science Standards, recognize the importance of understanding models. Primary sources from the Library of Congress collections invite students to explore how different models of the universe have developed over time, and to think a bit more generally about interpreting models.
An Earth-Centered System
For Aristotle, and many of the ancient Greeks, the sphere of the world was made up of four elements: earth, water, air and fire. These elements shifted around a nested set of solid, crystalline spheres. The rest of the universe, and all of its stars, were on the last crystal sphere. If you watch the sky at night, you can see that this idea of crystalline spheres explains a lot about the motion that you observe.
This 1613 diagram is an example of an Earth-centered model. Around the Earth you can see air (aer) and fire (fyer). From there you can see spheres for each of the “wandering stars” or the planets. Note that the sun (Sonne) and the moon (mone) are presented as having their own spheres like the planets and speculate on what can be inferred from these details about what the creator of this model understood about the universe.
A Sun-Centered Cosmos
The 1543 publication of the Polish astronomer Nicolaus Copernicus’ On the Revolutions of the Heavenly Spheres proposed a different model. Instead of the Earth at the center of the universe it was the sun. The idea that the sun was at the center was revolutionary, but Copernicus still saw the universe in terms of rotations of solid heavenly spheres.
In this diagram, Copernicus presents his sun-centered model of the heavens. The sun (Sol) is at the center of the diagram, Terra (Earth) is depicted as the third planet orbiting the Sun, and the moon (illustrated as a small crescent) is shown orbiting the Earth. Note that the outermost circle, Stellariem Fixarum (the fixed stars) is described as immobile (immobilis).
- Look at the publication dates of the two drawings. What do they tell you about how different models of the universe are developed?
- Compare the two drawings and identify which characteristics they have in common, as well as the differences.
- Look for a few major differences between Copernicus’ model and what we know about the solar system today. What evidence led to these changes in the model? How did scientists gather that evidence?
- Look at a current model of the solar system. Think of a future discovery that would require that you change this model of how the system works—not just add or subtract from it.
Models certainly aren’t limited to the sciences. What models do you use in your teaching?